Control device and control method of pulse constant current source

Abstract

The invention provides a control device and a control method of a pulse constant current source. The device comprises a first power supply, a second power supply, a micro-control unit, a digital-to-analog converter, a first switch, a second switch and a third switch, the voltage of the first power supply is greater than that of the second power supply; the micro-control unit, the digital-to-analog converter, the third switch and the pulse constant-current source are connected in series to form a loop; the micro-control unit is used for controlling the digital-to-analog converter to output an analog quantity corresponding to the target pulse current after controlling the first switch and the third switch to be closed, and determining the actual pulse current output by the pulse constant current source based on the voltage at the two ends of the current detection resistor and the detected device. And under the condition that the actual pulse currents within the first target duration after the analog quantity is output are determined to be the same, the second switch is controlled to be closed, and the first switch is controlled to be switched off after the second target duration. According to the invention, the problem of serious heating of a power device of the pulse constant-current source caused by an existing control scheme of the pulse constant-current source can be solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronics, in particular to a control device and a control method of a pulse constant current source. BACKGROUND

[0002] With the development of semiconductor power devices and lasers towards high power, it is required to drive the power devices and lasers with greater pulse current to test the electrical characteristics of the devices. When testing the electrical characteristics of the power devices or lasers, faster and narrower pulse current can reduce the heat of the devices and the influence of temperature on the electrical characteristics of the devices.

[0003] In addition, to improve the testing efficiency, multiple power devices are connected in series as a load, and the same constant current source is used to drive the load for testing. According to the number of series loads, the output voltage is also required to be different.

[0004] To realize faster pulse and multi-group series testing of the measured objects, to realize greater output current and faster rising edge, a common method is to set a higher voltage output power supply to supply power to the pulse constant current source, which will result in very low working efficiency of the power supply during the pulse output time of the pulse constant current source, and further result in serious heat of the power devices of the pulse constant current source. SUMMARY

[0005] Therefore, it is necessary to provide a control device and a control method of a pulse constant current source to solve the technical problem that the existing control scheme of the pulse constant current source will cause serious heat of the power devices of the pulse constant current source.

[0006] To solve the above problems, in a first aspect, the present application provides a control device of a pulse constant current source, comprising: a first power supply, a second power supply, a micro control unit, a digital-to-analog converter, a first switch, a second switch and a third switch; the voltage of the first power supply is greater than the voltage of the second power supply; The micro control unit, the digital-to-analog converter, the third switch and the pulse constant current source are connected in series to form a loop; The pulse constant current source is connected with the first power supply through the first switch, and connected with the second power supply through the second switch, and the pulse constant current source and the first switch and the second switch are further provided with a series-connected current detection resistor and a measured device; The micro control unit is configured to control the digital-to-analog converter to output an analog quantity after the first switch and the third switch are controlled to be closed, control the pulse constant current source to output a target pulse current through the analog quantity, determine an actual pulse current output by the pulse constant current source based on the voltage across the current detection resistor and the voltage across the device under test, and control the second switch to be closed in a case where the actual pulse current is the same within a first target time duration after the output analog quantity is determined, and control the first switch to be opened after the second switch is closed for a second target time duration.

[0007] In a possible implementation, the control device of the pulse constant current source further includes a first analog-to-digital converter, a second analog-to-digital converter, and a third analog-to-digital converter. The first analog-to-digital converter is connected to the input end of the current detection resistor and the micro control unit respectively. The second analog-to-digital converter is connected in series between the pulse constant current source and the micro control unit. The third analog-to-digital converter is connected to the input end of the device under test and the micro control unit respectively. The micro control unit is further configured to determine the voltage across the device under test and the voltage across the current detection resistor based on voltage signals collected by the first analog-to-digital converter, the second analog-to-digital converter, and the third analog-to-digital converter respectively, and determine the actual pulse current output by the pulse constant current source based on the voltage across the device under test, the voltage across the current detection resistor, the internal resistance of the device under test, and the current detection resistor.

[0008] In a possible implementation, the number of pulse constant current sources is at least two, and the at least two pulse constant current sources are connected in parallel.

