WO2014005399A1

WO2014005399A1 - Overvoltage protection circuit and current sampling device

Info

Publication number: WO2014005399A1
Application number: PCT/CN2012/085914
Authority: WO
Inventors: 周翔; 潘健; 罗轶峰
Original assignee: 湖南三一智能控制设备有限公司; 三一重工股份有限公司
Priority date: 2012-07-06
Filing date: 2012-12-05
Publication date: 2014-01-09
Also published as: CN102751714B; CN102751714A

Abstract

An overvoltage protection circuit and a current sampling device provided with the overvoltage protection circuit. The overvoltage protection circuit comprises: an enable switch, arranged for controlling a sampling resistor switch which is connected to a sampling resistor in series to switch on and off; and a comparer, one end of the comparer being provided with an external interface, and the other end of the comparer being connected to an enable end of the enable switch, and arranged for determining whether the enable end of the enable switch is enabled according to a comparison result. Therefore, the overvoltage protection circuit and the current sampling device can effectively protect the sampling resistor, and not only is short in response time, but also simple in design, and low in cost.

Description

This invention claims priority to Chinese Patent Application No. 201210234438.7, entitled "Overvoltage Protection Circuit and Current Sampling Device", filed on July 6, 2012, with the Chinese Patent Office, which is entitled "Overvoltage Protection Circuit and Current Sampling Device". The entire content is incorporated herein by reference. Technical field

The present invention relates to the field of construction machinery, and in particular to an overvoltage protection circuit and a current sampling device. Background technique

At present, in the controller for construction machinery, the current type AD (A/D) sampling scheme is generally: Open a 250Ω sampling resistor through a circuit switch, so that the 4~20mA current signal is converted into 0~5V. The voltage signal is sampled by the AD port of the microcontroller.

However, during the actual wiring process, the user may mistakenly connect the AD port to the 24V driving power supply, so that the power of the sampling resistor will increase sharply to 2.3W or more. More seriously, in the vehicle power supply, the voltage may be When it reaches 28V, the power will increase to more than 3.1W. However, the sampling resistor (1206 package) on the controller typically has only 1/4W of power, in which case the sample resistor will burn out. Summary of the invention

In view of this, the present invention provides an overvoltage protection circuit and a current sampling device to solve the problem that the sampling resistor in the sampling circuit is burned out due to overvoltage.

In order to achieve the above object, the technical solution of the present invention is achieved as follows:

In one aspect, the present invention provides an overvoltage protection circuit, the circuit comprising: an enable switch configured to control opening and closing of a sampling resistor switch in series with a sampling resistor; and a comparator having one end The other end is connected to the enable end of the enable switch; the comparator determines whether the enable end of the enable switch is enabled according to the comparison result.

In the above circuit, the comparator includes: a first comparison circuit, one end of which is provided with a first external interface, and the other end is respectively connected with an enable end of the enable switch; and the second comparison circuit has a second end The external interface is connected to the enable end of the enable switch.

The first comparison circuit includes: a first comparison amplification circuit connected to the first external interface, the first comparison amplification circuit configured to compare an access voltage value and a specified voltage value of the first external interface And the output of the first comparison amplifying circuit is deflected when the access voltage value of the first external interface is higher than the specified voltage value; the first clamping circuit is compared with the first comparison The circuit is connected, and the first clamping circuit is configured to control a voltage value of the enable terminal of the enable switch.

The second comparison circuit includes: a second comparison amplification circuit connected to the second external interface, the second comparison amplification circuit configured to compare an access voltage value and a specified voltage value of the second external interface And the output of the second comparison amplifying circuit is deflected when the access voltage value of the second external interface is higher than the specified voltage value; and the second clamping circuit is compared with the second comparison The second clamp circuit is configured to control a voltage value of the enable switch enable end; wherein the specified voltage value is a reference value determined to be mis-connected according to the access voltage value.

Preferably, the circuit further includes: a controller, the controller is connected to the output ends of the first and second comparison amplifying circuits, and configured to monitor whether there is misconnection information to determine the first and second external Whether the interface is misconnected.

Preferably, the above circuit further comprises: an alarm, the alarm is connected to the controller, and the alarm is configured to perform a fault alarm when the controller detects the misconnection information.

