DEVICE AND METHOD FOR DETECTING AC VOLTAGE

Description

[0001] The invention relates to a device for detecting alternating voltage, comprising input terminals for applying an alternating voltage to be detected and output terminals at which the presence of the alternating voltage is indicated. The input terminals and the output terminals are galvanically isolated from each other by a coupler, wherein a control element of the coupler is supplied with operating current by the applied alternating voltage and a switching element of the coupler is connected to the output terminals. The invention further relates to a method for detecting alternating voltage, which can be carried out in particular with the aforementioned device.

[0002] Such devices are used to verify the presence of alternating current at specific points in an electrical installation by reading the output terminals, for example, from a higher-level monitoring system. A common application is to determine the correct functioning of devices such as relays or contactors, circuit breakers, residual current devices, or load break switches by detecting the presence of alternating current (on the output side) at switching contacts and comparing it to the specified switching state of the device under test.

[0003] The alternating voltage to be detected is typically a low voltage with a voltage above 60 V, such as mains lighting voltage. The higher-level monitoring device can be, for example, an industrial automation system used for monitoring or control purposes. The monitoring device can also be a gateway or a data logger, through which monitoring is then performed.

[0004] The inputs of such monitoring devices are usually designed to detect a switching signal or a protective extra-low voltage, for example 12 volts (V) or 24 V.

[0005] The aforementioned device for detecting alternating current transmits information about the presence of the alternating current to the monitoring device in the form of a switching signal, using a galvanically isolated connection. Typically, an optocoupler with a light-emitting diode (LED) as the control element for galvanic isolation is used. The LED of the optocoupler is supplied with operating current by the applied alternating current, causing the switching element of the optocoupler, usually a phototransistor, to become conductive.

[0006] In its simplest form, a series circuit consisting of a rectifier diode and a series resistor can be used to power the optocoupler's LED with the AC voltage. The series resistor can be a simple resistor and / or a capacitor with its AC resistance. While this results in a very simple setup, the disadvantage is that the LED's series resistor dissipates power in the range of tens to hundreds of milliwatts (mW), leading to energy-inefficient operation of the device. Furthermore, such an arrangement also detects an input DC voltage, which is not desirable in all applications.

[0007] Document EP 2 665 182 A2 describes a zero-crossing detector with input terminals for applying an alternating voltage and with output terminals galvanically isolated by means of an optocoupler.

[0008] It is therefore an object of the present invention to provide a device and a method for detecting alternating voltage of the type mentioned above, which, with a simple design and robust operation, operate as energy-efficiently as possible with low power losses and selectively detect alternating voltage.

[0009] This problem is solved by a device and a method with the respective features of the independent claims. Advantageous embodiments and further developments are the subject of the dependent claims. A device according to the invention of the type mentioned above has a zero-crossing detector for the alternating voltage to be detected, which supplies the control element of the coupler with the operating current in pulses. According to the invention, the device operates particularly energy-efficiently because the switching element is not continuously energized when the alternating voltage is applied, but is only briefly energized by means of the zero-crossing detector. In addition, only alternating current, and not direct current, is advantageously detected in this way.For example, if, after detection of a zero crossing, the switching element is only switched for a period of 10% of the half-wave of the alternating voltage following the zero crossing, this results in a saving of 90% of the energy required to control the switching element compared to continuously controlling the switching element.

[0010] In the method according to claim 8, the galvanically isolating coupler can operate inductively, capacitively, or optically, and in particular can be formed by an optocoupler. The control element in this case is a light-emitting diode.

[0011] In an advantageous embodiment of the device, an input voltage converter is connected upstream of the zero-crossing detector. This converter transforms the AC voltage into an AC voltage or pulsed DC voltage with a lower voltage value. The zero-crossing detector can be constructed more easily at lower voltages. The input voltage converter can include a rectifier with at least one upstream capacitor and / or series resistor. In particular, the aforementioned combination of rectifier, capacitor, and series resistor allows for the construction of a cost-effective voltage converter with relatively low power losses. Preferably, at least one bridge rectifier is used.

[0012] In a further advantageous embodiment of the device, the zero-crossing detector includes a diode and a smoothing capacitor to convert the AC voltage or pulsed DC voltage at its input into a DC voltage for supplying the control element. A switching transistor is connected in series with the control element via its switching path, with one control input of the switching transistor being coupled to the AC voltage or pulsed DC voltage at the input of the zero-crossing converter. The switching transistor is controlled such that it only conducts during the zero-crossing region.

