JPH0372273A - Measuring device for contact resistance - Google Patents

Abstract

PURPOSE:To enable highly-precise measurement of a contact resistance value of a wide range at a prescribed current value by separating analog and digital circuits electrically by using a photocoupler, and by displaying the result digitally. CONSTITUTION:Being controlled by a digital drive circuit 1, a prescribed current is supplied from a constant current circuit 3 to a contact 10 to be measured and a measured voltage proportional to a resistance value of the contact 10 is amplified 4, subjected to A/D conversion 5 and displayed digitally in a display element 9, while determination of the quality in respect to a reference value is displayed in a determination display element 8. On the occasion, transmission of signals between an analog signal system of the circuit 3, a differential amplifier 4, etc. and a digital circuit such as the circuit and a comparator circuit 7 is executed optically by photocouplers 2 and 6. Thereby it is made possible to separate electrically the two circuit systems from each other and to separate a power source and the earth completely. Accordingly, the lowering of precision in measurement due to the superposition of a commercial frequency, switching noise, impulse noise or the like on an analog signal is eliminated and highly precise measurement can be executed with a prescribed measuring current.

Description

[Detailed Description of the Invention] [Summary] This invention relates to a contact resistance measuring device that accurately measures the contact resistance, etc. of a switch, and is capable of measuring a wide range of contact resistance values with high accuracy at a specified measurement current value, and is a low-cost contact. The object of the present invention is to provide a resistance measuring device, which includes a drive circuit that controls the timing of supplying a measurement current, a constant current circuit that supplies a constant measurement current to an object to be measured in accordance with the timing, and a resistance measurement device that controls the measurement current by the measurement current. a differential amplifier that amplifies the voltage generated across the object to be measured; a double-integral AD conversion circuit that converts the output of the differential amplifier from analog to digital; and a display unit that displays the digital output of the AD conversion circuit. , a comparison circuit that compares the digital output with a preset reference value, and a judgment display unit that displays a pass/fail judgment with a lamp based on the comparison result of the comparison circuit, and the constant current circuit is controlled by the drive circuit. is carried out via a first photocoupler, and the constant current circuit has a series circuit of a transistor, a Zener diode connected to the base of the transistor, and a temperature compensation diode, and further includes a series circuit of a transistor, a Zener diode connected to the base of the transistor, and a temperature compensation diode. The digital output is transmitted to the display section and the comparison circuit through a second photocoupler.

[Industrial application field]

The present invention relates to a contact resistance measuring device that measures a low resistance value that fluctuates over a wide range, such as the contact resistance of a switch, using a constant current value.

[Conventional technology]

The contact resistance of electrical contacts such as pushbutton switches built into keyboards varies over a wide range from the order of mΩ to several Ω or more. Conventionally, these contact resistances have been measured using a ohmmeter that can widen the resistance measurement range by switching the measurement range. Measurements were made using a combination of a constant current power supply and a digital multimeter that digitally measures voltage, current, and resistance.

[Problems to be Solved by the Invention] The contact resistance value of a contact changes over a wide range from the order of mΩ to several Ω or more depending on the presence of an insulating film on the surface of the contact. Therefore, when measuring contact resistance, a small constant current of, for example, 1 or 5 mA is passed through the contact and the voltage drop across the contact is detected so that the contact state is not affected by the measurement current. It is specified to measure the contact resistance value.

However, the milliohmmeter mentioned above changes the measurement range by changing the measurement current value depending on the contact resistance of the sample to be measured so that the voltage drop value stays within a certain range, so it is not possible to measure at a constant current value. There wasn't. Furthermore, since the measurement value is read from the amount of deflection of the meter pointer, there is a problem in that there are individual differences in readings between the measurers and measurement errors are likely to occur.

On the other hand, digital multimeters have the problem that the equipment is expensive because they have extra measurement functions such as voltage and current in addition to resistance values, and it is uneconomical to use them only for measuring contact resistance.

It is conceivable to configure it by combining a constant current source and a digital voltmeter, but the normal voltage drop due to contact resistance is μV.
It is extremely small on the order of magnitude, and it has been difficult to measure minute DC voltages with high precision without being affected by noise using an inexpensive circuit.

The present invention was created in view of the above-mentioned problems, and it is an object of the present invention to provide a contact resistance measuring device that can measure a wide range of contact resistance values with high precision using a specified measurement current value and that has reduced cost.

