JP2011220699A - Voltage-current measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a voltage-current measuring apparatus which can accurately measure voltage even if a common-mode voltage is superimposed, thereby overcoming the problem seen in previous voltage-current measuring apparatus that voltage cannot be measured when a common-mode voltage is superimposed on an input signal since one end of an input terminal is grounded.SOLUTION: The voltage-current measuring apparatus comprises: analog switches 39 and 40 for switching the input of a differential amplifier 17; and MOSFETs 34 and 35 for detaching a current detecting resistance 38 from input terminals 31a and 31b. Since the input terminals are not directly grounded, voltage can be measured, even if a common-mode voltage is superimposed.

Description

  The present invention relates to a measuring apparatus capable of measuring both voltage and current, and more particularly to a voltage / current measuring apparatus suitable for use in measuring an output signal of an industrial instrument.

  The output signals of industrial instruments are standardized, and a voltage signal of 1 to 5 V or a current signal of 4 to 20 mA is used. For this reason, in order to check the output signal of an industrial instrument, a measuring device capable of measuring both voltage and current is required.

  4 and 5 show the configuration of a voltage / current measuring apparatus that can measure both the voltage and the current. FIG. 4 shows a configuration for measuring a voltage, and FIG. 5 shows a configuration for measuring a current.

  In FIG. 4, reference numeral 10 denotes a voltage / current measuring device, which includes input terminals 11 and 12, a MOSFET 13, a control unit 14 that controls on / off of the MOSFET 13, a resistor 15, an analog switch 16, a differential amplifier 17, and an AD converter 18. Is done. An industrial instrument 20 that outputs a voltage signal is connected to the input terminals 11 and 12.

  One end of MOSFET 13 and a contact of analog switch 16 are connected to input terminal 11. The input terminal 12 is grounded.

  The other end of the MOSFET 13 and one end of the resistor 15 are connected to the b contact of the analog switch 16. The other end of the resistor 15 is grounded.

  The c contact of the analog switch 16 is connected to one input terminal of the differential amplifier 17. The other input terminal of the differential amplifier 17 is grounded. The output signal of the differential amplifier 17 is input to the AD converter 18 and converted into a digital value. This digital value is displayed on a display (not shown).

  When measuring the voltage, the analog switch 16 is operated to connect the contact a to the contact c, and the MOSFET 13 is turned off. In FIG. 4, since the N-channel MOSFET is used, the control unit 14 outputs a low level signal to turn off the MOSFET 13.

  Since the b contact of the analog switch 16 is open and the MOSFET 13 is off, the resistor 15 is disconnected from the industrial instrument 20. The output voltage of the industrial instrument 20 is input to the differential amplifier 17 via the a and c contacts of the analog switch 16 and amplified, and converted into a digital value by the AD converter 18.

  FIG. 5 is a configuration diagram when the current is measured. In addition, the same code | symbol is attached | subjected to the same element as FIG. 3, and description is abbreviate | omitted.

  In FIG. 5, 21 is an industrial instrument that outputs a current signal, and is connected to the input terminals 11 and 12. Further, the b and c contacts of the analog switch 16 are connected and the a contact is opened. Further, the MOSFET 13 is turned on. In FIG. 4, the control unit 14 outputs a high level signal and turns on the MOSFET 13.

  The output current of the industrial instrument 21 flows to the resistor 15 via the MOSFET 13 and is converted into a voltage signal. When the resistance value of the resistor 15 is R15 and the output current of the industrial instrument 21 is IIN, the voltage across the resistor 15 is IIN × R15.

  The voltage across the resistor 15 is input to the differential amplifier 17 via the analog switch 16 and amplified, and converted into a digital value by the AD converter 18.

  Thus, since both the voltage and the current can be measured by the voltage / current measuring device 10, the output signal of the industrial instrument can be evaluated by one device.

JP 2009-282050 A

However, such a voltage / current measuring apparatus has the following problems.
In the voltage / current measuring apparatus of FIG. 4, the input terminal 12 is grounded. For this reason, when the common mode voltage is superimposed on the output signal of the industrial instrument 20, measurement cannot be performed, and the industrial instrument 20 cannot be evaluated.

