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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Capacitance meter for electrolytic capacitors with leakage test. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology One of the most common reasons for the failure of electronic equipment or the deterioration of its parameters is a change in the properties of electrolytic capacitors. Sometimes, when repairing equipment (especially manufactured in the former USSR), made with the use of certain types of electrolytic capacitors (for example, K50-...), in order to restore the device's operability, they resort to complete or partial replacement of old electrolytic capacitors. All this has to be done due to the fact that the properties of the materials included in the electrolytic (precisely electrolytic, because electrolyte is used in the composition) capacitor, under electrical, atmospheric, thermal influences change over time. And thus, the most important characteristics of capacitors, such as capacitance and leakage current, also change (the capacitor "dries out" and its capacitance increases, often even by more than 50% of the original, and the leakage current increases, i.e. internal resistance shunting the capacitor decreases), which naturally leads to a change in characteristics, and in the worst case, to a complete failure of the equipment.
We bring to your attention a diagram and an example of the design of a capacitance meter for electrolytic capacitors with a leakage test. I’ll make a reservation right away - the original idea of \u1b\uXNUMXbthe circuit is not mine, but was developed [XNUMX], I corrected one error, added built-in calibration and a test for capacitor leakage, developed a design option and manufactured it with tuning and testing. The excellent results of the device made me share the information with you. The meter has the following qualitative and quantitative characteristics: 1) capacitance measurement on 8 subranges:
2) assessment of the capacitor leakage current by the LED indicator;
Theory The essence of the device is to measure the voltage at the output of the differentiating circuit, Fig.1.
Resistor voltage: Ur = i*R,
Because the circuit is differentiating, then its current: i \uXNUMXd C * (dUc / dt),
The scheme is presented on Fig. 2. In the initial position, the tested capacitor Cx (or calibration C1 with the SA2 toggle switch on) is discharged through R1. The measuring capacitor, on which (not on the subject directly) the voltage proportional to the capacitance of the test Cx is measured, is discharged through contacts SA1.2. When the SA1 button is pressed, the subject Cx (C1) is charged through the corresponding subrange (switch SA3) resistors R2 ... R11. In this case, the charging current Cx (C1) passes through the VD1 LED, whose brightness makes it possible to judge the leakage current (resistance shunting the capacitor) at the end of the capacitor charge. Simultaneously with Cx (C1), the measuring (known to be good and with low leakage current) capacitor C1 is also charged through a source of stabilized current VT2, VT14, R15, R2. VD2, VD3 are used to prevent the discharge of the measuring capacitor through the supply voltage source and the current stabilizer, respectively. After charging Cx (C1) to the level determined by R12, R13 (in this case, to a level of about half the voltage of the power source), the comparator DA1 turns off the current source, synchronous with Cx (C1), the charge C2 stops and the voltage from it is proportional to the capacitance of the test Cx (C1) is indicated by the PA1 microammeter (two scales with multiples of 3 and 10, although it can be adjusted to any scale) through the DA2 voltage follower with a high input resistance, which also ensures long-term charge retention on C2. Setting When setting the position of the calibration variable resistor R17 is fixed in any position (for example, in the middle). By connecting reference capacitors with precisely known capacitance values in the appropriate range, resistors R2, R4, R6-R11 calibrate the meter - such a charge current is selected so that the reference capacitance values correspond to certain values on the selected scale. In my circuit, the exact values of the charging resistances at a supply voltage of 9 V were:
After calibration, one of the reference capacitors becomes the calibration C1. Now, when the supply voltage changes (changes in the ambient temperature, for example, when the finished debugged device is strongly cooled in the cold, the capacitance readings turned out to be underestimated by 5 percent) or just to control the measurement accuracy, it is enough to connect C1 with the SA2 toggle switch and, by pressing SA1, with the calibration resistor R17 adjustment of PA1 to the selected capacitance value C1. Design Before starting the manufacture of the device, it is necessary to select a microammeter with a suitable scale (s), dimensions and current of the maximum deflection of the needle, but the current can be any (of the order of tens, hundreds of microamperes) due to the possibility of setting and calibrating the device. I used an EA0630 microammeter with Inom = 150 μA, accuracy class 1.5 and two scales 0 ... 10 and 0 ... 30. The board has been designed to be attached directly to the microammeter using nuts on its leads. This solution ensures both mechanical and electrical integrity of the structure. The device is placed in a case of suitable dimensions, sufficient to accommodate also (except for the microammeter and the board): - SA1 - button KM2-1 of two small-sized switches;
SA1, SA2, SA3, R17, VD1 are fixed on the top cover (panel) of the device and are located above the board (the battery is fixed with a wire frame directly on the board), but connected to the board with wires, and all other radio elements of the circuit are located on the board (and under microammeter directly too) and are connected by printed wiring. I didn’t provide for a separate power switch (and it wouldn’t fit in the selected case), combining it with the wires for connecting the tested capacitor Cx in the SG5 type connector. The "mother" XS1 of the connector has a plastic case for installation on a printed circuit board (it is installed in the corner of the board), and the "father" XP1 is connected through a hole in the end of the device case. When connecting the "male" connector with its contacts 2-3, it turns on the power of the device. It’s a good idea to attach a connector (block) of some design to the Cx wires in parallel for connecting individual sealed capacitors. Working with the device When working with the device, you need to be careful with the polarity of connecting electrolytic (polar) capacitors. With any polarity of the connection, the indicator shows the same value of the capacitance of the capacitor, but with the wrong polarity of the connection, i.e. "+" of the capacitor to the "-" of the device, the VD1 LED indicates a high leakage current (after the capacitor is charged, the LED continues to burn brightly), while with the correct polarity of the connection, the LED flashes and gradually goes out, demonstrating a decrease in the charging current to a very small value, almost to full decay (should be observed for 5-7 seconds), provided that the capacitor under test has a low leakage current. Non-polar non-electrolytic capacitors have a very low leakage current, which can be seen from the very fast and complete extinction of the LED. And if the leakage current is large (the resistance shunting the capacitor is small), i.e. the capacitor is old and "flows", then the glow of the LED is already visible at Rleaks = 100 kOhm, and with lower shunt resistances, the LED burns even brighter. Thus, it is possible to determine the polarity of electrolytic capacitors by the glow of the LED: when connected, when the leakage current is less (the LED is less bright), the polarity of the capacitor corresponds to the polarity of the device.
Important notice! For greater accuracy of readings, any measurement should be repeated at least 2 times, because. for the first time, part of the charge current goes to create an oxide layer of the capacitor, i.e. capacitance readings are slightly underestimated. Literature
Publication: cxem.net
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