Capacity meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Electrolytic capacitors, due to a decrease in capacitance or a significant leakage current, are often the cause of radio equipment malfunction. An electronic tester, the circuit of which is shown in the figure, allows you to determine the feasibility of further use of the capacitor, which was supposedly the cause of the malfunction. Together with a multi-limit avometer (at the limit of 5 V) or a separate measuring head (100 μA), a tester, it is possible to measure capacitances from 10 μF to 10 μF, as well as to qualitatively determine the degree of leakage of capacitors.
The tester is based on the principle of controlling the residual charge on the poles of the capacitor, which was charged with a current of a certain value for a certain time. For example, a capacitance of 1 F. charged with a current of 1 A for 1 s will have a potential difference across the plates equal to 1 V. The practically constant charge current of the tested capacitor C is provided by a current generator assembled on transistor V5. On the first capacitance range, you can measure up to 100 μF (capacitor charge current 10 μA), on the second - up to 1000 μF (100 μA) and on the third - up to 10 μF (000mA). The charge time Cx is chosen equal to 1 s and is counted either automatically using a time relay or using a stopwatch.
Before starting the measurement in the switch position S2 "Discharge", the potentiometer R8 sets the balance of the bridge formed by the base-emitter junctions of transistors V6 and V7, resistors R8, R9, R10 and diodes V3. V4 used as a low voltage reference. Then switch S1 selects the expected capacitance measurement range. If the capacitor is not marked or has lost part of the capacitance, measurements are started in the first range.
The switch of the type of work S2 before the measurement is set to the position ". Discharge", in this case, the connected capacitance Cx is immediately discharged through the resistor R9. In the "Charge" position, the switch S2 is held for 5 seconds, and then transferred to the "Count" position and the measurement result is immediately read. The capacitance value (in μF) is inversely proportional to the voltage divisions (V) printed on the instrument scale and is determined by the formula C \u50d A / U, where A is a constant equal to 500, 5000, XNUMX, respectively, for the first, second and third measurement ranges. If the capacitor is faulty and has a large leakage current, the meter needle will quickly return to zero on the scale. The magnitude of the leakage current is not determined.
Setting up the tester is easy and comes down mainly to setting the previously indicated charge currents with the potentiometers R2, R4, R6 using the microammeter included in the Cx terminals.
Note. In the capacitance meter, diodes KD202B and transistor KT340V can be used. An additional resistor should be connected in series with the microammeter to obtain a range of 5 V on the full scale, or use an avometer connected to the appropriate measurement limit.
Literature
- "Radio, fernsehen, etektronik" (GDR). 1978, no. 2
Publication: N. Bolshakov, rf.atnn.ru
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