  An object of the present invention is to realize a voltage / current measuring apparatus capable of accurately measuring an input voltage even when a common mode voltage is superimposed on a voltage signal input to an input terminal.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In a voltage / current measuring apparatus capable of measuring a voltage signal and a current signal,
First and second input terminals to which a voltage signal or a current signal is applied;
A first switch having one end connected to the first input terminal;
A second switch having one end connected to the second input terminal;
A current detection resistor having both ends connected to the other ends of the first switch and the second switch;
A first selection switch that receives a signal applied to the first input terminal and a signal at one end of the current detection resistor, and selects and outputs these signals;
A second selection switch for inputting a signal applied to the second input terminal and a signal at the other end of the current detection resistor, and selecting and outputting these signals;
A differential amplifier that receives a signal selected by the first and second selection switches and outputs a voltage related to a differential voltage between the input signals;
It is equipped with. Even if a common mode voltage is superimposed on the voltage signal to be measured, the voltage can be measured.

The invention according to claim 2 is the invention according to claim 1,
An output signal of the differential amplifier is input, and an AD converter that converts the input signal into a digital value is provided. Processing of measured values is simplified.

The invention according to claim 3 is the invention according to claim 1 or 2,
A first diode disposed between the first input terminal and one end of the first switch; and a second diode disposed between the second input terminal and one end of the second switch. The diode is provided. Even if there are parasitic diodes in the first and second switches, accurate measurement can be performed.

The present invention has the following effects.
According to the first, second, and third aspects of the present invention, the resistor for measuring current is connected to the input terminal via the first and second switches, and the input terminal is connected using the first and second selection switches. The applied voltage and the voltage across the current detection resistor are selected and input to the differential amplifier.

  Since the current detection resistor is connected to and disconnected from the input terminal via the first and second switches, the input terminal is not directly grounded and operates as if the input terminal is insulated from the ground. To do. For this reason, even if the common mode voltage is superimposed on the voltage signal to be measured, there is an effect that the voltage can be measured accurately.

  Further, by disposing the first and second diodes between the input terminal and the first and second switches, an element having a parasitic diode such as a MOSFET may be used as the first and second switches. There is an effect that no current flows when the switch is turned off.

  Further, by converting the output of the differential amplifier into a digital value by the AD converter, there is an effect that the subsequent signal processing can be simplified.

It is the block diagram which showed one Example of this invention. It is the block diagram which showed one Example of this invention. It is the block diagram which showed the other Example of this invention. It is a block diagram of the conventional voltage current measuring apparatus. It is a block diagram of the conventional voltage current measuring apparatus.

  Hereinafter, the present invention will be described in detail with reference to the drawings. 1 and 2 are configuration diagrams showing an embodiment of a voltage / current measuring apparatus according to the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 3, and description is abbreviate | omitted. FIG. 1 shows a configuration for measuring voltage, and FIG. 2 shows a configuration for measuring current.

  In FIG. 1, reference numeral 30 denotes a voltage / current measuring device, which inputs input terminals 31a and 31b, diodes 32 and 33, MOSFETs 34 and 35, control unit 36, resistors 37 and 38, analog switches 39 and 40, and differential signals. It comprises a differential amplifier 17 and an AD converter 18 for outputting. The analog switches 39 and 40 have three contacts a to c, and the c contact is selected and connected to either a or b.

  The input terminals 31a and 31b correspond to first and second input terminals, respectively, and the MOSFETs 34 and 35 correspond to first and second switches, respectively. The analog switches 39 and 40 correspond to first and second selection switches, respectively, and the diodes 32 and 33 correspond to first and second diodes, respectively. Further, the resistor 38 corresponds to a current detection resistor.

  Reference numeral 41 denotes an industrial instrument that outputs a voltage signal, and reference numeral 42 denotes a common mode voltage. The industrial instrument 41 is connected to the input terminals 31a and 31b. The common mode voltage 42 is generated between the input terminal 31b and the ground.

  The input terminal 31 a is connected to the a contact of the analog switch 39 and the anode of the diode 32. The input terminal 31 b is connected to the contact “a” of the analog switch 40 and the cathode of the diode 33.

  One end of the MOSFET 34 is connected to the cathode of the diode 32, and the other end is connected to the b contact of the analog switch 39. One end of the MOSFET 35 is connected to the anode of the diode 33, and the other end is connected to the b contact of the analog switch 40.

