Capacity meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Oxide capacitors have the unpleasant property of losing capacitance - "drying out", which is one of the main causes of failures in radio equipment that is in long-term operation. I propose a diagram of a device for measuring the capacitance of capacitors. The device allows you to measure capacitance in five subranges:

• 1. 0,1...3,1 ±0,1 uF;
• 2. 1,0...31 ±1 uF;
• 3. 3.10...310 ±10µF;
• 4. 4. 100...3100 ±100µF;
• 5. 5.1000...31000 ±1000uF.

The schematic diagram is shown in Fig.1. A reference frequency generator is assembled on the VT1 transistor and the DD1 chip. The generator runs constantly, and at a high level at pin 10 DD1.4, the reference frequency pulses are fed to a five-bit binary counter DD5, DD6, DD4.2. Information about the number of pulses received at the input of the counter (pin 11) DD5.1 ​​is displayed by the LEDs VD2 ... VD6 in binary code.

(click to enlarge)

On the elements DD2 and DD3.1, a second generator is assembled, the pulse sequence of which is formed with the participation of the capacitor Cx, the capacitance of which must be measured. A high level at pin 5 DD2.3 allows it to work.

An indication pulse generator is assembled on VT2, DD3.2, DD3.3, DD3.4. During one period of natural oscillations of this generator, information is not supplied to the counter, the counter indicates its state, the LED VD1 "Measurement" is extinguished.

RS-trigger DD4.1 is used as a mode switch for measurement/indication. The trigger is switched at the input R by pulses from the indication generator, at the input S - by pulses from the generator Cx. Timing diagrams of the circuit operation are shown in Fig.2.

Depending on the expected capacitance Cx, a certain generation frequency is selected by switch SA1, adjusted by resistors R2 ... R6. If the capacitance Сц is exactly equal to the upper limit of measurements on any of the subranges, then the number of pulses received by the counter should be equal to 31 (the total capacitance of the counter). With a smaller value of Cx, the number of pulses received by the counter will be correspondingly smaller.

Let the measurement be carried out on the 5th subrange. The LEDs flashed code 1, 4,16. In this case, the capacity is defined as follows:

Сх=(1+4+16)x1000=21000 (мкФ).

The measurement error in this subrange is ±1000 μF. If this accuracy is insufficient, it is necessary to carry out one more measurement on the 4th subrange. In this case, the counter will work with overflow:

21000:3100=6 and 24 in the remainder (31 is the measurement step on the 100th subrange). The rest will be highlighted with numbers 4, 8.

Сх=(31x6+8+16)x100=21000±100(мкФ).

If there are no installation errors, the device setup is reduced to its calibration. You need to start from the 5th subrange.

First you need to connect a capacitor with a capacity of several tens of thousands of microfarads to the Cx sockets. It is better to make it up from several capacitors connected in parallel. Instead of a tuning resistor R2, temporarily solder a variable with a nominal value of 100 ... 200 kOhm and achieve meter readings corresponding to the capacitance of the connected capacitor. Measure the resistance of the resistor and replace it with a constant and trimmer connected in series (the value of the trimmer should be approximately 1/3 of the required value). Specify the value of the tuned resistor by connecting С„ of other ratings.

These operations should be done on the remaining subranges. The display time is set by selecting R13.

For accurate operation of the device, a good power source is required, in which the capacitance of the filter capacitor in the rectifier after the diode bridge must be at least 2000 uF. Good results are obtained if 142 series microcircuits are used in the stabilizer.

The printed circuit board drawing is shown from the side of the printed conductors (Fig. 3). Jumpers are marked with a dashed line. The parts are located on the opposite side of the board, the "x" marks the pins of the parts soldered to a common wire, the wiring of which is not shown. The device uses microcircuits of the 155 series, the use of less energy-intensive series is undesirable. This will require revision of the schema. Transistors - KT315, LEDs - AL102, AL307.

When developing the device, the main criterion was simplicity. Binary counters can be replaced by decimal counters on the K155IE2 IC, supplementing them with appropriate decoders and indicators, but the design will become noticeably more complicated. C1, C2 - K52, K53; C3, C4, C5 - MBM, K73, KM [3]. Other parts - any type with a tolerance of ± 20%. SA1 - biscuit or P2K. C1 ... C4, R2 ... R6 are installed on the free places of the board. From the wiring is not shown, as they may have different sizes than the author. The LEDs are soldered so that when the board is installed, they protrude on the front panel. As sockets for C, single sockets from ShR connectors are used.

Literature

1. Designs of Soviet and Czechoslovak radio amateurs. - M.: Radio and communication, 1987.
2. Ovechkin M.A. Amateur TV games. - M.: Radio and communication, 1985.
3. To help the radio amateur: Collection. Issue 109.-M.: Patriot, 1991.
4. Maltsev L.A. etc. Fundamentals of digital technology. - M.: Radio and communication, 1987.

Author: V.Kalendo, Minsk; Publication: N. Bolshakov, rf.atnn.ru

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