[0009] In a possible implementation, the control device of the pulse constant current source further includes a first anti-reverse diode and a second anti-reverse diode. The first anti-reverse diode is connected in series between the first power supply and the first switch, and the second anti-reverse diode is connected in series between the second power supply and the second switch.

[0010] In a possible implementation, the micro control unit is further configured to control the second switch to be opened after a preset pulse duration ends, set the output of the digital-to-analog converter to zero, and control the third switch to be opened.

[0011] In a possible implementation, the control device of the pulse constant current source further includes a first capacitor and a second capacitor. The first capacitor is connected in parallel to the first power supply, and the second capacitor is connected in parallel to the second power supply.

[0012] In a possible implementation, the pulse constant current source comprises a first amplifier, a second amplifier, an output resistor and a power device. The first amplifier and the second amplifier are coupled in parallel with the power device, and an output end of the power device is connected with the output resistor.

[0013] In a second aspect, the present application provides a control method of a pulse constant current source, which is applied to the control device described above, and the control method comprises the following steps. The micro control unit controls the digital-to-analog converter to output an analog quantity corresponding to a target pulse current size after controlling the first switch and the third switch to be closed, and controls the pulse constant current source to output the target pulse current through the analog quantity. The micro control unit determines an actual pulse current size output by the pulse constant current source based on a voltage across the current detection resistor and a voltage across the device under test, and controls the second switch to be closed in the case that the actual pulse current size is the same within a first target time length after the output analog quantity is determined, and controls the first switch to be opened after the second switch is closed for a second target time length.

[0014] In a possible implementation, the control method of the pulse constant current source further comprises the following steps. The micro control unit determines the voltage across the device under test and the voltage across the current detection resistor based on voltage signals collected by the first analog-to-digital converter, the second analog-to-digital converter and the third analog-to-digital converter, and determines the actual pulse current size output by the pulse constant current source based on the voltage across the device under test, the voltage across the current detection resistor, an internal resistance of the device under test and the current detection resistor.

[0015] In a possible implementation, the control method of the pulse constant current source further comprises the following steps. The micro control unit controls the second switch to be opened after a preset pulse duration ends, sets an output of the digital-to-analog converter to zero, and controls the third switch to be opened.

[0016] The beneficial effect of the above implementation manner is that the control device and the control method of the pulse constant current source provided by the application, after the micro control unit controls the first switch and the third switch to be closed, the analog quantity is output by the digital-to-analog converter, the target pulse current is output by the pulse constant current source through the analog quantity, the actual pulse current size output by the pulse constant current source is determined based on the voltage across the current detection resistor and the voltage across the measured device, and in the case that the actual pulse current size in the first target time length after the output analog quantity is determined is the same, it can be determined that the output of the pulse constant current source reaches a steady state, so that the second switch is closed, and after the second switch is closed for a second target time length, the first switch is opened, and the switching between the first power supply and the second power supply is realized. The application realizes faster edge output of the pulse constant current source through one power supply (i.e. a high-voltage power supply), after the output of the pulse constant current source reaches a steady state, another power supply (i.e. a low-voltage power supply) is switched to realize steady-state output of the pulse constant current source, the heat dissipation power consumption of the pulse constant current source is reduced, and a large heat sink does not need to be additionally added, thereby solving the technical problem that the existing control scheme of the pulse constant current source causes the power device of the pulse constant current source to heat seriously. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments description. Obviously, the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0018] Figure 1 The structural schematic diagram of one embodiment of the control device of the pulse constant current source provided by the application; Figure 2 The flow chart of one embodiment of the control method of the pulse constant current source provided by the application; Figure 3 The flow chart of another embodiment of the control method of the pulse constant current source provided by the application; Figure 4 The control timing diagram of the pulse constant current source provided by the application. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the application will be described clearly and completely in the following with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only some embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0020] In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0021] The terms "comprising" and "having" and any variations thereof appearing in the embodiments of the present application are intended to cover inclusive rather than exclusive inclusion, for example, a process, method, apparatus, product, or device that comprises a list of steps or modules as an example is not necessarily limited to those steps or modules which are clearly recited, but can include other steps or modules that are not expressly listed or inherent to such processes, methods, products, or devices.

