Thermostat for hot water tank

Abstract

The invention discloses a thermostat (100, 200, 300) having an input terminal (110, 152) and an output terminal (112). A load switch (114) for selectively connecting an input terminal to an output terminal is provided, the load switch including an electromagnetic relay configured to interrupt an electrical connection between the input and output terminals when a coil (115) of the load switch is powered off. The thermostat comprises a temperature sensor (120) and a controller circuit (130) arranged to derive a temperature from the sensor. The controller circuit (130) is configured to selectively energize the coil (115) of the load switch (114) based on the temperature value to connect the input and output terminals. A thermal fuse (140) is provided and arranged to irreversibly disconnect the load switch coil (114) when the thermal fuse (140) is subjected to a temperature above a threshold, thereby disconnecting the input and output terminals.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to UK Patent Application No. 2206504.9 filed on May 4, 2022, which is incorporated herein by reference. Technical Field

[0003] The invention disclosed herein relates to thermostats and may find particular application in thermostats for controlling the temperature of hot water cylinders. Background Art

[0004] A thermostat is a device that can be configured to maintain the temperature of a particular heating or cooling process at a selected temperature, or at least within a specific temperature range. In electric heating applications, the thermostat is configured to supply power to the heating element if the measured temperature is below a lower threshold, and to interrupt the supply of power to the heating element if the measured temperature is above an upper threshold. In electric cooling applications, the thermostat is configured to supply power to the cooling system if the measured temperature is above an upper threshold, and to interrupt the supply of power to the heating element if the measured temperature is below a lower threshold.

[0005] A well-known example of an electrical heating process is a hot water cylinder. A hot water cylinder will always be equipped with a thermostat to maintain the temperature of the water in the hot water cylinder at a selected temperature. Depending on the water temperature, the thermostat will connect or interrupt the supply of electricity to the heating element as required, so that the water temperature will fluctuate around the selected temperature. Such thermostats are mostly electromechanical components that rely on the principle of bimetallic mechanical displacement to open and close a set of contacts as the measured water temperature changes. Most residential hot water cylinders have a waterproof pocket into which the bimetallic stem of the thermostat operably extends. The bimetallic stem is therefore in close proximity to the contents of the hot water cylinder, allowing the thermostat to react to changes in water temperature.

[0006] As the water in the hot water cylinder cools or heats, the center rod of the bimetallic valve stem contracts or expands as the case may be. This enables the center rod in the valve stem to actuate a set of bimetallic electrical contacts of the thermostat to open (disconnect the power supply from the heating element) when the temperature acting on the valve stem exceeds the set temperature, and to close (connect the power supply to the heating element) when the temperature acting on the valve stem drops below the set temperature, thereby regulating the temperature within a temperature range around the set temperature.

[0007] Thermostats also feature thermal safety cut-outs as a fail-safe mechanism in the event of a malfunction. These thermal safety cut-outs are configured to disconnect the power supply from the heating element in the event of an overheat, which could be caused by the contacts of the thermostat getting stuck in the closed state and causing the water in the hot water cylinder to heat up uncontrollably. Typically, the thermal protection is performed by an actuator consisting of a bimetallic element. The actuation produced by the bimetallic stem causes a set of thermal protection contacts to open irreversibly, thereby irreversibly disconnecting the power supply to the heating element, if the stem is subjected to temperatures exceeding a safe value. Such events require human intervention and usually require complete replacement of the thermostat.

[0008] It is well known that hot water cylinders are one of the largest consumers of household electricity (if not the largest), contributing up to 50% of household electricity consumption. To address this, various energy-saving technologies related to hot water cylinders have emerged. These technologies vary greatly, from insulation to disconnecting the power supply to the unit manually or with the help of a timer switch.

[0009] The trend towards integrating hot water cylinders into the rest of the "smart home" has become a hot topic in recent years, particularly with the advent of the fourth industrial revolution and the Internet of Things. To achieve this integration, a smart controller needs to interface with the hot water cylinder to measure the water temperature and be able to control the supply of electricity to the heating element. Most, if not all, of these "smart thermostats" rely at least in part on digital processing to perform, perhaps with the aid of a microprocessor.

[0010] Recalling the thermal safety cut-outs described above, it is a safety requirement (and more broadly a regulatory requirement) that these thermal safety cut-outs operate passively. In South Africa, thermostats for hot water cylinders must comply with South African National Standard (SANS) 181. SANS 181 specifically requires that closed water heaters should incorporate a thermal cut-out that provides an all-pole disconnect and operates independently of the thermostat (although appliances intended to be connected to fixed wiring are not required to disconnect the dead conductors).

[0011] That is, the operation of these safety mechanisms should not be dependent on the operation of, for example, a microprocessor, as a failure of the microprocessor could render the safety mechanism ineffective. If the microprocessor ceases to execute while the relay is in the closed state, this would cause the hot water cylinder to heat uncontrollably, which could result in a dangerous, catastrophic failure. Therefore, if a "smart thermostat" utilizes a microprocessor-controlled relay to control the supply of power to the heating element, it would also require a passive thermal cut-off device as the most important fail-safe.

[0012] In some jurisdictions, regulatory requirements may allow for software-based operation of thermal safety cut-off mechanisms. However, the requirements for robustness and redundancy of such software-based mechanisms are onerous and may render their implementation prohibitively expensive. Such systems are also subject to regulatory testing and approval, which can also be a time-consuming and expensive undertaking that needs to be undertaken with each system revision.

[0013] Furthermore, with the cost of photovoltaic panels within reach of consumers, residential solar hot water cylinder installations are becoming increasingly popular. Many households that are considering converting their hot water cylinder to solar already have an existing electric hot water cylinder installed. Rather than replacing the existing electric hot water cylinder with, for example, a thermosyphon hot water system, it may be cheaper to retrofit the existing installation and power the electric hot water cylinder directly from photovoltaic (PV) power.

[0014] The contacts on thermostats used in mains-powered hot water cylinders are generally not suitable for switching direct current (DC). The travel of the switch contacts between their closed and open positions is typically about 2 mm. If the contacts open at the instant when the mains supply voltage is close to zero crossing, no arcing will occur across the open contacts. However, if the mains supply voltage is high enough at this point, an arc may form across the open contacts and will remain arcing until the voltage approaches zero crossing again (approximately ±10 V, depending on the travel of the switch contacts). This arcing increases contact wear and shortens the life of the relay.