Preferably, the circuit further includes: a fault analyzer, the fault analyzer is connected to the controller, and the fault analyzer is configured to enter the fault when the controller detects the misconnection information Diagnosis.

In the above circuit, the first comparison amplifying circuit includes: a first voltage dividing regulator connected to the first external interface, and the first voltage dividing regulator configured to be externally connected according to the first external interface The magnitude of the voltage value selects a different voltage dividing resistor and the specified voltage value; the first operational amplifier has a negative terminal at the input end thereof connected to the first voltage dividing regulator, and a positive electrode connected to the power supply voltage, the first operational amplifier The output is coupled to the first clamp circuit.

In the above circuit, the second comparative amplifying circuit includes: a second voltage dividing regulator connected to the second external interface, and the second voltage dividing regulator configured to be externally connected according to the second external interface The magnitude of the voltage value selects a different voltage dividing resistor and the specified voltage value; the second operational amplifier has a negative terminal connected to the second voltage dividing regulator, a positive electrode connected to the power supply voltage, and a second operational amplifier The output is connected to the second clamp circuit.

In the above circuit, the enable switch includes: a first enable switch and a second enable switch; the sampling resistor switch includes a first sampling resistance switch and a second sampling resistance switch; wherein the first enable switch The enable end is connected to the comparator, the first enable switch is configured to control opening and closing of the first sampling resistance switch; and the enable end of the second enable switch is connected to the comparator, The second enable switch is configured to control opening and closing of the second sampling resistor switch.

In the above circuit, the sampling resistor includes: a first sampling resistor, one end of which is connected to the first sampling resistor switch, and the other end is connected to the first external interface; and the second sampling resistor has one end and the second The sampling resistor switch is connected, and the other end is connected to the second external interface.

In another aspect, the present invention provides a current sampling device provided with any of the overvoltage protection circuits described above.

Compared with the prior art, the present invention has the following advantages:

In the overvoltage protection circuit of the present invention, the comparator can realize two short circuit detection, and each current detection is realized by setting a voltage dividing resistor, a clamp diode and an operational amplifier. Therefore, the present invention can not only effectively protect the sampling resistor, but also design a single cartridge, which is low in cost. And the present invention The designed overvoltage protection circuit combines the comparator and the enable switch. The response time is short, and the sampling resistor is not turned off when the heating tube is turned off. It is worth mentioning that even if the sampling resistor is connected to the 24V driving power supply for a long time, it will not generate heat and has good reliability.

The current sampling device provided by the present invention is provided with the overvoltage protection circuit. Since the overvoltage protection circuit has the above technical effects, the current sampling device provided with the overvoltage protection circuit should also have a corresponding technology. effect. DRAWINGS

The accompanying drawings, which are incorporated in the claims In the drawing:

1 is a schematic diagram of an enable switch of an embodiment of an overvoltage protection circuit of the present invention;

2 is a block diagram showing the electrical principle of an embodiment of the overvoltage protection circuit of the present invention;

3 is a circuit diagram of an embodiment of an overvoltage protection circuit of the present invention;

4 is a schematic block diagram of an electrical principle of another embodiment of the overvoltage protection circuit of the present invention; FIG. 5 is a circuit diagram of another embodiment of the overvoltage protection circuit of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION It should be noted that the embodiments of the present invention and the features of the embodiments may be combined with each other without conflict. The invention will be described in detail below with reference to the drawings in conjunction with the embodiments.

The basic idea of the present invention is: The overvoltage protection circuit proposed by the present invention is designed with an enable switch to turn on the function of the sampling resistor, and a comparator is also designed to realize two short circuit detections, when the user accidentally accesses the non-current sampling. After the power is turned on, the enable switch is automatically turned off, and the sampling resistor switch is turned off to protect the sampling resistor from being burnt. Therefore, the present invention can not only automatically turn off the sampling resistor switch in a short response time, effectively protect the sampling resistor, and design the cartridge, which is low in cost. Here, the non-current sampling power supply is a driving power source with a voltage other than 0~5V in the current sampling mode, for example: the non-current sampling power supply may be a power supply with a power supply voltage of 12V, 19V, 24V, or 36V.