[0022] The naming or numbering of the steps appearing in the embodiments of the present application does not mean that the steps in the method flow must be performed in the time / logical order indicated by the naming or numbering, and the flow steps that have been named or numbered can change the execution order according to the technical purpose to be achieved, as long as the same or similar technical effects can be achieved.

[0023] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative embodiments to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] The present application provides a control device and a control method of a pulse constant current source, which are described below respectively.

[0025] As shown in Figure 1 , the present application provides a control device of a pulse constant current source, comprising: a first power supply V H , a second power supply V L , a micro control unit (MCU), a digital-to-analog converter (DAC), a first switch S WH , a second switch S WL and a third switch S W ; the voltage of the first power supply V H is greater than the voltage of the second power supply V L ; The micro control unit, the digital-to-analog converter, the third switch S W and the pulse constant current source are connected in series to form a loop; The pulse constant current source is connected to the first power supply V H through the first switch S WH , connected to the second power supply V L through the second switch S WL , and the pulse constant current source is connected to the first switch S WHand the second switch S WL A current sensing resistor Rs and the device under test V are also connected in series. dut ; The microcontroller unit is used to control the first switch S WH and the third switch S W After closing, the analog-to-digital converter is controlled to output an analog signal, which in turn controls the pulse constant current source to output a target pulse current. The actual pulse current magnitude output by the pulse constant current source is determined based on the voltage across the current sensing resistor and the voltage across the device under test. If the actual pulse current magnitude remains the same for the first target duration after the analog signal is determined, the second switch S is controlled. WL Close, and at the second switch S WL After the second target duration has elapsed, control the first switch S. WH disconnect.

[0026] It is understandable that the output current of the pulse constant current source is set. I parasitic inductance of the circuit L Power supply voltage V 1. Circuit parasitic resistance R p, load voltage V The rise time t of the pulse constant current source and the actual output current sensing resistor are: R s; During the rise phase of the pulse current: power supply voltage V The relationship between 1 and each parameter is expressed as follows: (1) When the output current reaches a stable period, the corresponding supply voltage is required. V 2. Relational expression: (2) When faster edge speeds are required, and the series connection of the device under test results in a large parasitic inductance in the circuit, a higher power supply voltage is required.

[0027] When a single power supply is used to power the pulse constant current source, while meeting the requirement of faster edge speed, it is found that, by comparing formulas (1) and (2), during the stable pulse current time, the higher bus voltage leads to a higher voltage across the power device, resulting in a lower power of the constant current source and severe overheating of the constant current source power device.

[0028] In order to solve the problem, the application designs two power supplies, i.e. a first power supply and a second power supply, the first power supply can be a high-voltage power supply, and the second power supply can be a low-voltage power supply, and the two power supplies are controlled to be turned on and turned off through a first switch and a second switch, one of the power supplies can realize faster output edges, and the other power supply can realize stability, thereby reducing the heat dissipation power consumption of the pulse constant current source.

[0029] In some embodiments, the control device of the pulse constant current source further comprises a first analog-to-digital converter, a second analog-to-digital converter and a third analog-to-digital converter. The first analog-to-digital converter, i.e. ADC1 in the first analog-to-digital converter, Figure 1 is connected with the input end of the current detection resistor and the micro control unit, respectively. The second analog-to-digital converter, i.e. ADC2 in the second analog-to-digital converter, Figure 1 is connected in series between the pulse constant current source and the micro control unit. The third analog-to-digital converter, i.e. ADC3 in the third analog-to-digital converter, Figure 1 is connected with the input end of the device under test and the micro control unit, respectively. The micro control unit is further used to determine the voltage across the device under test and the voltage across the current detection resistor based on the voltage signals collected by the first analog-to-digital converter, the second analog-to-digital converter and the third analog-to-digital converter, and determine the actual pulse current output by the pulse constant current source based on the voltage across the device under test, the voltage across the current detection resistor, the internal resistance of the device under test and the current detection resistor.