[0015] In the case of a DC supply, such as from a photovoltaic module, the contacts will inevitably open at a non-zero voltage, and since the supply voltage will have no zero crossing, the arc will continue until the supply voltage is removed, drops below a certain voltage (for example, when the incident sunlight on the photovoltaic module decreases), or until the switch contacts fail catastrophically.

[0016] Therefore, retrofitting PV power on electric water cylinders also typically requires an inverter that converts the DC output of the PV modules (usually stored in a battery bank) to an AC power source (i.e., mains voltage) designed for the thermostat. The required capital expenditure increases dramatically with the addition of the required inverter, battery bank, and charger components, making this technology inaccessible to many low- and middle-income households that would benefit most from a PV-powered water cylinder.

[0017] Furthermore, with this modification, the mains power may be disconnected completely and the hot water cylinder may not be provided with any backup power. If adverse weather conditions continue, the hot water cylinder will run cold.

[0018] The applicant is aware of European patent EP 2 766 670 B1 ("EP'670"), which discloses a power switchgear for a boiler, comprising two contactors for selectively connecting direct current and alternating current power to a load, the coils of which are both controlled by an alternating current (AC) power supply. In this regard, EP'670 refers to live and dead connections, which a person skilled in the art understands to refer to the mains power supply (or public electricity), e.g. 230V AC or 120V AC EP'670 further discloses a thermal fuse for preventing thermostat failure. Figure 1 It is shown that the "thermal fuse" (marked "TP") is actually a set of normally closed two-pole contacts (e.g., a two-pole relay or contactor). If a preset temperature is reached, the normally closed contacts ("Y1" and "Y2" of the "TP") will open, thereby interrupting the AC power supply to the contactor coil. As mentioned earlier, since both contactor coils are controlled by the AC power supply, the EP'670 device requires a continuous AC power connection.

[0019] The applicant believes that there is still room for improvement.

[0020] The foregoing discussion of the background to the invention is intended only to facilitate understanding of the invention. It should be appreciated that the discussion does not constitute an acknowledgement or admission that any of the material referred to was part of the common general knowledge in the field at the priority date of the application. Summary of the invention

[0021] According to the present invention, there is provided a thermostat comprising:

[0022] at least one set of input terminals for connecting power to the thermostat;

[0023] output terminals for connecting the thermostat to the heating element;

[0024] a load switch for selectively connecting at least one of the set of input terminals to the output terminal and comprising an electromagnetic relay configured to interrupt an electrical connection between the at least one set of input terminals and the output terminal when a coil of the load switch is de-energized;

[0025] Temperature sensor;

[0026] a controller circuit in communication with the temperature sensor and arranged to derive the temperature from a signal received from the temperature sensor, the controller circuit further configured to selectively energize the coil of the load switch to connect the at least one input terminal to the output terminal based on the derived temperature value; and

[0027] A thermal fuse is arranged to irreversibly de-energize the coil of the load switch, thereby disconnecting the connection between the at least one set of input terminals and the output terminals, when the thermal fuse is subjected to a temperature above a temperature threshold.

[0028] These features allow the thermostat to have an electronic temperature measurement and control mechanism, while the thermal safety cut-out is not electronically controlled. This therefore prevents the possibility of the controller circuit failing or getting stuck in a state where the load switch remains energized and therefore continues to supply power to the heating element. The thermal fuse causes the coil of the load switch to be irreversibly de-energized, which is caused by environmental factors (i.e. temperature) rather than electronic control (whether analog or digital).

[0029] De-energizing the coil of the load switch may include opening a circuit including the excitation signal source and the load switch coil, additionally or alternatively disabling the excitation signal source. For example, if a dedicated direct current (DC) power supply is used to power the load switch coil, the DC power supply may be turned off. This may be accomplished by disconnecting the input power of the DC power supply or by manipulating an enable / disable input on a switch mode power supply assembly.

[0030] The thermostat may be a valve stem type thermostat including a housing and an elongated hollow valve stem extending from the housing, wherein the temperature sensor and the thermal fuse are located inside the valve stem and near a distal end of the valve stem.

[0031] The low voltage rating and low current rating of the load switch coil make the thermal fuse and wiring small enough to be placed inside the valve stem. For example, using a mains rated voltage coil will require a thermal fuse and wiring that is physically large enough to handle such a high voltage. Similarly, using a thermal fuse to disconnect (high) load currents (rather than low coil currents) will also require a large thermal fuse and wiring. In both cases, the thermal fuse and wiring will be too large to be placed inside the valve stem.

[0032] These features may enable the thermostat to be used with hot water cylinders that were designed for use with conventional bimetallic stem style thermostats. The location of the temperature sensor and thermal fuse at the distal end of the valve stem may facilitate more accurate sensing and / or melting of the thermal fuse at the rated temperature. This is true because during use, they will be located deeper within the valve stem area and, therefore, deeper within the hot water cylinder. The thermal fuse features are arranged to de-energize the coil, rather than cut off high load currents, which enables the thermal fuse to be placed inside the valve stem. This is due to the current of the switched coil being much lower than the rated load current, thereby requiring a thinner conductor. Using a thermal fuse to cut off the load current would require wire that is too thick to fit within the inner diameter of the valve stem.

[0033] The at least one set of input terminals of the thermostat may include a first set of input terminals for connecting a first power source to the thermostat, and may further include a first power switch, the first power switch including an electromagnetic relay and having contacts interposed between the first set of input terminals and contacts of the load switch, the first power switch configured to interrupt electrical connection with the first set of input terminals when the coil of the first power switch is de-energized, wherein the thermal fuse is arranged to irreversibly de-energize the load switch coil and the first power switch coil when the thermal fuse is subjected to a temperature above a temperature threshold, thereby returning the contacts of both the load switch and the first power switch to an open position. The rated voltage of the coil of the first power switch may be the same as the rated voltage of the coil of the load switch.

[0034] The first power source may be an alternating current (AC) power source, such as a mains power source. The first power switch may have at least two switch contacts that work in conjunction with each other, so that energizing and de-energizing its coil connects and disconnects all poles of the first power source, respectively.