[0013] In a further advantageous embodiment of the device, its output terminals are connected to the terminals of a phototransistor of the optocoupler. The (pulsating) switching state of the phototransistor can then be queried at the output terminals. It is important to consider that the switching state is monitored over a period of at least half (or a full) cycle of the alternating voltage to determine whether an alternating voltage is present at the input terminals. With a digital evaluation of the phototransistor's switching state, for example via a microcontroller, this can easily be implemented programmatically.

[0014] In the device according to claim 1, an RC circuit is arranged in parallel to the terminals of the phototransistor of the optocoupler.

[0015] The RC circuit smooths the signal available at the output terminals, allowing it to be easily determined whether an AC input voltage is present or not, even without evaluation by a microcontroller. Alternative implementations of the method can use other timing elements (analog or digital) to provide a signal at the output terminals that maintains a specific level as consistently as possible when an AC voltage is present at the input terminals, and a different level when no AC voltage is present at the input terminals.

[0016] A method according to the invention serves to detect alternating voltage with a device that has input terminals for applying an alternating voltage to be detected and output terminals at which the presence of the alternating voltage is indicated. The input terminals and the output terminals are galvanically isolated from each other by a coupler, wherein a control element of the coupler is supplied with operating current by the applied alternating voltage and a switching element of the coupler is connected to the output terminals. The method is characterized in that the control element of the coupler is supplied with operating current in pulses, with a pulse time parameter depending on a zero crossing of the alternating voltage. The pulse time parameter can be understood as a start time, an end time, and / or a pulse length. The method can be carried out, for example, with the device described above.The advantages stated in connection with the device result.

[0017] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying figures. The figures show: Figure 1 a schematic block diagram of a device for detecting alternating voltage; and Figure 2 Another device for detecting alternating voltage in a more detailed circuit diagram.

[0018] Figure 1 Figure 1 shows an embodiment of a device 1 for detecting alternating voltage in a block diagram.

[0019] The device 1 has input terminals 2 to which an alternating voltage Vin to be detected can be applied. The device 1 further has two potential-free output terminals 3, which are galvanically isolated from the input terminals 2, as described in the Figure 1 as indicated by the dotted line.

[0020] A higher-level monitoring device can use the output terminals 3 to query whether the AC voltage Vin is present at the input terminals 2. Due to the galvanic isolation, the query can be performed at a voltage level independent of the AC voltage Vin, in particular at the level of a very low DC voltage. The AC voltage Vin at the input terminals 2 is typically an AC extra-low voltage with a voltage level of 60 V or more.

[0021] At the in Figure 1In the example shown of device 1, the output terminals are connected to a switching element 132, symbolically represented as a switch. To generate a voltage signal for the higher-level monitoring device, one of the output terminals 3 is connected to a ground terminal GND, and the other output terminal 3 is connected to a positive potential Vc via a pull-up resistor 4. This potential Vc can be provided, for example, by the higher-level monitoring device. In this configuration, the output terminals 3 form a voltage output at which a voltage Vout is present, depending on the switch position of the switching element 132. Due to the arrangement of the pull-up resistor 4, Vout is approximately equal to Vc when the switching element 132 is not switched and approximately zero when the switching element 132 is conducting.

[0022] Device 1 can be used, for example, to check whether a power supply circuit switched by a relay or contactor, a circuit breaker, a residual current device, or a load break switch is actually live. By comparing the voltage signal Vout with a predefined switching state of the aforementioned device, the correct functioning of the device can be verified.

[0023] According to the application, the device 1 operates with particular energy efficiency because the switching element 132 is not permanently switched on when the alternating voltage Vin is applied, but is only briefly switched to conduction once per half-cycle of the alternating voltage Vin using a zero-crossing detector 12. For example, it can be provided that the switching element 132 is switched to conduction for only one millisecond (ms) or less after a zero crossing is detected. Thus, a pulse time parameter, here the pulse start, is made dependent on a detected zero crossing. An alternating voltage with an assumed frequency of 50 Hz has a period of 20 ms and accordingly a zero crossing every 10 ms. If the switching element 132 is then only switched for, for example, 1 ms at a time, only 10% of the switching energy for the switching element 132 is required compared to continuous switching.