[0030] It can be understood that the voltage across the current detection resistor can be determined based on the voltage signals collected by the first analog-to-digital converter and the second analog-to-digital converter, and the voltage across the device under test can be determined based on the voltage signals collected by the first analog-to-digital converter and the third analog-to-digital converter. According to the values of the voltage and resistance between the pulse constant current source and the two power supplies, the actual pulse current output by the pulse constant current source can be determined according to Ohm's law.

[0031] In some embodiments, the number of the pulse constant current sources is at least two, and the at least two pulse constant current sources are connected in parallel.

[0032] It can be understood that each pulse constant current source can be equipped with an independent feedback control loop to stabilize the output current by itself, support independent operation or time-sharing switching of each channel, and avoid long-term large current fatigue damage of the components.

[0033] In some embodiments, the control device of the pulse constant current source further comprises: Figure 1 a first anti-reverse diode D H and a second anti-reverse diode D L . The first anti-reverse diode is connected in series between the first power supply and the first switch, and the second anti-reverse diode is connected in series between the second power supply and the second switch.

[0034] It can be understood that the first anti-reverse diode and the second anti-reverse diode can prevent the first power supply and the second power supply from being reversely charged during switching.

[0035] In some embodiments, the micro control unit is further configured to control the second switch to be turned off after the preset pulse duration ends, set the output of the digital-to-analog converter to zero, and control the third switch to be turned off.

[0036] It can be understood that after the preset pulse duration ends, the pulse constant current source no longer needs to output, at this time, the second switch can be controlled to be turned off first, then the output of the digital-to-analog converter is set to zero to stop the work of the pulse constant current source, and finally the third switch is turned off.

[0037] In some embodiments, the control device of the pulse constant current source further comprises: Figure 1 The first capacitor V Hcap and the second capacitor V Lcap are connected in parallel with the first power supply and the second power supply, respectively. The first capacitor is connected in parallel with the first power supply, and the second capacitor is connected in parallel with the second power supply.

[0038] It can be understood that the capacitors connected in parallel across the power supply can suppress the sudden change of the power supply voltage, realize power-on delay (slow charging) and power-off delay (slow discharging), and protect sensitive devices.

[0039] In some embodiments, referring to Figure 1 , the pulse constant current source comprises a first amplifier OP1, a second amplifier OP2, an output resistor Rs, and a power device Qg. The first amplifier and the second amplifier are coupled and connected in parallel with the power device, and the output end of the power device is connected with the output resistor.

[0040] It can be understood that the output end of the first amplifier is connected with the power device, the positive input end of the second amplifier is connected with the power device, and the positive input ends of the first amplifiers of different pulse constant current sources are connected with each other to realize parallel connection of the pulse constant current sources.

[0041] The application further provides a control method of a pulse constant current source, which is applied to the control device as described in any one of the above embodiments, as shown in Figure 2 The method comprises the following steps: S201, after the micro control unit controls the first switch and the third switch to be closed, the micro control unit controls the digital-to-analog converter to output an analog quantity corresponding to a target pulse current size, and controls the pulse constant current source to output the target pulse current through the analog quantity; S202, the micro control unit determines an actual pulse current size output by the pulse constant current source based on the voltage across the current detection resistor and the voltage across the device under test, and controls the second switch to be closed in a case where the actual pulse current size is the same within a first target time length after the output analog quantity is determined, and controls the first switch to be opened after the second switch is closed for a second target time length.

[0042] In some embodiments, the control method of the pulse constant current source further comprises: The micro control unit determines the voltage across the device under test and the voltage across the current detection resistor based on the voltage signals collected by the first analog-to-digital converter, the second analog-to-digital converter and the third analog-to-digital converter, and determines the actual pulse current size output by the pulse constant current source based on the voltage across the device under test, the voltage across the current detection resistor, the internal resistance of the device under test and the current detection resistor.

[0043] In some embodiments, the control method of the pulse constant current source further comprises: The micro control unit controls the second switch to be opened after the preset pulse duration ends, sets the output of the digital-to-analog converter to zero, and controls the third switch to be opened.