[0035] In one embodiment, the at least one set of input terminals of the thermostat may further include a second set of input terminals for connecting a second power source to the thermostat, and may further include a second power switch, the second power switch including an electromagnetic relay configured to selectively connect contacts of the first power switch or the second set of input terminals to contacts of the load switch, the second power switch arranged to connect contacts of the first power switch to contacts of the load switch when the coil of the second power switch is de-energized, and wherein the thermal fuse is arranged to irreversibly de-energize the load switch coil, the first power switch coil, and the second power switch coil when the thermal fuse is subjected to a temperature above a temperature threshold. The rated voltage of the coil of the second power switch may be the same as the rated voltage of the coil of the load switch.

[0036] In another embodiment, the at least one set of input terminals may further include a second set of input terminals for connecting a second power source to the thermostat, and may further include a second power switch, the second power switch including an electromagnetic relay, the electromagnetic relay configured to selectively connect the first set of input terminals or the second set of input terminals to the contacts of the first power switch, the second power switch being arranged to connect the first set of input terminals to the contacts of the first power switch when the coil of the second power switch is de-energized, and wherein the thermal fuse is arranged to irreversibly de-energize the load switch coil, the first power switch coil, and the second power switch coil when the thermal fuse is subjected to a temperature above a temperature threshold, wherein the rated voltage of the coil of the second power switch is the same as the rated voltage of the coil of the load switch.

[0037] These features may enable a thermal fuse fault event to return all switches to a static configuration in which both the first and second power supplies are effectively fully disconnected.

[0038] The second power source may be a direct current (DC) power source, such as a photovoltaic (PV) power source.

[0039] The coils of the load switch, the first power switch and the second power switch may have the same rated voltage. The thermostat may further include a power supply unit arranged to output an excitation voltage corresponding to the rated voltage of the coil from either or both of the first power supply and the second power supply. De-energizing the coil may include opening a circuit including the power supply output and the coil.

[0040] The controller circuit may further include a power selection component arranged to selectively energize the coils of the first power switch and the second power switch to selectively connect the first or second power source to the contacts of the load switch.

[0041] These features can enable the thermostat to selectively power the heating element, possibly based on the availability of either or both power sources. For example, in an embodiment where the second power source is PV power, the power selection component can be used to switch to the first power source (possibly AC mains power) in low light conditions or inclement weather conditions.

[0042] The thermostat may further include an arc protection component arranged to reduce or prevent arcing of the load switch contacts. The arc protection component may include an AC zero crossing detection component arranged to cause switching of the load switch contacts during or near a zero crossing of the power supply when the power selection component selects the AC power supply. The arc protection component may further include a DC short circuit component arranged to temporarily short circuit the power supply before the load switch contacts are disconnected when the power selection component selects the DC power supply.

[0043] These features can reduce arcing by switching the load switch contacts only near the zero crossing when AC power is selected. At such low voltages, arcs are prevented from forming or only last for a short time, at most until the zero crossing. When DC power is selected, these features can also reduce or prevent arcing by temporarily shorting the DC power supply before opening the load switch contacts. This causes the DC supply voltage to zero or near zero, thereby causing zero or small load current to flow, which will not cause or sustain an arc when the load switch contacts are open. These features can extend the life of the load switch.

[0044] The rated voltage of the coil of the load switch may be a direct current (DC) voltage of 60V or less or an alternating current (AC) voltage of 60V or less in magnitude.

[0045] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In the attached picture:

[0047] Figure 1 is a schematic diagram of a first embodiment of a thermostat for use with a hot water cylinder and powered by a mains supply;

[0048] Figure 2 is a schematic diagram of a second embodiment of a thermostat for use with a hot water cylinder and connected to two power supplies;

[0049] Figure 3 is a schematic diagram of a third embodiment of a thermostat for use with a hot water cylinder and connected to two power supplies;

[0050] Figure 4 is a schematic diagram of a circuit including an excitation voltage, a coil of various electromagnetic relays of a thermostat, and a thermal fuse arranged to irreversibly de-energize the coil upon an overheating event;

[0051] Figure 5 is a schematic diagram of an alternative circuit for a coil of various electromagnetic relays including an excitation voltage, a thermostat, and a thermal fuse arranged to irreversibly de-energize the coil upon an overheating event;

[0052] Figure 6 is the power supply unit of the thermostat and has Figure 5 A schematic diagram of the arrangement of the circuit;

[0053] Figure 7 is a schematic diagram of a stem-type thermostat for use with a hot water cylinder, showing an enlarged view with hidden detail of the distal end of its stem;

[0054] Figure 8 is a block diagram showing the functional units of a controller circuit of a thermostat;

[0055] Fig. 9 is a graph showing a zero-crossing envelope of an AC power source; and

[0056] Fig.10 is a schematic diagram further including a DC short-circuit component Figure 2 Thermostat. DETAILED DESCRIPTION

[0057] Embodiments of the thermostat are described below. The thermostat may have at least one set of input terminals for connecting a power source to the thermostat. The input terminals may be screw terminals with appropriate ratings for current carrying capacity, creep, and clearance. In some embodiments, an AC mains power source may be connected to the input terminals. The thermostat further includes output terminals for connecting the thermostat to a heating element (typically a hot water cylinder with similar appropriate ratings).

[0058] The thermostat includes a load switch, an electromagnetic relay, and thus has a coil and a set of contacts that selectively open and close depending on whether the coil is energized. In an embodiment with a multi-pole single-throw load switch (e.g., a double-pole single-throw), the load switch has one or more normally open contacts. In an embodiment with a load switch having a single normally open contact, the load switch is typically arranged to disconnect or connect the power supply to the output terminal on the power supply side (and if connected, therefore to the heating element), rather than the return side. In an embodiment using a double-pole single-throw load switch with a normally open contact, the connection to the output terminal (and therefore to any heating element connected thereto) is interrupted on all poles. Since the load switch is normally open, the load switch is configured to interrupt the electrical connection between at least one set of input terminals and output terminals when its coil is de-energized.

[0059] The thermostat further includes a temperature sensor. A digital temperature sensor is preferred, for example, DS18B20 (from Maxim Integrated TM ), has a 1-Wire digital communication interface and provides a digital temperature reading with a checksum value for error checking. In some embodiments, the temperature sensor can be a passive component, such as a thermistor, which may require another analog chain and digitization stage to obtain a digital temperature value. In some embodiments, an analog signal obtained from a temperature sensor can be used.

