R, C, L meter on microcircuits. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed device provides measurement of resistances of resistors, capacitances of capacitors and inductances of coils in a fairly wide range with an accuracy of no worse than 1.5...2%. The measurement results are counted by a pointer indicator with a linear scale.

Main technical characteristics:

• Measured resistance of resistors, Ohm.......10-2...106
• Measured capacitance of capacitors, pF.......10...107
• Measured inductance, H.......10-3...103
• Power consumption, W, no more.......10

At the heart of the measurement of parameters R, C; L lies the method of forming a voltage drop on the measured element, proportional to the value of its parameter. We will consider the principle of operation of the device using the example of measuring the resistance of a resistor. A fragment of the circuit explaining the pa6oty of the meter is shown in Fig.1. When a voltage of a fixed value U and frequency f is applied to a chain consisting of additional Rd and measured Rx resistors (moreover, Rx is much less than Rd), the voltage drop across the resistor Rx, (the large input resistance of a millivoltmeter has practically no effect on the circuit parameters) is: Ux =Urx/(Rd+Rx) Denoting the ratio of constant values ​​U/Rd through the coefficient K and providing the condition Rx/Rd much less than 1 in the entire range of resistance measurements, the expression is simplified to the form Ux~KRx, (with an error not exceeding the measurement accuracy) , which shows that the measured voltage is proportional to the value of the measured resistance of the resistor.

(click to enlarge)

Before measurement, it is necessary to calibrate the millivoltmeter scale by setting the voltage U at which the voltage drop across the calibration resistor Rx (when SA is turned on and Rx is turned off) will cause the instrument pointer to deviate to the final division of the scale. In this case, the entire scale of the device will correspond to the value of the calibration resistor Rx.

When measuring inductance, the same patterns as when measuring the resistance of a resistor, only instead of a calibration inductance coil, they include a resistor equivalent to the reactance of the coil for the frequency of the supply voltage.

Measuring the capacitance of a capacitor differs in that the voltage drop is measured from the current flowing through it on an additional resistor Rd connected in series with the capacitor. In this case, the instrument scale is calibrated using calibration capacitors. The resistance of the additional resistor in this case should be much less than the reactance of the capacitor at the measurement frequency. The voltage drop measured across the additional resistor is proportional to the value of the capacitance of the capacitor.

The meter consists of a switching unit for calibration resistors and capacitors, a generator that generates fixed frequencies of 159Hz and 15,9 kHz, and an alternating current millivoltmeter.

The switching unit includes a switch for measurement limits SA1, a switch for the type of work SA2 and a switch (or button) for calibration SA3. In the diagram below, the switch positions are shown for measuring resistors at the 1 MΩ limit. In the device circuit, resistors R7 - R13 are calibration resistors when measuring the resistance of resistors to coil inductances, and R14 - R20 are additional. When measuring the capacitances of capacitors, resistors R1 - R6 are additional, and capacitors C1 - C6 are calibration.

The generator (node ​​A) is made on microcircuits: DA1 is a master oscillator according to the scheme with a Wien bridge in a positive figurative connection circuit, DA2 is a non-inverting amplifier with a gain of 2, DA3 is an integrator. Changing the frequency of the generator is achieved by switching capacitors C7 - C10. In the seven upper positions of the SA1 switch according to the scheme, the generator provides oscillations with a frequency of 159 Hz, and in the two lower ones - 15,9 kHz. To obtain a sufficiently powerful measuring signal at the output of a non-inverting amplifier, a current amplifier based on a VT2 transistor was used. Resistor R30 (when the switch SA3 is closed) calibrates the device before taking measurements. The generator is stable in operation and has a harmonic coefficient of no worse than 0,05%.

An AC millivoltmeter (node ​​B) is made on a VT3 transistor and a DA4 chip. The FET cascade, made according to the source follower circuit, increases the input resistance of the device to 100 MΩ. The pointer meter RA1 is connected at the output of the amplifier to the diagonal of the rectifier bridge on diodes VD3, VD4 and resistors R44, R45. The scale of the millivoltmeter is linear, the measurement error is practically determined by the class of the pointer meter used.

In the design of the device, a pointer meter of the M906 type with a total deflection current of 50 μA was used. Switches SA1 and SA2 are biscuit, type PGG - 9P6N and 3P1N, respectively. Switch SA3 type TV1-1.

Resistors C2-10, C-13, C2-14 were used as calibration resistors, the remaining resistors are of the MLT or OMLT type. Capacitors KT-1, KSO, MBM, K73-17, K50-6, K50-20, other types can also be used. The measurement accuracy of the device to a certain extent depends on the selection of calibration capacitors, additional and calibration resistors, so they must be selected with an accuracy of no worse than ±0,5%. If these elements are used with an accuracy of ± 0,1 ... 0,25%, then the measurement error will practically be reduced to the accuracy of the microammeter measuring head used.

Operational amplifiers K574UD1 and K140UD8 can be used with any letter indices and their mutual replacement is possible without changing the printed circuit board design. In addition, instead of the K574UD1 chip, you can use the K544UD2 chip, and instead of the K553UD2 chip, the K153UD2 chip, but for each of these cases, you will need to change the pattern of the current-carrying tracks of the board.

(click to enlarge)

In addition to the types of diodes indicated in the diagram, diodes D311A, D18, D9 can be used. The KP103M transistor can be replaced with any transistor from the KP103 group, and KP303V with KP303G or KP303E. As a transistor VT2, any transistor from the KT815 or KT817 groups is applicable.

All calibration and additional elements are soldered directly to the terminals of the SA1 switch, and the generator and millivoltmeter elements are placed on two printed circuit boards made of foil fiberglass with one-sided metallization. On the generator board, the VT2 transistor should be placed on a heat sink with a heat dissipating surface area of ​​50 cm2. The millivoltmeter board is attached directly to the output terminals of the pointer measuring head.

The adjustment of the meter should begin with the adjustment of the generator. With a properly completed installation and serviceable elements, by rotating the engine of the tuning resistor R26, the generator is set to a stable operating mode. It is convenient to observe the setting of the generator on the oscilloscope screen, and determine the frequency using an electronic counting frequency meter.

To set the generator to a frequency of 159 Hz, switch SA1 is placed in any of the seven upper positions according to the scheme and, using trimmer resistors R21 and R22, adjust the frequency value. If pairs of capacitors C7, C10 and C8, C9 are selected with an accuracy of no worse than ± 1%, then tuning to a frequency of 15,9 kHz is not required, it is provided automatically. It should be noted that the exact setting of the frequencies is not necessary, it is only important that they differ from each other by 100 times. The effect of frequency setting inaccuracy is easily compensated for when calibrating the instrument.

Setting up a millivoltmeter comes down to setting the microammeter needle to the last division of the scale with a tuned resistor R43 when a voltage of 0,05 V with a frequency of 159 Hz is applied to the input of the millivoltmeter. Then, the conformity of the deviation of the arrow of the device is checked when a voltage of 0,05 V with a frequency of 15,9 kHz is applied to the input. With serviceable circuit elements, this is provided automatically, no adjustments are required.

For the convenience of reading the readings, the scale of the microammeter should be made at 100 divisions or use a 100 μA ready-made microammeter from a similar microammeter, setting it instead of the 50 μA scale.

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