[0044] In some embodiments, the control method of the pulse constant current source provided by the application is as shown in Figure 3 In order to achieve a larger output current, multiple pulse constant current source modules are commonly used in parallel to achieve output, as shown in Figure 1 1~n are used in parallel; The N constant current source modules share two groups of power supplies V H , V L , and the pulse constant current source is powered by high-speed switches S WH , S WL The high-speed switches are realized by electronic switches, which can realize nS level closing and opening, and D H , D L are anti-reverse diodes, i.e., first anti-reverse diode and second anti-reverse diode; to prevent the high-voltage power supply D L from reverse charging the D H power supply when switching the low-voltage power supply D L .

[0045] ADC1, ADC2, ADC3 are three high-speed analog-to-digital converters, the sampling rate is greater than 10M, and the voltage Vdut across the DUT (measured device or measured load) and the voltage across the current detection resistor Rs are collected respectively, and the voltage of the DUT and the actual output current size Is of the pulse constant current source can be obtained through operation.

[0046] When the MCU (micro control unit) receives the current size Is to be output and the pulse width t, the MCU controls the switch S WH is closed through the communication interface to control the DAC (digital-to-analog converter) to output the corresponding analog quantity size, and the switch S W is closed. At this time, the constant current source unit module starts to output the current size, and the current gradually rises, corresponding Figure 4 to the rising edge phase t1 of the pulse current; in this phase, the power supply V H provides power, and theoretically, the higher the voltage of the power supply V H , the faster the rising edge of the pulse current satisfies the definition of formula (1), the loop equivalent resistance R, the load voltage V, the pulse current size I and the loop parasitic inductance are certain, at this time, the voltage of the power supply V is proportional to the voltage of the power supply V H , the higher the voltage of the power supply V , the faster the rising edge of the pulse current is; When the MCU obtains the voltages V ADC1 , V ADC2 , V ADC3 collected by the three ADCs, according to the sampling resistor Rs, the pulse current size can be calculated, when the MCU continuously detects the same output pulse current value size, and after the sampling delay t2, the MCU internally passes through the time timer (sampling delay) and the read-back current setting comparator. Control the size of the power supply V L (V L >Vdut+10V, 10V is a reserved margin for line impedance, and can be set) to trigger the MCU to control the switch S WL to be closed, after a time t4, the switch S WH is opened, and the subsequent power supply V L is supplied; By the above mode, within the pulse width t, two different power supplies are controlled to supply power to the constant current source, the voltage drop of the pulse constant current source power device Qg when the constant current is output is reduced, the power consumption of the power device Qg is reduced, and a faster rising edge can be realized.

[0047] The present application has the following beneficial effects: The two power supply voltages in the application can be adjusted, can be set in advance, one power supply realizes faster output edge, one power supply realizes steady state, reduces the heat dissipation power consumption of the pulse constant current source. In the output DC constant current source mode, a single power supply can be considered to be used, and the cost is reduced. The application mainly solves the problem that in the pulse mode, faster pulse edges are realized, and the heat dissipation power consumption of the constant current source is reduced, and the efficiency is improved.

[0048] The application uses high-speed electronic switch S WH , S WL Switching power supply, at different times, the pulse constant current source is powered, although a single power supply can also realize switching, but to achieve nS level adjustment of the output voltage of the power supply, the control is complex and the cost is high, the application adopts two power supplies to realize switching, which can reduce the complexity of control, thereby reducing the cost.

[0049] The application acquires the voltage through three groups of ADC, reads the voltage difference between the measured device, reads the current size, uses the comparison method, and can automatically adjust the steady state power supply size.

[0050] The pulse constant current source control device and the control method provided by the application are described in detail, specific examples are applied in this paper to describe the principle and implementation mode of the application, and the above examples are only used to help understand the method and the core idea of the application; meanwhile, for those skilled in the art, according to the idea of the application, the specific implementation mode and application range will be changed, and the above description should not be understood as the limitation of the application.