[0060] The thermostat further includes a controller circuit in communication with the temperature sensor and arranged to derive the temperature from a signal received from the temperature sensor. In embodiments where the controller circuit includes digital processing, it may include a controller circuit arranged to communicate with the temperature sensor via a digital communication interface (e.g., 1-Wire, I 2 C, etc.) in communication with a digital temperature sensor. In the case of a passive temperature sensor, an on-board analog-to-digital converter ("A2D") may be used to obtain a digital temperature value. Alternatively, an external A2D component may be inserted between the microprocessor and the passive temperature sensor. In embodiments where the control circuit is an analog circuit, a suitable analog chain including amplifiers, filters, and comparators (preferably with hysteresis) may be utilized.

[0061] The controller circuit is configured to selectively energize the coil of the load switch based on the derived temperature value so as to connect at least one input terminal to the output terminal. A suitable electronic switch, such as a transistor, can be used to selectively connect or disconnect the excitation voltage (or current) to or from the coil of the load switch. For example, the electronic switch can be a bipolar junction transistor (BJT) or a field effect transistor (FET).

[0062] The thermostat further includes a thermal fuse, sometimes referred to as a "thermal link", which is arranged to irreversibly de-energize the coil of the load switch when the thermal fuse is subjected to a temperature above a temperature threshold, thereby disconnecting the electrical connection between one or more input terminals and the output terminals (and thereby disconnecting any load, i.e., a heating element, connected to the output terminals).

[0063] The thermostat may be a stem-type thermostat, which includes a housing (housing input terminals, output terminals, controller circuitry, etc.) and an elongated hollow stem extending from the housing. The stem should be made of a thermally conductive material such as copper. A temperature sensor and thermal fuse are located inside the hollow stem at or near its distal end and are connected to components in the housing by wires.

[0064] The working principle of thermal links is based on the concept that when the ambient temperature reaches dangerous levels, the thermal element will melt and cut off the flow of electricity. In the context of this specification, and with reference to the technology disclosed herein, the term "thermal fuse" or "thermal link" refers to an electrical component that conducts electrical current under normal circumstances and has a thermal element that melts in the event of an overheating event, thereby causing an open circuit and preventing the flow of current.

[0065] An exemplary high current rating variant uses a thermal block placed within a metal housing and is designed to trigger a cut-out function in response to an abnormal temperature condition. Due to the higher current rating, this variant may have a larger housing (approximately 4 mm in diameter) and thicker leads (approximately 1 mm thick), so it is not suitable for installation inside a hollow valve stem when used in conjunction with the wiring required to carry the higher current.

[0066] An exemplary low current rating variant utilizes a fusible material within the housing. This variant may have a smaller housing (e.g., 1.6 mm) and thinner leads (about 0.5 mm), enabling it to be used inside a hollow valve stem.

[0067] As previously mentioned, utilizing a low current variant of the thermal fuse enables it to be located inside the valve stem, preferably as close to the outer tip (distal end) of the valve stem as possible. The first effect of locating the thermal fuse close to the outer tip of the valve stem is that, during use, the thermal fuse will be deeper into the hot water cylinder, thereby being as close to the actual water temperature as possible. For the same reason, it is preferred to also place the temperature sensor in the same location, i.e. inside the valve stem, as close to the outer tip as possible. However, since the space inside the valve stem is very small, it leaves little room for all the conductors required to run to and from both the temperature sensor and the thermal fuse.

[0068] This is why it is advantageous for a thermal fuse to interrupt the load switch coil energization signal without interrupting the load current conductor. The coil energization signal is much lower current (on the order of milliamps) compared to the load current of several amps, so only relatively thin conductors are required to and from the thermal fuse. This enables the temperature sensor, thermal fuse and its conductor to be placed inside the hollow stem. If a high current rated variant is used, the conductor required to carry the load current will be very thick.

[0069] In addition, the thermal fuse is arranged in the circuit so that it operates not only at low current, but also at low voltage. Therefore, the switch coil of the thermal fuse arranged to irreversibly de-energize it (when the thermal fuse is subjected to a temperature above a temperature threshold) can be rated to operate at a low DC voltage (for example, 5V or 12V). The combination of thermal fuses arranged to perform their functions in the low current, low voltage part of the circuit may be advantageous because it enables the size of the thermal fuse and the wiring to be selected to be small enough to be installed inside the valve stem as described above. Low current but (relatively) high voltage applications may still require a physically larger thermal fuse with a high voltage rating. Therefore, a combination of low voltage and low current may be preferred.

[0070] Requirements often state that the thermal safety features of the thermostat must not rely on electronic control, especially software control. If the microcontroller fails, or if its firmware gets stuck in an infinite loop, when the load switch coil is energized and the load switch contacts are closed, this could result in a catastrophic and dangerous failure. The thermal fuse interrupting the coil energization signal returns the thermostat to a non-powered state, completely uncontrolled by any electronic device.

[0071] In some embodiments, the thermostat can be a dual power thermostat. That is, two different power sources can be connected to the thermostat, and the two power sources are selectively used according to operating conditions and requirements.

[0072] In such an embodiment, the thermostat may have a first power switch that is also an electromagnetic relay. The first power switch preferably has multiple poles, one for each connection, conductor or terminal of the associated power supply. Thus, for an AC mains power supply consisting of energized and de-energized connections, the first power switch should be a normally open double-pole single-throw relay so that the two contacts switch in unison and all poles of the power supply are disconnected when the first power switch is in the de-energized state.

[0073] The contacts of the first power switch are interposed between the first input terminal and the load switch. This should be understood as being interposed in an electrical sense, since the first power switch is operable to switch power to the contacts of the load switch, which in turn is operable to switch power to the load.

[0074] In such an embodiment, the thermostat may further include a second set of input terminals. It is contemplated that in some embodiments, the first power source (which may be connected to the first set of input terminals) may be AC ​​mains (e.g., 240V at 50Hz). AC ), the second power source (which can be connected to the second set of input terminals) can be a DC power source, which may come from a photovoltaic power source. However, both the first and second power sources can be AC ​​power sources (one is a mains power source, and the other may be a photovoltaic power source after an inverter stage).

[0075] In such an embodiment, the thermal fuse is configured to irreversibly de-energize the coils of the load switch, the first power switch, and the second power switch when the thermal fuse is exposed to a temperature exceeding its rated operating temperature (in the hot water cylinder). This returns the thermostat to a non-powered state, disconnecting the load and all poles of both power supplies. Thus, this introduces redundancy because if for some reason the load switch contacts remain closed (e.g., because the contacts have melted for some reason), the first and second power switches are used to disconnect the power. The first power switch and the second power switch may be collectively referred to as "power switches" hereinafter.