Based on the above, an embodiment of the present invention provides an overvoltage protection circuit, and the overvoltage protection circuit includes: an enable switch and a comparator. The enable switch is configured to control opening and closing of the sampling resistance switch connected in series with the sampling resistor; one end of the comparator is provided with an external interface, and the other end is connected with an enable end of the enable switch. And, the comparator determines, according to the comparison result, whether the enable end of the enable switch is enabled.

It should be noted that the external interface may be an analog input (AI) interface, but is not limited to the AI interface. Other types of interfaces that meet the sampling requirements may also be used in this embodiment. In this embodiment, when the external interface (such as the AI interface) enters the current sampling mode, the enable switch turns on the function of the sampling resistor, and when the switch is turned on, the enable end of the enable switch is set to be high. level.

Referring to FIG. 1, there is shown a schematic diagram of an example of a circuit for enabling a switch in this embodiment. The enable switch is an electronic switch composed of a diode or a logic gate circuit, such as a BTS3408 electronic switch. As shown in Fig. 1, the diode constituting the enable switch is grounded at one end and connected to a logic gate circuit at the other end. Sampling resistor 1 has one end connected to the sampling resistor switch and the other end connected to the AI interface. The one end connected to the sampling resistor switch is grounded. In this embodiment, the enable switch is configured to control opening and closing of the sampling resistor switch, and the enable switch can be integrated with the sampling resistor switch in the chip. The enable end of the enable switch is connected to the comparator, and the other end is connected to a controller such as a microcontroller control terminal.

Alternatively, the enable switch can also be implemented by means of a relay connected to the sampling resistor or other control unit.

Referring to Figure 2, there is shown an electrical block diagram of an embodiment of the overvoltage protection circuit of the present invention. As shown in FIG. 2, in the overvoltage protection circuit of this embodiment, there are two enable switches, namely: a first enable switch and a second enable switch. Correspondingly, there are two sampling resistance switches, namely: Sampling resistor switch and second sampling resistor switch.

The enable end of the first enable switch is connected to the comparator, the first enable switch is configured to control opening and closing of the first sampling resistor switch; and the enable end of the second enable switch Connected to the comparator, the second enable switch is configured to control opening and closing of the second sampling resistor switch. The sampling resistor includes: a first sampling resistor and a second sampling resistor. One end of the first sampling resistor is connected to the first sampling resistor switch, and the other end is connected to the first external interface. One end of the second sampling resistor is connected to the second sampling resistor switch, and the other end is connected to the second external interface.

In this embodiment, in order to implement two short circuit detections by using one comparator, the comparator further includes: a first comparison circuit and a second comparison circuit. The first comparison circuit is provided with a first external interface at one end and an enable end of the enable switch at the other end. One end of the second comparison circuit is provided with a second external interface, and the other end is respectively connected to an enable end of the enable switch.

Further, the first comparison circuit includes: a first comparison amplification circuit and a first clamp circuit. The first comparison amplifying circuit is connected to the first external interface, and the first comparison amplifying circuit is configured to compare an access voltage value of the first external interface with a size of a specified voltage value, and in the When the access voltage value of the external interface is higher than the specified voltage value, the output of the first comparison amplifying circuit is deflected. The first clamp circuit is coupled to the first comparison amplifier circuit, and the first clamp circuit is configured to control a voltage value of the enable switch enable terminal.

It should be noted that the specified voltage value is a reference value determined to be misconnected according to the access voltage value. In this embodiment, the specified voltage value can be set according to a voltage value of a non-current sampling power source that may be erroneously accessed. Generally, when the access voltage value of the first external interface is higher than the specified voltage value, the output of the first comparison amplifying circuit is deflected, and the first clamping circuit turns on the enabling switch. The voltage of the energy terminal is set to a low level, so that the enable switch is turned off, thereby protecting the sampling resistor. For example: The resistance of the sampling resistor R is 250 ohms, the maximum power is 1/16W, then the root P=U*U/R, which gives the maximum voltage value U _max that can be tolerated at both ends of the sampling resistor. Generally, it can be agreed that the reference voltage of the forward input pin of the comparator is 2.5V, and the voltage dividing resistors of the negative input pin are respectively Rl and R2, then the calculation formula is U _max x (R2/(R1+R2) ) = 2.5V, resulting in the values of Rl, R2.