[0024] In the example of the Figure 1An input voltage converter 11 is connected upstream of the zero-crossing detector 12. This converter transforms the AC voltage Vin into a lower AC voltage (or a pulsating DC voltage). The zero-crossing detector 12 is then powered with this lower voltage. Since the input voltage converter 11 can introduce a phase shift, the switching element 132 is not necessarily switched at the zero crossing of the AC voltage Vin, but rather once per half-cycle or once per period, depending on whether a zero crossing is detected during both falling and rising voltages, or only during either falling or rising voltages.

[0025] The zero-crossing detector 12 pulses the signal Vout. When the signal Vout is detected at the output terminals 3, it must be taken into account that the measurement is performed for more than half (or a full) period of the AC voltage to determine whether the signal Vout is zero. If the signal Vout is zero at any point within a period exceeding half (or a full) period, the AC voltage Vin is present. Only if a non-zero signal Vout is detected for at least half (or a full) period can it be concluded that no AC voltage Vin is present at the input terminals 2. When the signal Vout is evaluated as a digital signal, for example by a microcontroller, this can be easily detected programmatically.

[0026] In an alternative embodiment, an RC circuit with a time constant adapted to the period of the input AC voltage Vin can be arranged at the output terminals 3, smoothing the signal Vout. When the AC voltage Vin is applied, the signal Vout then decreases to a minimum value close to zero with the time constant. When the AC voltage Vin is no longer applied, the signal Vout rises to a maximum value close to Vc with this time constant. An embodiment in which a corresponding RC circuit is already integrated into the device 1 is shown in the Figure 2 shown.

[0027] Figure 2 shows a second embodiment of a monitoring device 1 in a more detailed circuit diagram than Figure 1 Except for the previously mentioned RC circuit, the circuit design of the Figure 2 to the exemplary embodiment of Figure 1 be transferred. The external circuit of device 1 is involved in the Figure 2analogous to the Figure 1 .

[0028] In the device 1 according to Figure 2 The coupler 13 is, for example, an optocoupler, which will also be referred to as optocoupler 13 below. The switching element 132 is a phototransistor of the optocoupler 13. An RC circuit 133 is connected in parallel to the switching path of the phototransistor. This circuit serves to smooth the output signal Vout and comprises a capacitor and a resistor in series. Alternatively, a coupler operating on a different transmission principle can be used instead of the optocoupler 13, for example, an inductive or capacitive coupler.

[0029] The optocoupler 13 incorporates a light-emitting diode (LED) as a control element 131, which, when current flows, switches the phototransistor, i.e., the switching junction 132, to conduction. The control element 131 is subsequently also referred to as LED 131. The LED 131 is supplied with operating current in pulses via a zero-crossing detector 12. An input voltage converter 11 is connected upstream of the zero-crossing detector 12 as an input stage, which is supplied by the applied AC voltage Vin.

[0030] The input voltage converter 11 is designed as a capacitor power supply. It comprises two capacitors 111 connected in series with a series resistor 112 and a bridge rectifier 113, directly connected to the input terminals 2. Alternatively, instead of the combination of capacitors and resistors, an arrangement with one or more resistors without a capacitor can be used. Several resistors can be connected as a voltage divider or in a cascade configuration. An input voltage converter implemented solely with resistors is somewhat more lossy but exhibits high overvoltage protection. An optional suppressor diode connected in parallel to the input terminals 2 serves as a protective element 114 against short-term high voltage pulses at the input terminals 2.

[0031] A Zener diode 121 and a resistor 122 are connected in parallel to the outputs of the bridge rectifier 113. A pulsating voltage Vpuls is applied to this parallel circuit, the timing of which corresponds to successive half-waves of the alternating voltage Vin, with negative half-waves being "flipped up" by the bridge rectifier 113. The voltage Vpuls is applied via a diode 123 to a smoothing capacitor 124, which charges accordingly to a DC voltage V=, the amplitude of which corresponds to the voltage Vpuls.

[0032] Capacitor 124 is connected to LED 131 of optocoupler 13 via a series resistor 125 and a transistor 126. In the illustrated example, transistor 126 is a PNP bipolar transistor whose control input is connected via another series resistor 127 to the node where the pulsating voltage Vpuls is applied.

[0033] When a positive voltage is applied to this node, i.e., during a half-cycle of the pulsating voltage Vpuls, transistor 126 is off and capacitor 124 charges to the voltage V=. As soon as the voltage Vpuls drops to a value close to zero, transistor 126 becomes conductive and capacitor 124 discharges through series resistor 125, transistor 126, and LED 131, which lights up and switches switching element 132 on. As soon as the voltage Vpuls rises again, transistor 126 switches off again and LED 131 goes out. This is true at least if the capacitance of smoothing capacitor 124 is large enough to supply LED 131 with operating current until transistor 126 switches off again.The capacitance of the smoothing capacitor 124 can also be chosen such that the illumination time of the LED 131 is limited by the energy stored in the smoothing capacitor 124 and the LED 131 is already extinguished when the transistor 126 is switched off again via its control input.