[0076] The coils of the load switch and the power switch can be connected in various configurations. For example, they can be connected in parallel and switched using a common electrical switch (e.g., a BJT or FET). Using an NPN BJT, one side of each coil can be connected to an excitation power supply, such as a DC voltage of 5V or 12V, depending on the coil rating. Ideally, the load switch and the power switch should have coils of the same rating (e.g., DC 12V), and the other side of the coil is connected to the collector of the BJT, which switches each coil during operation. Multiple BJTs can also be used, with each coil connected to a separate BJT, and the same signal is used to drive the base of each BJT during use.

[0077] An alternative configuration for de-energizing the coils of the load and power switches is to disable the coils from which the excitation signal (e.g., 12V DC or 5V DC ) power supply, provided that disabling is achieved at low voltage and low current levels. Many voltage regulators or switch mode power supply (PSU) integrated circuits (ICs) have an enable / disable input. For example, a switch mode PSU IC may have an enable line that must be tied to ground to enable its regulated output. In this configuration, a thermal fuse can be connected between the enable / disable input of the switch mode PSU and ground. A pull-up resistor can be connected between the enable / disable input and a low voltage (e.g., 5V). If an overtemperature event occurs and the thermal fuse blows, the enable input will be pulled up, thereby disabling the output and, in turn, disabling the excitation signal (i.e., the regulated output voltage of the switch mode PSU).

[0078] To ensure that one of the power sources is a DC power source, it is preferred that the load switch is selected so that its contacts have a DC rating, as DC loads the contacts more than AC. This enables the load switch to switch either AC or DC power sources.

[0079] Preferably, the second power switch is a double pole double throw (DPDT) relay with the same coil rating as the first power switch and the load switch. The DPDT contacts enable at least two features. First, the double pole enables the two conductors of the second power supply to be disconnected. Second, it enables the second power supply to be connected to the normally open contact, and the output of the first power switch (i.e., the switch side of the contact, not the "hot" side) can be connected to the normally closed contact of the second power switch. The common terminal of the second power switch is then connected to the load switch. This acts as a mutually exclusive selection switch between the first and second power supplies.

[0080] It will be appreciated that various configurations of the three coils (i.e., the coils of the load switch, the first power switch, and the second power switch) may be implemented to simultaneously de-energize them in response to the blowing of the thermal fuse, mutatis mutandis as previously described.

[0081] In a dual power supply embodiment, the controller circuit of the thermostat further includes a power supply selection component arranged to selectively energize the coils of the first power switch and the second power switch to selectively connect the first or second power supply to the contacts of the load switch.

[0082] In addition, in the dual power supply embodiment, the thermostat also includes an arc protection component, which is arranged to reduce or prevent the arcing of the load switch contacts. As previously mentioned, direct current puts a greater load on the contacts because when current flows through the contacts, contact disconnection will inevitably occur. In the case of alternating current, the power supply passes through the zero point again at most 10ms after switching to the 50Hz mains power supply. Ideally, the AC power supply should also be switched to a position close to the zero crossing point. The arc protection component may include an AC zero crossing detection component, which is arranged to cause the switching of the load switch contacts during or near the power supply zero crossing when the power supply selection component selects the AC power supply. Therefore, if the situation requires that the load switch contacts are disconnected when the AC power supply is selected, the arc protection component will monitor or sample the power supply voltage and determine the time to disconnect the load switch contacts near the zero crossing point.

[0083] The arc protection assembly further includes a DC short circuit assembly, which is arranged to temporarily short the power supply before the load switch contacts are disconnected when the DC power supply is selected by the power selection assembly. This causes the DC power supply voltage to become zero volts or close to zero volts for a brief interval, thereby causing zero or small load current to flow through the switch contacts. Therefore, disconnecting the load switch contacts during this interval prevents or at least reduces arcing. These features can extend the life of the load switch. A typical DC relay without an arc protection assembly may have a rated life of 10,000 cycles (switching under load). However, a thermostat may require a capacity of 50,000 cycles. Since the contacts will not be subjected to the common arcing action associated with switching full-load DC current, the arc protection assembly can significantly increase the life or cycle rating in the manner described above.

[0084] Figure 1 A first embodiment of a thermostat (100) is shown. Figure 1 An overview schematic diagram of a thermostat (100) in use with a hot water cylinder (101) having an internal heating element (102) is shown. In this embodiment, the thermostat (100) is configured to be powered by a 50Hz 240V AC The power source (103) supplies power.

[0085] Figure 1 A detailed schematic diagram of the thermostat (100) is also shown. The thermostat (100) has a set of (first) input terminals (110) for connecting a power source (103) to the thermostat (100). The thermostat (100) further comprises output terminals (112) for connecting the thermostat (100) to a heating element (102) of a hot water cylinder (101).

[0086] The thermostat (100) includes a load switch (114). The load switch (114) is an electromagnetic relay having a coil (115) and a set of normally open contacts (116). The load switch (114) is arranged to selectively make or break an electrical connection between an input terminal (110) (and thus a power source (103)) and an output terminal (112) (and thus a heating element (102)). In the case where the load switch (114) contacts (116) are normally open, the connection is broken when its coil (115) is de-energized.

[0087] The thermostat further comprises a temperature sensor (120) arranged to measure the water temperature of the hot water cylinder (101). The thermostat (100) further comprises a controller circuit (130) in communication with the temperature sensor (120) and arranged to derive the temperature from a signal received from the temperature sensor. The controller circuit (130) comprises a microprocessor (132) arranged to communicate with the temperature sensor (120) and obtain a temperature value.

[0088] The controller circuit (130) is configured to selectively energize the coil (115) of the load switch (116) based on the derived temperature value. If the set water temperature of the hot water cylinder (101) is, for example, 50°C, the controller circuit (130) may energize the coil (115) when a temperature below 50°C (e.g., 48°C or lower) is measured, and may de-energize the coil (115) when a temperature above 50°C (e.g., 52°C or higher) is measured. This may introduce hysteresis to prevent relay chattering.

[0089] A power supply unit (PSU) (134) provides an excitation signal (VCC) which is connected to one end of the coil (115). The other end of the coil (115) is connected to the collector of an NPN BJT (136) via a thermal fuse (140). The base of the BJT (136) is connected to the output of a microprocessor (132), and the output of the microprocessor is operable to energize the coil (115) by turning on the BJT (136). If an overheating event occurs, the thermal fuse (140) connected in series between the coil (115) and the BJT (136) will irreversibly interrupt the current through the coil (115), regardless of the state of the BJT (136).

