Capacitance meter - attachment to the tester. Scheme, description

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Capacitance meter - attachment to the tester. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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I propose a capacitance meter (Fig. 1), with which you can measure capacitors without even unsoldering them from the circuit. The main nodes of the meter are:

DD1.1, DD1.2, DD1.4 - triangular voltage generator;
VT4...VT6, DD1.6 - measuring amplifier;
VT7 - comparison node and detector;
VT8 - current amplifier;
DD1.3, DD1.5, VT9 - a switch that connects the capacitance of the solar cell to the output of the detector within the measurement limits "500 uF" and "5000 uF" (when the frequency of the master oscillator is very low);
VT1...VT3 - voltage stabilizer with short circuit protection and low battery indication.

Capacitance meter - attachment to the tester. Schematic diagram of the S-meter attachment

Fig.1. Schematic diagram of the S-meter attachment (click to enlarge)

The output voltage of the stabilizer is approximately 3,9 V. The stabilization mode is maintained until the input voltage exceeds 4 V. The degree of saturation of the regulating transistor VT1 is set by resistor R9; R8 is used to start the stabilizer. LED VD3 and diodes VD4, VD5 are used as a zener diode. An LED can be of any type, but even instances of the same type have a noticeable variation in ignition voltage. Therefore, to accurately set the output voltage of the stabilizer, it is necessary to select the resistor R11. The input voltage of 4 V is chosen with the expectation that if batteries (4 pcs.) are used for power supply, then when they are completely discharged, each battery should not be less than 1 V (otherwise their resource is sharply reduced). If the input voltage drops below 4 V, the stabilization mode breaks and the LED goes out. Capacitor C12 serves to suppress parasitic RF oscillations. The generator DD1.1, DD1.2, DD1.4 generates a triangular voltage of different frequencies (each measurement limit has its own frequency). The larger the capacitance to be measured, the lower the oscillator frequency should be. The output voltage of the generator through the divider R6-R7 (1:100) is applied to the measured capacitance. Its value at the "Cx" terminals is approximately 35 mV. Therefore, the elements of the circuit in which this capacitance is located do not affect the accuracy of the measurement. The exception is low-resistance resistors or inductance connected in parallel with the capacitance, which is very rare.

A broadband amplifier on VT4 ... VT6 and a reference voltage source on DD1.6 amplify these 35 mV to a voltage of about 3 V. If the measured capacitance is not connected, at the ends of the resistor R17 there are two voltages of the same frequency and approximately the same amplitude, but opposite in phase , since the amplifier inverts the output voltage of the generator. Resistor R17 balances the input of the detector, achieving the minimum readings of the microammeter. Pre-resistor R22 (DC balance), the arrow of the tester should be brought to the middle of the scale. After balancing with R17, resistor R22 returns the instrument pointer to "0" of the scale.

The device is ready to work. When switching the measurement limits, the balancing is preserved, but when you turn it on again after a long period of time, an imbalance may occur, which is restored after 2 ... 3 minutes. At the limits of "500 uF" and "5000 uF", the arrow is set to "0" longer, because a large capacitance C7 is connected to the output of the VT9 detector.

The measured capacitor Cx is included in the feedback circuit of the measuring amplifier, reducing its gain at a given frequency in proportion to its capacitance. The output voltage of the amplifier decreases and no longer compensates for the antiphase exemplary voltage of the generator. The unbalance value on R17 is fixed by VT7, the VT8 emitter follower amplifies the current signal and feeds it to the meter. The arrow deviates in proportion to the measured capacitance. The oscillator frequencies are selected in such a way that for a device with a total deflection current of 100 μA at the first measurement limit, the full-scale deviation of the arrow causes a capacitance of 0,1 μF.

If a 50µA tester is used, the maximum capacitance measured at the first limit will be 0,05µF. In the diagram, the measurement limits and elements are indicated for a 50 μA head. The circuit works quite linearly and with a 100µA head. There are testers with measuring heads for 60 or 75 μA. The resistance of the frames for all testers is different. Therefore, if non-linearity occurs at the end of the scale, a current-limiting resistor R24 should be selected, and within a small range, the generator frequency.