[0034] In any case, the illumination duration of LED 131 will be short, comparable to half the period of the AC voltage Vin. As shown in Figure 1, the illumination duration can be set in the range of milliseconds or less.

[0035] While LED 131 is illuminated, the phototransistor (switching element 132) becomes conductive and discharges the capacitor of the RC circuit 133. A minimum voltage Vout then appears at output terminals 3, indicating the presence of the AC voltage Vin. If LED 131 remains off for an extended period (several periods), the voltage Vout at output terminals 3 rises, indicating that no AC voltage Vin is present at input terminals 2. Reference symbol list

[0036] 1 Monitoring device 2 Input terminal 3 Output terminal 4 Pull-up resistor 11 Input voltage converter 111 Capacitor 112 Series resistor 113 Bridge rectifier 114 Protection element 12Zero crossing detector 121Zener diode 122Resistor 123Diode 124Smoothing capacitor 125Series resistor 126Switching transistor 127Series resistor 13 Coupler / Optocoupler 131 Control element / Light-emitting diode (LED) 132 Switching element 133 Smoothing capacitor Vin AC voltage Vout Output signal Vc Supply voltage V=DC voltage Vpulse pulsed DC voltage

Claims

1. A device (1) for detecting alternating voltage, comprising input terminals (2) for the application of an alternating voltage (Vin) to be detected and output terminals (3), at which application of the alternating voltage (Vin) is indicated, wherein the input terminals (2) and the output terminals (3) are galvanically separated from each other by means of a coupler (13) which is designed as an optocoupler, and wherein a light-emitting diode as a control element (131) of the coupler (13) is supplied with operating current from the applied alternating voltage, and a switching element (132) of the coupler (13) is connected to the output terminals (3), characterized in that the device (1) has a zero-crossing detector (12) for the alternating voltage (Vin) to be detected, which zero-crossing detector supplies the control element (131) of the coupler (13) with the operating current in pulses, and in that an RC element (133) is arranged parallel to the terminals of the phototransistor of the optocoupler (13) in such a way that it smoothes the signal which can be tapped at the output terminals in an analog manner.

2. The device (1) according to claim 1, in which an input voltage converter (11), which converts the alternating voltage (Vin) into an alternating voltage or pulsed direct voltage (Vpuls) with smaller voltage value, is connected upstream of the zero-crossing detector (12).

3. The device (1) according to claim 2, in which the input voltage converter (11) has a rectifier with at least one capacitor (111) connected upstream and / or series resistor (112) connected upstream.

4. The device (1) according to claim 3, in which the rectifier of the input voltage converter (11) is at least one bridge rectifier (113).

5. The device (1) according to any one of claims 1 to 4, in which the zero-crossing detector (12) has a diode (123) and a smoothing capacitor (124), in order to form, from the alternating voltage or pulsed direct voltage (Vpuls), at its input, a direct voltage (V=) for supplying the control element (131).

6. The device (1) according to any one of claims 1 to 5, in which the zero-voltage detector (12) has a switching transistor (126), whose switching path is connected in series with the control element (131) and whose control input is coupled to the alternating voltage or the pulsed direct voltage (Vpuls) at its input.

7. The device (1) according to any one of claims 1 to 6, in which the output terminals (3) are connected to the terminals of a phototransistor of the coupler (13) which is designed as an optocoupler.

8. A method for detecting alternating voltage, with a device which has input terminals (2) for the application of an alternating voltage (Vin) to be detected and output terminals (3), at which application of the alternating voltage (Vin) is indicated, wherein the input terminals (2) and the output terminals (3) are galvanically separated from each other by means of a coupler (13), and wherein a control element (131) of the coupler (13) is supplied with operating current from the applied alternating voltage, and a switching element (132) of the coupler (13) is connected to the output terminals (3), characterized in that the control element (131) of the coupler (13) is supplied with the operating current in pulses, wherein a pulse time parameter is dependent upon a zero cross of the alternating voltage (Vin).

9. The method according to claim 8, in which the pulse time parameter is a point in time of a beginning of one of the pulses, a point in time of an end of one of the pulses and / or a length of the pulses.