Device for testing electrolytic capacitors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Device for testing electrolytic capacitors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed device for testing electrolytic capacitors on ESR contains a minimum of details and, despite the external similarity of the circuit to previously published ones, has, in my opinion, the best characteristics. Range of measured resistances (1 - 6) Ohm. The scale is practically linear and straight, i.e. zero is on the left. Powered by two nickel-cadmium batteries, current consumption - (0.3 - 0.7) mA. The circuit consists of a master oscillator with a frequency of about 70 kHz, made on MS 561LN2, a transformer and a measuring head with a rectifier.

The transformer is connected in parallel with the generator, shunted with a relatively low resistance of the latter. The inductance of the primary winding of the transformer is large enough.

(click to enlarge)

All these factors save the circuit from parasitic resonances during measurements. TMS 15 was used as a transformer (probably from some old TV). Its primary winding has an inductance of 45 mH, a resistance of 14 ohms. Of the other two windings, the smaller one is used, with an inductance of 0.11 mH. By the way, the use of a larger winding makes it easy to shift the range of measured resistances upwards. The rectifier diode operates at a voltage of about 2 volts, which makes the scale almost linear. The rectifying diode must be pulsed (high frequency) and high-voltage (so that it does not break when a charged capacitor is connected).

A capacitor should not be connected in parallel with the head, because. it will be charged by voltage peaks that occur at the edges of the generator voltage. The setting consists in setting the frequency (approx. 70 kHz) and setting the arrow to the end of the scale with the probes open. The frequency of the generator is highly dependent on the supply voltage, however, the batteries hold the voltage very stably until almost completely discharged. Probes - 20 cm.

Supplement, especially for the book "Repair Secrets"

This device was repeated in many copies and proved to be the best. When repeating the device, some changes were tested, which are described below.

1. Some difficulties in the manufacture of the device were associated with the type of transformer used. At this point, any transformer with a primary winding inductance of at least 10 mH and a transformation ratio of 20 - 40 works well. It is very convenient to use an adapter transformer from a computer power supply on a TL - 494 chip. Its primary winding has a lead from the midpoint (not used). The value of the inductance is 15 - 20 mH. One of the two secondary windings has a tap from a very small number of turns (this winding has three terminals). So, as a secondary winding in this device, use this small part.

2. To power the device, it is convenient to use (if assembled in a miniature case) a 3-volt lithium battery of the CP 2032 type (they are on motherboards, for example). To increase the resolution of the device when measuring small (less than 0.15 Ohm) ESR values, you can replace the BA 159 diode with a Schottky diode (1N5819) and increase the supply voltage to 4.5 - 5 volts. The current consumption will then increase to 2-3 mA.

3. It is convenient to assemble the device in a case from a cheap Chinese tester. The tester head has a full deflection current of 50 µA, therefore, in series with it, you will need to connect a resistor of several kilohms (see below). It can also be assembled as a prefix to the tester. Use a limit of 0.5 volts or 50 µA.

4. If there is a need for longer (40 - 50 cm) probes, then they should be made as twisted probes of four wires. Two by two wires (diameter in insulation about one mm) are twisted together, and then two pigtails are twisted together and connected in parallel. With a length of 40 cm, the introduced error is about 0.1 Ohm.

5. The device is practically insensitive to charged capacitors, but the presence of two back-to-back diodes in parallel with the probes will not hurt.

Setting

Set the frequency to about 70 kilohertz. Close the probes together. Set the resistor connected in series with the microammeter to the minimum resistance position. By rotating the resistor slider, through which the generator voltage is supplied to the measuring part, to achieve a deviation of the arrow to the right from zero by 3-5 divisions (at 50 divisions of the scale). Open the probes and with a resistor connected in series with the microammeter, put the arrow just to the right of the 50th division of the scale. After that, you can calibrate.

Author: Ilya Lipavsky, Kiryat Ata, Israel; Publication: cxem.net

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