This adjustment is conveniently carried out at the 2nd, 3rd or 4th limit. Let's say, at the 3rd limit, we connect an exemplary capacitance of 2 microfarads. The tester needle (100 µA limit enabled) is set to "20". We check the accuracy in the middle of the scale by measuring the capacitance of 5 microfarads. If at all points the measured values \u10b\u90bcorrespond to the ratings, and at the end of the scale, for example, the exemplary capacitance of 24 microfarads gives "3", then R3 must be slightly reduced. In this case, the readings for all points will shift upwards. To move all the points back, you should slightly lower the frequency of the generator at the 17rd limit, i.e. increase the capacity of CXNUMX. After adjusting the linearity on one of the limits, it remains on the rest, but frequency correction in one direction or another may be required. By lowering the frequency, we get a decrease in readings, and vice versa. At the beginning of the scale, the linearity of the measurements depends on how accurately the balancing is carried out using RXNUMX.

To check the operation of the measuring amplifier, it is necessary to unsolder R4 from pin 4 of DD1.2 and solder it to pin 6 of DD1.4. We measure the constant voltage at pin 6 DD1 and the collector VT6 relative to the "common" wire - it should be the same (differ no more than 100 ... 200 mV). The adjustment is made by selecting R14 (when it decreases, the voltage on the VT6 collector rises).

Measurements should be carried out 5-10 minutes after soldering the elements, so that the thermal regime of the circuit can be restored. After adjusting the voltage, the connection R4 with pin 4 DD1 is restored. At limit 3, alternating voltages are measured at both terminals of R17. If they differ somewhere by 200 mV, then this is enough.

The positive half-wave of the triangular voltage is used to detect the signal, so it is important that the instrumentation amplifier does not saturate when the positive half-wave is amplified. If there is no oscilloscope, then this can be checked like this. Turning on the lower limit and comparing the fluctuations of the arrow of the tester, measure the output voltage of the generator at terminal 6 DD1 and at the collector VT6. DC voltage should be measured, because the period of oscillation of the arrow is about 1 s. The measuring amplifier will not saturate if the oscillation amplitude at the VT6 collector is 100 ... 200 mV less than at pin 6 of DD1. This is easily compensated by balancing the R17. The amplitude of the voltage at the output of the amplifier is regulated by resistors R14, R15 (with decreasing values, the gain decreases).

All these adjustments are described in detail to obtain improved measurement accuracy. In most cases, this will not be required (the error is within 10%).

At the limit of 6, small fluctuations of the instrument pointer are possible, which in most cases does not affect the measurement accuracy.

Details. DD1 - K561LN2, 564LN2, K176LN2. It is better to use transistors KT3102 ... KT3107, but, in principle, any silicon ones will do.

Diodes - any silicon. All resistors are MLT-0,125 or 0,25 W, except for R7. It is desirable to discharge the capacitor before measurement. If accidentally caught not discharged, R7 should have a power reserve. When the measured capacitance has a small charge, the device does not throw the arrow, because. R18 limits the charge rate of SU (C9), introducing VT7 into saturation. During this time, R7 discharges Cx, and the readings are set smoothly. To increase the speed of movement, the arrows R18 can be reduced.

Power switch SA2 and limit switch SA1 - any type. Resistors R17, R22 - preferably group A, of any type.

The device is assembled on a board made of thin non-foil fiberglass. The holes for the conclusions of the parts are pierced with an awl. The elements are connected by their terminals - to reduce the mounting capacity. C1 ... C6 are soldered on the switch. The design fit in the housing from the pocket radio "Electron". On the front panel are SA1, SA2, VD3, R17, R22, sockets "Cx" and "uA". With a supply voltage of 4,5 V, the current consumption of the set-top box is approximately 15 mA.

Author: V. Bognar, Kharkov; Publication: radioradar.net

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