Further, the first comparison amplifying circuit comprises: a first voltage dividing regulator and a first operational amplifier. The first voltage dividing regulator is connected to the first external interface, and the first voltage dividing regulator is configured to select different voltage dividing resistors according to the magnitude of the external voltage value of the first external interface. The specified voltage value. The negative terminal of the input end of the first operational amplifier is connected to the first voltage dividing regulator, the positive pole is connected to the power supply voltage, and the output end of the first operational amplifier is connected to the first clamping circuit.

Similar to the structure of the first comparison circuit, the second comparison circuit includes: a second comparison amplification circuit and a second clamp circuit. The second comparison amplifying circuit is connected to the second external interface, and the second comparison amplifying circuit is configured to compare an access voltage value of the second external interface with a size of a specified voltage value, and in the When the access voltage value of the external interface is higher than the specified voltage value, the output of the second comparative amplifying circuit is deflected. The second clamp circuit is coupled to the second comparison amplifier circuit, and the second clamp circuit is configured to control a voltage value of the enable switch enable terminal.

Further, the second comparison amplifying circuit comprises: a second voltage dividing regulator and a second operational amplifier. The second voltage dividing regulator is connected to the second external interface, and the second voltage dividing regulator is configured to select different voltage dividing resistors and the specified voltage value according to the magnitude of the external voltage value of the second external interface . A cathode of the input terminal of the second operational amplifier is connected to the second voltage dividing regulator, a positive pole is connected to the power supply voltage, and an output terminal of the second operational amplifier is connected to the second clamping circuit.

It should be noted that when the first comparison circuit and/or the second comparison circuit in the comparator are wrong When the non-current sampling power supply is connected, the first enable switch and the second enable switch are all turned off to turn off the first and second sampling resistance switches, thereby protecting the first and second sampling resistors. Therefore, in this embodiment, by using two comparison circuits having a current detecting function and an enable switch, overvoltage protection of the sampling resistor can be realized.

Thus, the comparator of this embodiment can achieve two short circuit detections. Here, in combination with the circuit diagram shown in FIG. 3, the above embodiment is further illustrated by an example:

Referring to FIG. 3, a schematic diagram of an example of an overvoltage protection circuit is shown. The example of the overvoltage protection circuit mainly includes an enable switch and a comparator. The comparator includes two comparison circuits. Comparison circuit, second comparison circuit. In this example, there are two said enable switches, namely: a first enable switch and a second enable switch. Correspondingly, there are two sampling resistance switches, namely: a first sampling resistance switch and a second sampling resistance switch; and, two sampling resistors, namely: a first sampling resistor R sampling and a second sampling resistor R sampling ₂ . One end of the first sampling resistor R is connected to the first sampling resistor switch, and the other end is connected to the AI interface 1. One end of the second sampling resistor R sample 2 is connected to the second sampling resistor switch, and the other end is connected to the AI interface 2, as shown in FIG.

In this example, the first comparison circuit is provided with a voltage dividing resistor R11, a voltage dividing resistor R12, a clamping diode D1, and an operational amplifier; wherein, the voltage dividing resistor R11 is connected to the AI interface 1 and the other end is connected to the negative terminal of the operational amplifier. The voltage divider resistor R12 is connected to the negative terminal of the operational amplifier and the other terminal is grounded. The operational amplifier anode is connected to the power supply, such as the 3.3V power supply 3V3, the output terminal of the operational amplifier is connected to the clamp diode D1, the other end of the clamp diode D1 and the enable end of the first enable switch K1 and the second enable switch K2. The connection controls the opening and closing of the first enable switch K1 and the second enable switch Κ2, that is, controls the opening and closing of the first sampling resistor switch and the second sampling resistor switch. The further connection relationship of the components of this embodiment can be referred to FIG. 3, and will not be described in detail herein.

Similar to the first comparison circuit, the second comparison circuit is provided with a voltage dividing resistor R21, a voltage dividing resistor R22, a clamping diode D2, and an operational amplifier; wherein, the voltage dividing resistor R21 is terminated The AI interface 2 is connected, and the other end is connected to the negative terminal of the operational amplifier; the voltage dividing resistor R22 is connected to the negative terminal of the operational amplifier, and the other end is grounded. The connection relationship of the remaining components is similar to the connection relationship of the corresponding components of the first comparison circuit. Therefore, the further connection relationship of the remaining components can be referred to FIG. 3, and will not be described in detail herein.

