EPS indicator of oxide capacitors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Recently, in the technical literature, including in the journal "Radio", descriptions of devices that are very useful in amateur and professional practice - indicators or meters of ESR have been published (for example, in the article by A. Shchusya "ESR meter of oxide capacitors." - Radio, 2006 , No. 10, pp. 30, 31). The author of the proposed article set himself the task of developing a simpler and more economical device. Indeed, if in the above ESR meter, an alternating voltage of the order of tens of millivolts is applied to the capacitor under test, and the current through the microammeter does not exceed 0,5 mA, then the current consumed by the meter reaches 20 mA.

Further analysis showed that in some EPS meters this parameter is affected by the quality factor of the equivalent circuit formed by the inductance of the measuring transformer, the capacitances of the tested and decoupling capacitors, as well as the measured EPS value. The rectangular signal applied to this circuit serves to excite damped oscillations in it at the resonant frequency of the circuit. The alternating voltage is rectified and fed to a measuring device - a microammeter (or milliammeter).

Measurement at the resonant frequency is convenient because in this case the total reactance of all elements of the circuit becomes equal to zero and the active resistances of the elements, including the ESR of the tested capacitor, come to the fore.

As a result, several samples of indicators have been developed, in which the measurement of ESR at the resonant frequency is applied. It was not shock excitation of the measuring circuit by external pulses, but its inclusion in a generator with self-excitation, which greatly simplified the design, was used. It turned out that it is more expedient to use continuous generation, which increases the sensitivity and efficiency of the device. The principle of operation of such an indicator is based on the fact that the amplitude of the generated voltage depends on the energy losses in the resonant circuit, i.e., on the active resistance of its constituent elements, which include the ESR of capacitors.

Main Specifications

EPS indication limits, Ohm .. .0,1 ...23
Generation frequency, kHz ....... 12 ... 16
Supply voltage, V ..... 1,25 ... 1,6
Consumption current, mA
in standby mode .....0,15....0,4
in measuring mode
and calibration ...............1...1.5

Fig. 1

The indicator scheme is shown in fig. 1. On the transistor VT1, according to the capacitive three-point circuit, an oscillator is assembled, on the transistor VT2 - a detector, the load of which is the milliammeter RA1. Capacitor C4 smoothes the detected voltage ripple, resistor R5 is current limiting. The tested capacitor Cx is included as an integral element in an oscillatory circuit consisting of an inductor L1 and capacitors C1 and C2. The oscillator operates at a relatively low frequency of 12 ... 16 kHz, which is also an advantage of this indicator. Since the generation frequency is determined by the resonant frequency of the circuit, the effect of the capacitance of the controlled capacitor on the generated voltage is insignificant, while the effect of the ESR, on the contrary, is maximum and therefore can be easily determined. This function is performed by a detector on a transistor VT2; to simplify the design, it has a galvanic connection with an oscillator. Diodes VD1-VD4 are used to discharge (possibly charged) tested capacitors.

The device uses fixed resistors MLT, S2-23, variable - SPO, SP4-1, oxide capacitors - imported, capacitors C1, C2 - K73-17, MBM, C3 - K10-17. Transistors can be used in the KT315, KT342 series with any letter indices, diodes - in any KD510, KD521 series. The inductor is wound on a K10x6x3 magnetic circuit made of 2000NM ferrite and contains 50 turns of PEV-2 0,5 wire. Power switch - MT-1 or any small-sized one, you can also use a variable resistor with a switch. The full deviation current of the milliammeter can be from 0,3 to 15 mA, the current consumed by the device in the capacitor test mode will depend on this.

In one of the author's variants, an M381 ammeter (30 A) was used, from which the shunt and the internal fasteners related to it were removed. All parts, except for the power switch and galvanic battery, are mounted on a 65x77 mm printed circuit board, which is fixed inside the device case (Fig. 2). A 1,5 V battery of AAA size is placed in a plastic cassette and connected to the board and switch with mounting wires. The ammeter clamps are used to connect XP1 input probes. The axis of the variable resistor is brought out through a hole in the housing.

Before starting the measurement, it is necessary to close the probes of the "Cx" indicator and set the arrow to the final division of the scale with the resistor R2 - the indicator is ready for operation. The rejection of capacitors is very simple - the closer the milliammeter needle is to the maximum division of the scale, the lower the EPS. If, when connecting a controlled capacitor, the milliammeter needle is in the last third of the scale, such a capacitor is suitable for use. If the arrow is in the first two thirds of the scale, it is unusable. In accordance with this, the corresponding sectors can be highlighted in green and red colors. By connecting resistors with a resistance of 1 ... 30 Ohm instead of capacitors, it is possible to calibrate the indicator scale.

Fig. 3

To increase the thermal stability of the indicator readings, the resistor R4 can be replaced by a circuit of a tuning resistor and a diode (Fig. 3). With the probes closed, the resistor R4 slider is initially set to the middle position according to the scheme. If the readings of the milliammeter increase with increasing temperature, the trimming resistor engine is turned 10 ... 20 degrees up (according to the diagram) and the position of the arrow is restored with resistor R2. This procedure must be carried out several times until the desired result is obtained.

Author: Yu. Kurakin, Dimitrovgrad, Ulyanovsk Region; Publication: radioradar.net

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