[Means for Solving the Problems] The above problems include: a drive circuit that controls the supply timing of the measurement current; a constant current circuit that supplies a constant measurement current to the object under test in response to the tie shortening; a differential amplifier that amplifies the voltage generated across the object to be measured by the measurement current; a double-integral AD conversion circuit that converts the output of the differential amplifier from analog to digital; and a digital output of the AD conversion circuit. a display unit that displays a display, a comparison circuit that compares the digital output with a preset reference value, and a judgment display unit that displays a pass/fail judgment with a lamp based on the comparison result of the comparison circuit, Control of the constant current circuit is performed via a first photocoupler, and the constant current circuit has a series circuit of a transistor, a Zener diode connected to the base of the transistor, and a temperature compensation diode, and further This problem is solved by the contact resistance measuring device of the present invention, characterized in that the digital output is transmitted from the AD converter to the display section and the comparison circuit via a second photocoupler.

[For production]

A constant current is supplied from the constant current circuit to the device under test under the control of the drive circuit, and the measurement voltage proportional to the resistance value of the device is amplified by a differential amplifier, converted to a digital value by an AD converter, and displayed. A digital display is displayed in the section, and a pass/fail judgment based on a predetermined reference value is displayed in the judgment display section.

Signal transmission between analog signal systems such as constant current circuits and differential amplifiers that generate and amplify minute DC voltages and other digital circuits such as drive circuits and comparison circuits is performed optically using photocouplers. Since both circuit systems are electrically separated, it is possible to use completely separate power supplies and grounding.

This prevents commercial frequencies, switching noise, impulse noise, etc. from being superimposed on the analog signal and reducing measurement accuracy, allowing highly accurate measurement with a constant measurement current.

〔Example〕

The present invention will be described in detail below with reference to the accompanying drawings.

The figure is a block diagram of the contact resistance measuring device of the present invention, and the configuration and operation of each part are as follows.

In the figure, 1 is a digital drive circuit, 2 is a first photocoupler, 3 is a constant current circuit, 4 is a differential amplifier, and 5 is an AD
Converter, 6 is the second photocoupler, 7 is the comparison circuit, 8
9 is a judgment display section, 9 is a display section, and IO is a contact to be measured.

The digital drive circuit 1 turns on the ICII for a predetermined period of time in response to a gate signal from a control section (not shown) to cause the light emitting diode 21 of the photocoupler 2 to emit light.

The photocoupler 2 is composed of, for example, a light emitting diode 21 and a phototransistor 22, and the phototransistor 22 is turned on by the light emitted from the light emitting diode 21. Since the photocoupler 2 is connected by light, the driving side and the driven side are electrically connected. It acts as a completely separated switching element.

The constant current circuit 3 consists of a transistor 33 and three resistors, and the base of the transistor 33 is connected to the voltage RV through a series connection of a Zener diode 31 and a diode 32.
s, and is connected to ground through a series connection of a fixed resistor R3 and a phototransistor 22 of a photocoupler, its emitter is connected to a power supply V through a series connection of a variable resistor R8 and a fixed resistor R2, and its collector is connected to ground via the contact 10 to be measured. In this circuit, when the phototransistor 22 is turned on, Io-((Vz
+VF) -VIIE) / (R1+R2) (
Here, Vz; Zener voltage of Zener diode,
A collector current expressed by VF (forward voltage of the diode, Vgti) base-emitter voltage of the transistor flows, and this value is constant regardless of the contact resistance value RX of the contact 10 to be measured connected to the collector. Therefore, variable resistance R
By adjusting 1, the predetermined measurement current can be set to
0 can be supplied. This constant current circuit has 3 transistors, zener diodes, and diodes each.
Since it is a simple circuit consisting of only one resistor, it is low cost. Also, factors that cause the output current of a constant current circuit to fluctuate include fluctuations in the reference voltage Vs and circuit components (resistance, transistor).
Fluctuations are considered to be due to the temperature characteristics of In the present invention, a diode 32 and a Zener diode 31 (temperature coefficient: 10 ppm) are used to resolve fluctuations in the reference voltage Vs.

In order to offset the temperature coefficient of VIIE of the transistor 33, a diode 32 is connected between the Zener diode 31 and the base of the transistor 33.

Since the temperature is compensated in this way, high stability of 10 ppm or less can be obtained.