The advantage of this embodiment, for example, is that: the two comparator circuits are placed at the front end of the enable pin of the enable switch. When the current is sampled, the voltage value of the AI interface 1, and/or the AI interface 2 is higher than the specified value. At the voltage value, the output of the op amp is deflected if the 24V drive power supply is accidentally connected. At the same time, the clamping diode is used to set the voltage value of the enable terminal of the enable switch to a low level, thereby turning off the enable switch and turning off the sampling resistor switch, thereby protecting the sampling resistor from being burnt. It should be emphasized that the comparator and the enable switch designed in this embodiment can not only realize the current detecting function and the overvoltage protection of the sampling resistor, but also design a single tube, which is low in cost. Herein, a preferred embodiment of the present invention will be further described with reference to FIG. 4 and FIG. 5. As shown in FIG. 4, based on the foregoing embodiment, the overvoltage protection circuit of the embodiment is further preferably provided with a controller, and the control is further provided. The device is connected to the output ends of the first and second comparison amplifying circuits respectively for monitoring whether there is a misconnection information to determine whether the first and second external interfaces are misconnected.

It should be noted that the controller of this embodiment may be a microcontroller, a single chip microcomputer, etc., by connecting the output of the comparator to the pin of the microcontroller or the control end of the single chip microcomputer, the misconnection can be detected. information. The misconnected information is used to provide alarm information to the user and the controller processing unit, and the fault alarm indicating unit will give an alert, and the processing unit of the controller will quickly close the current switch.

Preferably, the overvoltage protection circuit of the embodiment is further provided with an alarm, the alarm being connected to the controller, and the alarm is configured to perform a fault alarm when the controller detects the misconnection information.

It should be noted that the alarm device provided in this embodiment can not only realize the fault alarm, but also improve the reliability of the overvoltage protection, and is more safe and reliable. More preferably, the overvoltage protection circuit of this embodiment is further provided with a fault analyzer, the fault analyzer is connected to the controller, and the fault analyzer is configured to detect when the controller detects the misconnection information. Diagnose the fault. Therefore, the embodiment provides a fault diagnosis function for the user's misconnection.

It should be noted that after the processor receives the fault alarm, the current switch is turned off, and the sampling mode is converted into the voltage mode, so that the port that the user has misconnected can be located, the function of the port will be closed, and the application program is also This information will be detected and the data on the port will be automatically masked without affecting the use of the entire motion controller. In the voltage sampling mode, when the user restores the normal wiring, the current switch can automatically switch to the normal mode and the control unit operates normally.

It can be seen from the above embodiments that the advantages of the various overvoltage protection circuit embodiments designed by the present invention are as follows:

In the overvoltage protection circuit of the present invention, the comparator can realize two short circuit detections, and each current detection is realized by setting a voltage dividing resistor, a clamp diode and an operational amplifier. Therefore, the present invention not only can effectively protect the sampling resistor, but also design a single cartridge, which is low in cost. Moreover, the overvoltage protection circuit designed by the present invention combines a comparator and an enable switch, and the response time is short, and the sampling resistor is not turned off when the heat is generated. It is worth mentioning that even if the sampling resistor is connected to the 24V drive power supply for a long time, it will not generate heat and has good reliability.

The embodiment of the present invention further provides a current sampling device provided with the overvoltage protection circuit described in any of the above embodiments. The overvoltage protection circuit according to any of the above embodiments has the above technical effects. Therefore, the current sampling device provided with the overvoltage protection circuit should also have corresponding technical effects, and the specific implementation process is similar to the above embodiment. I will not repeat them.

The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalents, improvements, etc., which are included in the spirit and scope of the present invention, should be included in the present invention. Within the scope of protection. Industrial applicability

The overvoltage protection circuit and the current sampling device provided by the invention can realize two short circuit detections, and each current detection is realized by setting a voltage dividing resistor, a clamping diode and an operational amplifier. Therefore, the present invention can not only effectively protect the sampling resistor, but also design a single cartridge, which is low in cost. Moreover, the overvoltage protection circuit designed by the present invention combines a comparator and an enable switch, and has a short response time, and the sampling resistor is not turned off when the heat is generated. It is worth mentioning that even if the sampling resistor is connected for a long time

On the 24V drive power supply, it will not generate heat and has good reliability. Therefore, the present invention has industrial applicability.