  A resistor 37 is connected between the input terminal 31b and the ground. The resistance value of the resistor 37 is a high resistance. A resistor 38 is connected between the sides of the MOSFETs 34 and 35 connected to the analog switches 38 and 39. The side of the resistor 38 to which the MOSFET 35 is connected is grounded.

  The output signal of the control unit 36 is output to the gates of the MOSFETs 34 and 35. The control unit 36 controls ON / OFF of the MOSFETs 34 and 35 based on this output signal.

  The c contacts of the analog switches 39 and 40 are connected to the two input terminals of the differential amplifier 17. The differential output signal of the differential amplifier 17 is input to the AD converter 18 and converted into a digital value. In addition, although there are a display for displaying a measured value or a switch for switching between voltage measurement and current measurement, the description is omitted.

  In the voltage measurement, the analog switches 39 and 40 are both configured to connect the c contact to the a contact. Further, both MOSFETs 34 and 35 are turned off. In FIG. 1, since an N-channel element is used as the MOSFET 34, the control unit 36 applies a low level signal to the gate of the MOSFET to turn it off. Further, since a P-channel element is used as the MOSFET 35, the control unit 36 turns off the MOSFET 35 by setting the gate of the MOSFET 35 to a high level.

  Next, the operation during voltage measurement will be described. Since the MOSFETs 34 and 35 are both off and the b contacts of the analog switches 39 and 40 are open, the resistor 38 is disconnected from the industrial instrument 41. For this reason, no current flows through the resistor 38. The output voltage of the industrial instrument 41 is input to the differential amplifier 17 via the analog switches 39 and 40.

  Since the resistor 37 is connected between the input terminal 31b and the ground, a current caused by the common mode voltage 42 flows. However, since the resistance 37 is a high resistance, this current can be ignored.

  In the conventional example of FIG. 4, one of the input terminals to which the industrial instrument is connected is grounded, and therefore, when the common mode voltage is superimposed on the output voltage of the industrial instrument, it cannot be measured. In this embodiment, since the input terminal is not directly grounded but only grounded via the resistor 37, the output voltage of the industrial instrument 41 can be measured even if the common mode voltage is superimposed.

  Further, since the output voltage of the industrial instrument 41 is input to the differential amplifier 17, the common mode voltage 42 superimposed on the output voltage is removed by the differential amplifier 17. For this reason, only the output voltage of the industrial instrument 41 is applied to the AD converter 18, and the voltage of the output signal can be measured accurately.

  FIG. 2 shows a configuration for current measurement. The current measurement will be described based on FIG. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  In FIG. 2, reference numeral 30 denotes a voltage / current measuring device, which has the same configuration as the voltage / current measuring device 30 of FIG. Reference numeral 43 denotes an industrial instrument that outputs a current signal, and is connected to the input terminals 31a and 31b.

  In the analog switches 39 and 40, both the c contact and the b contact are connected, and the a contact is opened. The control unit 36 controls the MOSFETs 34 and 35 to be turned on. Specifically, since the MOSFET 34 is an N-channel device, its gate voltage is set to a high level, and since the MOSFET 35 is a P-channel device, its gate voltage is set to a low level.

  The output current of the industrial instrument 43 flows through the resistor 38 via the diode 32 and the MOSFET 34, and returns to the industrial instrument 43 via the MOSFET 35 and the diode 33.

  For this reason, a voltage proportional to the output current of the industrial instrument 43 is generated at both ends of the resistor 38. When the resistance value of the resistor 38 is R38 and the output current of the industrial instrument 43 is IIN, the voltage generated across the resistor 38 is IIN × R38.

  The voltage across the resistor 38 is input to the differential amplifier 17 via the analog switches 39 and 40. The output voltage of the differential amplifier 17 is converted into a digital value by the AD converter 18. Since this digital value is proportional to the output current of the industrial instrument 43, the value of the output current of the industrial instrument 43 can be measured. A part of the output current of the industrial instrument 43 flows to the ground via the resistor 37, but since the resistance value of the resistor 37 is large, this current can be ignored.

  FIG. 3 shows another embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  In FIG. 3, reference numeral 50 denotes a voltage / current measuring device, which includes input terminals 31a and 31b, diodes 32 and 33, MOSFETs 34 and 35, control unit 36, resistors 37 and 38, analog switches 39 and 40, differential amplifier 17, AD conversion. The device 18 is configured. In addition, the description of the industrial instrument connected to the input terminal is omitted.