The differential amplifier 4 includes an operational amplifier 41 and four resistors R5+.
It is constructed using R61R71Rs, and is designed to amplify the microvoltage on the μV order that occurs across the contact 10 to be measured to a value that can be AD converted. Since this differential amplifier 4 has a large common mode rejection ratio (CMRR), ground potential fluctuations (which are superimposed as common noise on both + and - terminals) (
Common mode noise) is removed, and only the amount of voltage drop of the object to be measured is amplified to a predetermined value and input to the AD converter 5. The AD converter 5 is, for example, a 12-bit counting type AD converter using a double integration method that is inexpensive, highly accurate, and stable, and has a relatively slow conversion speed of several hundred m5.
Noise due to the commercial frequency with a period of 20 m5 is integrally removed and does not adversely affect the measured value. The 12-bit parallel output of this AD converter 5 drives a light emitting diode of a second photocoupler 6 connected to each bit line. The digital data transmitted via this photocoupler 6 is then manually input to a comparator circuit 7 and a display section 9.

The display unit 9 displays the amount of voltage drop at the contact to be measured in, for example, four decimal digits. Here, by setting the measurement current to 1 mA and setting the amplification degree of the differential amplifier 4 to a power of lO (for example, 100 times), it is possible to directly express the resistance value of the contact to be measured with a digitally displayed value. I'll come. The comparison circuit 9 compares the input measurement data with a preset upper limit value RU and lower limit value Rt of the allowable contact resistance of the contact to be measured, and outputs the determination result to the pass/fail display section 8 . The judgment display section 8 shows that the series circuit of the driving IC, the resistor, and the light emitting diode is connected to the digital power supply V, and if the contact resistance value of the contact to be measured is within the allowable range, the judgment display section 8 indicates "good". lamp 81, or “defective” lamp 82 if outside the tolerance range.
connected to be lit.

In the contact resistance measuring device configured as described above, signals are transmitted between the analog signal system of the constant current circuit and differential amplifier that generate and amplify minute DC voltages, and other digital circuits such as the drive circuit and comparison circuit. Transmission is carried out optically by a photocoupler, and both circuit systems are electrically separated, making it possible to use completely separate power supplies and grounding. Therefore, highly accurate measurement can be performed without deteriorating measurement accuracy due to commercial frequency, switching noise, impulse noise, etc. being superimposed on analog signals.

Furthermore, since the measured values are digitally displayed, there are no individual differences in reading, and measurement errors are reduced. Furthermore, since the constant current circuit has a simple configuration, the cost is low.

[Effects of the Invention] As explained above, the measuring device of the present invention uses a photocoupler to electrically separate an analog circuit and a digital circuit, and has a built-in temperature-compensated constant current circuit, and is capable of transmitting measurement results. Since the display is digital, it is possible to provide a contact resistance measuring device at a low cost that can measure the contact resistance with high precision while a constant measurement current is supplied to the contact to be measured.

[Brief explanation of drawings]

The figure is a block diagram of a contact resistance measuring device of the present invention. In the figure, 1 - digital drive circuit, 2 - first photocoupler, 21 - light emitting diode, 22
- Phototransistor, 3-...constant current circuit, 3■
... Zener diode, 32- Diode, 3
3-1-transistor, 4-differential amplifier, 5-A
D converter, bra, 8-judgment display section. 6-Second photocoupler 7...comparison circuit, 9-display section,

Claims (1)

[Scope of Claims] A drive circuit (1) that controls the supply timing of the measurement current; a constant current circuit (3) that supplies a constant measurement current to the object to be measured (10) in accordance with the timing; A differential amplifier (4) that amplifies the voltage generated across the object to be measured (10) by the measurement current, and a double integral AD converter (5) that converts the output of the differential amplifier (4) from analog to digital. ) and the AD converter (5
), a comparison circuit (7) that compares the digital output with a preset reference value, and a judgment lamp that displays a pass/fail judgment based on the comparison result of the comparison circuit. The constant current circuit (3) is controlled by the drive circuit (1) via a first photocoupler (2), and the constant current circuit (3) is a transistor. (33), a series circuit of a Zener diode (31) connected to the base of the transistor, and a temperature compensation diode (32),
Furthermore, the contact resistance measurement is characterized in that digital output is transmitted from the AD converter (5) to the display section (9) and the comparison circuit (7) via a second photocoupler (6). Device.