Claims

claims

1. An overvoltage protection circuit, characterized by including:

An enable switch, configured to control the opening and closing of a sampling resistor switch connected in series with the sampling resistor; A comparator, one end of which is provided with an external interface, and the other end is connected to the enable end of the enable switch; The comparator is configured according to The comparison result determines whether to enable the enable end of the enable switch.

2. The overvoltage protection circuit according to claim 1, characterized in that the comparator includes:

The first comparison circuit has a first external interface at one end, and the other end is connected to the enable end of the enable switch respectively;

The second comparison circuit has a second external interface at one end, and the other end is connected to the enable end of the enable switch respectively.

3. The overvoltage protection circuit according to claim 2, characterized in that,

The first comparison circuit includes:

A first comparison amplification circuit, which is connected to the first external interface. The first comparison amplification circuit is configured to compare the access voltage value of the first external interface with the specified voltage value, and output the signal to the first external interface. When the access voltage value of the interface is higher than the specified voltage value, the output of the first comparison amplifier circuit is deflected;

A first clamp circuit connected to the first comparison amplifier circuit, the first clamp circuit configured to control the voltage value of the enable end of the enable switch;

The second comparison circuit includes:

A second comparison amplification circuit, which is connected to the second external interface. The second comparison amplification circuit is configured to compare the access voltage value of the second external interface with the specified voltage value, and perform the second comparison amplification circuit on the second external interface. When the access voltage value of the interface is higher than the specified voltage value, the second comparison amplifier Output deflection of large circuits;

A second clamp circuit connected to the second comparison amplifier circuit, the second clamp circuit configured to control the voltage value of the enable end of the enable switch;

Wherein, the specified voltage value is a reference value for judging misconnection set according to the access voltage value.

4. The overvoltage protection circuit according to claim 3, characterized in that the circuit further includes: a controller, which is connected to the output terminals of the first and second comparison amplifier circuits respectively, and is configured to monitor whether there is Misconnection information is used to determine whether the first and second external interfaces are misconnected.

5. The overvoltage protection circuit according to claim 4, further comprising: an alarm, the alarm is connected to the controller, and the alarm is configured to detect misconnection information when the controller fault alarm.

6. The overvoltage protection circuit according to claim 4, further comprising: a fault analyzer, the fault analyzer is connected to the controller, and the fault analyzer is configured to detect when the controller detects Diagnose faults when misconnected information occurs.

7. The overvoltage protection circuit according to any one of claims 4 to 6, characterized in that the first comparison amplifier circuit includes:

A first voltage dividing regulator connected to the first external interface. The first voltage dividing regulator is configured to select different voltage dividing resistors and the voltage according to the value of the external voltage connected to the first external interface. Specify voltage value;

A first operational amplifier, the negative terminal of its input terminal is connected to the first voltage dividing regulator, the positive terminal is connected to the supply voltage, and the output terminal of the first operational amplifier is connected to the first clamping circuit; the second comparison Amplification circuit includes: A second voltage dividing regulator connected to the second external interface. The second voltage dividing regulator is configured to select different voltage dividing resistors and the voltage according to the value of the external voltage connected to the second external interface. Specify voltage value;

The negative terminal of the second operational amplifier is connected to the second voltage dividing regulator, the positive terminal is connected to the supply voltage, and the output terminal of the second operational amplifier is connected to the second clamping circuit.

8. The overvoltage protection circuit according to any one of claims 1 to 6, characterized in that the enable switch includes: a first enable switch and a second enable switch; the sampling resistor switch includes a first enable switch Sampling resistor switch and second sampling resistor switch;

Wherein, the enable end of the first enable switch is connected to the comparator, and the first enable switch is configured to control the opening and closing of the first sampling resistor switch; the enable end of the second enable switch Connected to the comparator, the second enable switch is configured to control opening and closing of the second sampling resistor switch.

9. The overvoltage protection circuit according to claim 8, characterized in that the sampling resistor includes:

A first sampling resistor, one end of which is connected to the first sampling resistor switch, and the other end is connected to the first pair of external interfaces;

The second sampling resistor has one end connected to the second sampling resistor switch and the other end connected to the second external interface.

10. A current sampling device, characterized in that the current sampling device is provided with the overvoltage protection circuit described in any one of the above claims 1 to 9.