  In this embodiment, the polarities of the diodes 32 and 33 are reversed from those in FIG. 1, and the MOSFETs 34 and 35 are exchanged. The side of the resistor 38 connected to the analog switch 39 is grounded. That is, the cathode of the diode 32 is connected to the input terminal 31 a and the anode is connected to one end of the MOSFET 35. The anode of the diode 33 is connected to the input terminal 31 b and the cathode is connected to one end of the MOSFET 34.

  For voltage measurement, the c contacts of the analog switches 39 and 40 are connected to the a contact, and the MOSFETs 34 and 35 are turned off. The voltage applied to the input terminals 31a and 31b is input to the differential amplifier 17 via the analog switches 39 and 40. The operation is the same as in FIG.

  At the time of current measurement, the c contacts of the analog switches 39 and 40 are connected to the b contacts, and the MOSFETs 34 and 35 are turned on. The current flowing from the input terminal 31 b flows through the path of the diode 33, MOSFET 34, resistor 38, MOSFET 35, and diode 32, and a voltage proportional to the value of the input current is generated at both ends of the resistor 38. This voltage is input to the differential amplifier 17 via the analog switches 39 and 40. Except for the direction of current, the operation is the same as in FIG.

  In these embodiments, the MOSFETs 34 and 35 are used as switches for disconnecting the resistor 38 from the input terminals 31a and 31b. A switch element such as a transistor or a mechanical switch can also be used.

  Further, although the analog switches 39 and 40 are used, they can be replaced with other switch elements or mechanical switches.

  Further, although an amplifier that outputs a differential output signal is used as the differential amplifier 17, a single-ended output amplifier may be used. In this case, the AD converter 18 also has a single end input.

  Further, the differential amplifier 17 may be any one that outputs an output signal related to the difference voltage between two input signals, and does not necessarily amplify the signal. A differential buffer with a gain of 1 or a gain of less than 1 may be used. The gain of the differential amplifier 17 is determined by the magnitude of the input signal and the characteristics of the AD converter 18.

  The diodes 32 and 33 are for preventing current due to the common mode voltage from flowing through the parasitic diodes of the MOSFETs 34 and 35 when the MOSFETs 34 and 35 are off. If switches that do not generate parasitic diodes are used instead of the MOSFETs 34 and 35, the diodes 32 and 33 can be omitted. In this case, the input terminals 31a and 31b and the MOSFETs 34 and 35 are directly connected.

  In these embodiments, the AD converter 18 is used to convert to a digital value. However, processing can be performed without converting to a digital value.

  Furthermore, in these embodiments, the output signal of the industrial instrument is measured, but it can also be used as a device for measuring the voltage or current of other signals.

17 Differential Amplifier 18 AD Converter 30, 50 Voltage Current Measurement Device 31a, 31b Input Terminal 32, 33 Diode 34, 35 MOSFET
36 Control unit 37, 38 Resistance 39, 40 Analog switch 41, 43 Industrial instrument 42 Common mode voltage

Claims (3)

In a voltage / current measuring apparatus capable of measuring a voltage signal and a current signal,
First and second input terminals to which a voltage signal or a current signal is applied;
A first switch having one end connected to the first input terminal;
A second switch having one end connected to the second input terminal;
A current detection resistor having both ends connected to the other ends of the first switch and the second switch;
A first selection switch that receives a signal applied to the first input terminal and a signal at one end of the current detection resistor, and selects and outputs these signals;
A second selection switch for inputting a signal applied to the second input terminal and a signal at the other end of the current detection resistor, and selecting and outputting these signals;
A differential amplifier that receives a signal selected by the first and second selection switches and outputs a voltage related to a differential voltage between the input signals;
A voltage / current measuring device comprising:   2. The voltage / current measuring apparatus according to claim 1, further comprising an AD converter that receives an output signal of the differential amplifier and converts the input signal into a digital value.   A first diode disposed between the first input terminal and one end of the first switch; and a second diode disposed between the second input terminal and one end of the second switch. 3. The voltage / current measuring apparatus according to claim 1, further comprising a diode.

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