[0090] Figure 2 A second embodiment of a thermostat (200) is shown. Figure 2Where the same reference numerals as before are used, such reference numerals represent the same, equivalent or substantially similar features or components. In this embodiment, the thermostat (200) is a dual power thermostat capable of being powered by an AC mains power source (103) or a DC power source (151) or both. The thermostat (200) further includes a power switch, referred to herein as a "first power switch (150)" to distinguish it from the components of the thermostat (200) further described below. The first power switch (150) is also an electromagnetic relay having a coil (153). However, the first power switch (150) is a double pole single throw (DPST) relay having two mating normally open contacts (154, 155). Each pole of the input terminal (110) (as well as the live and neutral wires of the power source (103)) is connected to a corresponding pole (154, 155) of the first power switch (150) on the "hot" side. The switch side of the first power switch (150) contacts (154, 155) are connected to the "hot" side of the load switch (114).

[0091] When the coil (153) of the first power switch is energized, the contacts (154, 155) switch in unison, whereas when in an off-state, the contacts open in unison.

[0092] The excitation voltage (VCC) of the PSU (134) is also connected to one side of the first power switch coil (153). Its other side is connected to the base of the BJT (136), so it is connected in parallel with the load switch coil (115) and can be controlled by the same BJT (136).

[0093] The thermostat (200) includes a second power switch (160), which is also an electromagnetic relay. The second power switch (160) is a double-pole double-throw (DPDT) relay, whose coil (162) has the same coil rating as the coil (153) of the first power switch and the coil of the load switch (115). The DPDT contacts (164, 165) enable the two conductors of the DC power supply (151) to be disconnected. It also enables the DC power supply (151) to be connected to the normally open contacts, and the output of the first power switch (150) (i.e., the AC power supply on the switch side of the contacts) to be connected to the normally closed contacts of the second power switch (160). The common terminal of the second power switch (160) is then connected to the load switch (114). This acts as a mutually exclusive selection switch between the DC and AC power supplies (151, 103).

[0094] The thermal fuse (140) is connected in series between the BJT (136) and the coils (115, 153, 162). Therefore, if an overheating event occurs, the thermal fuse (140) will irreversibly interrupt the current through each coil (115, 153, 162) regardless of the state of the BJT (136). This returns the thermostat to a non-powered state, disconnecting the heating element (102) and all poles of the two power supplies (103, 151).

[0095] Therefore, this introduces redundancy. If the load switch contacts (116) remain closed for some reason (eg, because the contacts are blown for some reason), the first power switch (150) and the second power switch (160) are used together to disconnect the power supply (103, 151).

[0096] Figure 3 A third embodiment of a thermostat (300) is shown. Figure 3 Where the same reference numerals as before are used, they represent the same, equivalent or substantially similar features or components. In this embodiment, the thermostat (300) is also a dual power thermostat capable of being powered by either an AC mains power source (103) or a DC power source (151) or both.

[0097] The thermostat (300) also includes a first power switch (150) and a second power switch (160), but the structures of the two are similar. Figure 2 Different. In this embodiment, the first power switch (150) is inserted between the second power switch (160) and the load switch (114), and the "hot" side of the first power switch contacts (154, 155) is connected to the switch side of the second power switch contacts (164, 165). On the switch side of the first power switch contacts (154, 155), one of the first power switch contacts (154) is connected to the "hot" side of the load switch contact (116), and the other of the first power switch contacts (155) is directly connected to one of the output terminals (112).

[0098] Each pole of the input terminal (110) (as well as the live and neutral wires of the power source (103)) is connected to a corresponding pole (154, 155) of the first power switch (150) on the "hot" side. The switch side of the first power switch (150) contacts (154, 155) is connected to the "hot" side of the load switch (114).

[0099] The second power switch (160) is a double pole double throw (DPDT) relay, whose normally closed contacts are connected to the first input terminal (110), thereby connecting to the AC power supply when connected to the thermostat (300). The normally open contacts of the second power switch (160) are connected to the second set of input terminals (152), thereby connecting to the DC power supply when connected to the thermostat (300). The common terminal or switch side of the contacts (164, 165) of the second switch is connected to the first power switch contacts (154, 155) as described above. At rest, that is, when the coil (162) of the second power switch (160) is not energized, the second power switch thus selects the first input terminal (110) as its input power supply. By energizing the coil (162) of the second power switch (160), the DC power supply can be selected as the input power supply. This acts as a mutually exclusive selection switch between the DC and AC power supplies (151, 103).

[0100] In this embodiment, the thermal fuse (140) is also connected in series between the BJT (136) and the coils (115, 153, 162). Therefore, if an over-temperature fuse event occurs, the thermal fuse (140) will irreversibly interrupt the current through each coil (115, 153, 162) regardless of the state of the BJT (136). This returns the thermostat to a non-powered state, disconnects the heating element (102), returns the second power switch (160) to utilizing AC power as its input, and also disconnects all poles by returning the first power switch (150) to its normally open state.

[0101] It should be appreciated that various configurations of the three coils (115, 153, 162) (ie, the coils of the load switch, the first power switch, and the second power switch) may be implemented to simultaneously de-energize them in response to the blowing of the thermal fuse. Figures 4 to 6 A selection of such variations is schematically shown in .

[0102] Figure 4 Three coils (115, 153, 162) connected in parallel are shown, with one end of each coil connected to an excitation voltage (VCC) and the other end connected to a BJT (136) via a thermal fuse (140). If the thermal fuse (140) experiences an overheat condition due to heat from the hot water cylinder (101), it will open the circuit, thereby de-energizing all three coils (115, 153, 162).

[0103] Figure 5Three coils (115, 153, 162) connected in parallel are shown, with one end of each coil connected to an excitation voltage (VCC) via a thermal fuse (140) and the other end connected to a BJT (136). If the thermal fuse (140) experiences an overheat condition due to heat from the hot water cylinder (101), it will disconnect the excitation voltage (VCC), thereby de-energizing all three coils (115, 153, 162). Figure 5 The structure may be Figure 4 If for some reason the thermal fuse (140) shorts to ground, then Figure 4 The arrangement will be able to maintain an (uncontrollable) current flow through the coil. However, Figure 5 The same short circuit in the arrangement will cause the coils (115, 153, 162) to be de-energized and return the thermostat to an unpowered state.