Claims

1. A control device for a pulsed constant current source, characterized in that, include: First power supply, second power supply, microcontroller unit, digital-to-analog converter, first switch, second switch, and third switch; The voltage of the first power supply is greater than the voltage of the second power supply; The microcontroller unit, the digital-to-analog converter, the third switch, and the pulse constant current source are connected in series to form a circuit; The pulse constant current source is also connected to the first power supply through the first switch and to the second power supply through the second switch. A current detection resistor and the device under test are connected in series between the pulse constant current source, the first switch, and the second switch. The microcontroller unit is configured to, after controlling the first switch and the third switch to close, control the digital-to-analog converter to output an analog signal, control the pulse constant current source to output a target pulse current through the analog signal, determine the actual pulse current magnitude output by the pulse constant current source based on the voltage across the current sensing resistor and the voltage across the device under test, and, if the actual pulse current magnitudes are the same within a first target duration after determining the output analog signal, control the second switch to close, and after a second target duration has elapsed since the second switch was closed, control the first switch to open.

2. The control device for the pulse constant current source according to claim 1, characterized in that, Also includes: First analog-to-digital converter, second analog-to-digital converter, and third analog-to-digital converter; The first analog-to-digital converter is connected to the input terminal of the current sensing resistor and the microcontroller unit, respectively. The second analog-to-digital converter is connected in series between the pulse constant current source and the microcontroller unit; The third analog-to-digital converter is connected to the input terminal of the device under test and the microcontroller unit, respectively. The microcontroller unit is further configured to determine the voltage across the device under test and the voltage across the current sensing resistor based on the voltage signals acquired by the first analog-to-digital converter, the second analog-to-digital converter, and the third analog-to-digital converter, and to determine the actual pulse current output by the pulse constant current source based on the voltage across the device under test, the voltage across the current sensing resistor, the internal resistance of the device under test, and the current sensing resistor.

3. The control device for the pulse constant current source according to claim 1, characterized in that, The number of pulse constant current sources is at least two, and at least two pulse constant current sources are connected in parallel.

4. The control device for the pulse constant current source according to claim 1, characterized in that, Also includes: First anti-reverse diode and second anti-reverse diode; The first anti-reverse diode is connected in series between the first power supply and the first switch, and the second anti-reverse diode is connected in series between the second power supply and the second switch.

5. The control device for the pulse constant current source according to claim 1, characterized in that, The microcontroller unit is further configured to, after the preset pulse duration ends, control the second switch to open, set the output of the digital-to-analog converter to zero, and control the third switch to open.

6. The control device for the pulse constant current source according to claim 1, characterized in that, Also includes: First capacitor and second capacitor; The first capacitor is connected in parallel with the first power supply, and the second capacitor is connected in parallel with the second power supply.

7. The control device for the pulse constant current source according to any one of claims 1-6, characterized in that, The pulse constant current source includes: a first amplifier, a second amplifier, an output resistor, and a power device; The first amplifier is coupled to the second amplifier and then connected in parallel with the power device, and the output terminal of the power device is connected to the output resistor.

8. A control method for a pulse constant current source, characterized in that, The method is applied to the control device according to any one of claims 1-7, and the control method includes: After the microcontroller controls the first and third switches to close, it controls the digital-to-analog converter to output an analog quantity corresponding to the magnitude of the target pulse current, and controls the pulse constant current source to output the target pulse current through the analog quantity. The microcontroller determines the actual pulse current output of the pulse constant current source based on the voltage across the current sensing resistor and the voltage across the device under test. If the actual pulse current is the same for the first target duration after the output analog quantity is determined, the microcontroller controls the second switch to close. After the second switch has been closed for the second target duration, the microcontroller controls the first switch to open.

9. The control method for the pulse constant current source according to claim 8, characterized in that, Also includes: The microcontroller unit determines the voltage across the device under test and the voltage across the current sensing resistor based on the voltage signals acquired by the first analog-to-digital converter, the second analog-to-digital converter, and the third analog-to-digital converter, and determines the actual pulse current output by the pulse constant current source based on the voltage across the device under test, the voltage across the current sensing resistor, the internal resistance of the device under test, and the current sensing resistor.

10. The control method for the pulse constant current source according to claim 8, characterized in that, Also includes: After the preset pulse duration ends, the microcontroller controls the second switch to open, sets the output of the digital-to-analog converter to zero, and controls the third switch to open.

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