[0104] Figure 6 An arrangement with dual power supplies is shown, namely an AC mains power supply (103) and a DC power supply (151). The PSU (134) receives both power supplies and derives the excitation voltage (VCC) from one of them. In the "AC branch" of the PSU (134), the mains power may be stepped down, rectified and regulated by circuits (174) known in the art.

[0105] In the "DC branch" of the PSU (134), the DC input can be regulated by circuits (176) known in the art with the same or similar output voltage. An "OR" arrangement (177) can be used to obtain the excitation voltage (VCC). Figure 5 The rest of the diagram shows the Figure 4 A similar arrangement wherein the coils (115, 153, 162) are connected in parallel, one side being connected to an excitation voltage (VCC) via a thermal fuse (140).

[0106] Figure 7 The thermostat (100, 200, 300) is shown installed in a hot water cylinder (101). The thermostat (100, 200, 300) is a valve stem type thermostat, including a housing (701) and an elongated hollow copper valve stem (702) extending from the housing. Most of the components of the thermostat (100, 200, 300) can be accommodated in the housing (701). Figure 7 As shown in the enlarged view of the hidden details in FIG. 7 , the temperature sensor (120) and the thermal fuse (140) can be located inside the valve stem (702) at the distal end (703). Using the thermal fuse (140) to cut off the relatively small coil current, rather than using it to cut off the relatively large load current, enables the use of a physically smaller thermal fuse (140) and a thinner conductor. This allows it to be installed inside the valve stem (702) side by side (or at least close to) the temperature sensor (120).

[0107] Figure 8 is a block diagram showing the functional components of the controller circuit (130). Figure 8 A microprocessor (132) is shown, which includes a memory (802) for storing and retrieving instructions and variables. The functions of the components described below can be provided by hardware or software units executed on the controller circuit (130). The software units can be stored in the memory (802) and can provide instructions to the microprocessor (130) to perform the functions of the components described.

[0108] The controller circuit (130) includes a power selection component (804) arranged to selectively energize the coils (153, 162) of the first power switch (150) and the second power switch (160), thereby selectively connecting the AC power source (103) or the DC power source (151) to the contacts (116) of the load switch (114).

[0109] The controller circuit (130) includes an arc protection component (806) arranged to reduce or prevent arcing of the load switch contact (116). The arc protection component includes an AC zero crossing detection component (808) arranged to switch the load switch contact (116) during or near a zero crossing of the AC power source (103) when the power source selection component selects the AC power source. Therefore, if the situation requires that the load switch contact (116) be disconnected when the AC power source (103) is selected, the arc protection component (806) will monitor or sample the power supply voltage and determine when the disconnection of the load switch contact (116) occurs near the zero crossing.

[0110] Fig. 9 A dashed envelope (900) is shown around the zero crossings of the AC power source (103) voltage within which the arc protection assembly (806) will cause switching to be performed.

[0111] Back to Figure 8 The arc protection assembly (806) further includes a DC short circuit assembly (810) which is arranged to temporarily short circuit the DC power supply (151) before the load switch contacts (116) are opened when the DC power supply is selected by the power selection assembly (804). This causes the DC power supply voltage to become zero volts or close to zero volts for a brief interval, thereby causing zero or small load current to flow through the load switch contacts (116). Therefore, opening the load switch contacts during this interval prevents or at least reduces arcing.

[0112] Fig.10An embodiment of the thermostat (200) is shown in which a FET (1001) is connected between the inputs of a DC power supply (151). The FET (1001) can be operated by the controller circuit (130) to temporarily short the DC power supply as described above, thereby preventing arcing.

[0113] The arc protection assembly (806) and its subassemblies can extend the life of the load switch (114). A typical relay with DC rated contacts may have a rated life of 10,000 cycles (switching under load). However, a thermostat may require 50,000 cycle capability. Because the contacts will not be subjected to the common arcing associated with switching full load DC current, the arc protection assembly (806) can significantly increase the life or cycle rating in the manner described above. Those skilled in the art will appreciate that Figure 3 Embodiments of the thermostat (300) may similarly include a FET (1001) between the inputs of the DC power source (151) and configured for use as described above.

[0114] Thus, embodiments of thermostats are provided whose form factor enables them to be used as replacement parts for conventional valve stem type bimetallic thermostats. A dual power supply thermostat is provided that is capable of selectively using AC and DC power supplies. The various thermal fuse configurations described above enable thermal fail-safe mechanisms to be implemented without the need for bulky bimetallic thermal cutouts and are not program or electronically controlled, thereby meeting regulatory requirements in this regard. Configurations with multiple relays also provide redundant protection. Embodiments of the thermostat further provide for full-pole disconnection of the power supply, which may also be required by regulation. The arc protection mechanism extends the life (switching cycles) of the load switch.

[0115] Embodiments of the thermostat disclosed herein also enable their use to selectively control and power a single load (ie, a single heating element) between two different power sources.

[0116] The low voltage rating and low current rating of the load switch coil as well as the power switch coil allow the thermal fuse and wiring to be small enough to be placed inside the stem of a stem-type thermostat. This enables the thermal fuse to be placed deep in the vessel of a hot water cylinder when in use. For example, using a mains rated voltage coil would require a thermal fuse and wiring that is physically large enough to handle such a high voltage. Similarly, using a thermal fuse to disconnect (high) load currents (rather than low coil currents) would also require a large thermal fuse and wiring. In both cases, the thermal fuse and wiring would be too large to be placed inside the valve stem.

[0117] It should also be understood that the embodiments of the thermostat disclosed herein can also be operated when connected to either or both of the AC and DC power sources. In addition, the safety features provided thereby are also independent of whether the thermostat is powered by AC, DC or both (and is operable).

[0118] The above description is presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise form disclosed. Those skilled in the relevant art will appreciate that many modifications and variations are possible in light of the above disclosure.

[0119] The language used in the specification is selected primarily for readability and indicative purposes and may not be selected to describe or limit the subject matter of the present invention. Therefore, the scope of the present invention is not intended to be limited by this detailed description, but by any claims based on the application filed herewith. Therefore, the disclosure of the embodiments of the present invention is intended to illustrate but not limit the scope of the present invention, which is set forth in the appended claims.

[0120] Finally, throughout the specification and the following claims, unless the context requires otherwise, the word "comprise" or variations such as "comprising" or "including", will be understood to imply the inclusion of a stated value or group of values ​​but not the exclusion of any other value or group of values.

Claims

1. A thermostat (100, 200, 300), comprising: at least one set of input terminals (110, 152) for connecting power to the thermostat; output terminals (112) for connecting the thermostat (100) to the heating element (102); a load switch (114) for selectively connecting at least one of a set of input terminals (110) to an output terminal (112) and comprising an electromagnetic relay configured to interrupt an electrical connection between the at least one set of input terminals and the output terminal when a coil (115) of the load switch is de-energized; Temperature sensor (120); a controller circuit (130) in communication with the temperature sensor (120) and arranged to derive the temperature from a signal received from the temperature sensor, the controller circuit further configured to selectively energize a coil (115) of a load switch (114) to connect at least one input terminal (110) to an output terminal (112) based on the derived temperature value; and A thermal fuse (140) is arranged to irreversibly de-energize a coil (115) of a load switch when the thermal fuse (140) is subjected to a temperature above a temperature threshold, thereby disconnecting a connection between at least one set of input terminals (110) and output terminals (112).

2. The thermostat (100, 200, 300) according to claim 1, wherein: The thermostat is a valve stem type thermostat, comprising a housing (701) and an elongated hollow valve stem (702) extending from the housing, wherein the temperature sensor (120) and the thermal fuse (140) are located inside the valve stem and near the distal end (703) of the valve stem.

3. The thermostat (200, 300) according to claim 1 or 2, wherein: The at least one set of input terminals (110, 152) includes a first set of input terminals (110) for connecting a first power source to the thermostat, and wherein the thermostat further includes a first power switch (150), the first power switch including an electromagnetic relay having contacts (154, 155) interposed between the first set of input terminals (110) and contacts (116) of a load switch (114), the first power switch (150) being configured to interrupt the first power supply when a coil of the first power switch (153) is de-energized. An electrical connection between a first set of input terminals (110) and a load switch (114), wherein the thermal fuse (140) is arranged to irreversibly de-energize the load switch coil (115) and the first power switch coil (153) when the thermal fuse (140) is subjected to a temperature above a temperature threshold, thereby returning contacts (116; 154, 155) of both the load switch and the first power switch to an open position, wherein the rated voltage of the coil (153) of the first power switch is the same as the rated voltage of the coil (115) of the load switch.

4. The thermostat (200, 300) according to claim 3, wherein: The first power switch (150) has at least two switch contacts (154, 155) that work in coordination, so that the coil (153) of the first power switch (150) is energized and deenergized to respectively connect and disconnect all poles of the first group of input terminals (110) with the first power source.

5. The thermostat (200, 300) according to claim 3, wherein: The at least one set of input terminals further comprises a second set of input terminals (152) for connecting a second power source to the thermostat, and wherein the thermostat further comprises a second power switch (160), the second power switch comprising an electromagnetic relay, the electromagnetic relay being configured to selectively connect contacts (154, 155) of the first power switch (150) or the second set of input terminals (152) to contacts (116) of the load switch (114), the second power switch (160) being arranged to ) connects the contacts (154, 155) of the first power switch (153) to the contacts (116) of the load switch (114) when the coil (162) of the second power switch (160) is de-energized, and wherein the thermal fuse (140) is arranged to irreversibly de-energize the load switch coil (115), the first power switch coil (153) and the second power switch coil (162) when the thermal fuse is subjected to a temperature above a temperature threshold, wherein the rated voltage of the coil (162) of the second power switch (160) is the same as the rated voltage of the load switch (114).

6. The thermostat (300) according to claim 3, wherein: The at least one set of input terminals further comprises a second set of input terminals (152) for connecting a second power source to the thermostat, and wherein the thermostat further comprises a second power switch (160), the second power switch comprising an electromagnetic relay, the electromagnetic relay being configured to selectively connect the first set of input terminals (110) or the second set of input terminals (152) to contacts (154, 155) of the first power switch (150), the second power switch (160) being arranged to connect a second power source to the thermostat when a coil of the second power switch (160) is connected to the thermostat. (162) connects the first set of input terminals (110) to the contacts (154, 155) of the first power switch (150) when power is off, and wherein the thermal fuse (140) is arranged to irreversibly de-energize the load switch coil (115), the first power switch coil (153) and the second power switch coil (162) when the thermal fuse (140) is subjected to a temperature above a temperature threshold, wherein the rated voltage of the coil (162) of the second power switch (160) is the same as the rated voltage of the load switch (114).

7. The thermostat (200, 300) according to claim 5 or 6, wherein: The first power switch (150) is arranged so that energizing and de-energizing the coil (153) of the first power switch (150) respectively connects and disconnects all poles of the first group of input terminals (110) to the first power source (103), and connects and disconnects all poles of the second group of input terminals (152) to the second power source (151).

8. The thermostat (200, 300) according to any one of claims 5 to 7, further comprising a power supply unit (134), which is arranged to output an excitation voltage corresponding to the rated voltage of the coil (115, 153, 162), and the excitation voltage is obtained from any one or both of the first power supply (103) and the second power supply (151).

9. The thermostat (200, 300) according to any one of claims 5 to 8, wherein: The controller circuit (130) includes a power selection component (804) arranged to selectively energize coils of a first power switch and a second power switch to selectively connect the first power source or the second power source to contacts of a load switch.

10. The thermostat (100, 200, 300) according to any one of claims 5 to 9, wherein: The controller circuit (130) further includes an arc protection assembly (806) arranged to reduce or prevent arcing of the load switch contacts (116).

11. The thermostat (200, 300) according to claim 10, wherein: The arc protection component (806) includes an AC zero-crossing detection component (808) arranged to switch the load switch contact (116) during or near the power supply zero-crossing when the power supply selection component (804) selects the AC power supply.

12. The thermostat (200, 300) according to claim 10 or 11, wherein: The arc protection assembly (806) further comprises a DC short circuit assembly (810) arranged to temporarily short circuit the power supply before the load switch contact (116) is opened when the DC power supply is selected by the power selection assembly (804).

13. Thermostat (100, 200, 300) according to any one of the preceding claims, wherein The rated voltage of the coil (115) of the load switch (114) is a DC voltage of 60V or less or an AC voltage of 60V or less.

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