Digital capacitance meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In amateur radio practice, it often becomes necessary to measure the capacitance of electrolytic capacitors, since their capacitance can change quite significantly over time. The device described in [1], according to the author, has a number of disadvantages - high power consumption, a narrow range of measured capacitances (10 ... 10000 μF), low accuracy of measuring small capacitances.

The proposed meter is free from these disadvantages. At the same time, leaving the number of microcircuits used unchanged, it was possible to significantly improve the accuracy and introduce a number of service functions that make it easier to work with the device. This device provides measurement of capacitance of capacitors from 0,01 to 10000 microfarads on four subranges with upper measurement limits of 10, 100, 1000 and 10000 microfarads. The subbands are switched automatically. The measurement result is presented in digital form on a four-digit indicator.

(click to enlarge)

The principle of operation of the device is based on counting the number of pulses in a time interval proportional to the capacitance of the capacitor. The "capacity-time" converter is made on a single vibrator DD5.3, DD5.4. The duration of the pulse generated by such a single vibrator is determined by the empirical formula from [2]:

Resistors R7 and R8 are chosen so that the duration of the pulses in milliseconds is numerically equal to the capacitance in microfarads. The single vibrator is started after pressing the SB1 button. To suppress the bounce of the button contacts, the driver DD5.1, DD5.2 is designed. It generates a pulse of negative polarity, the duration of which corresponds to the time of closing the contacts, and the front and fall of the pulse are somewhat delayed relative to the moments of closing and opening [1.4]. Inverter DD9 generates a reset signal that coincides in time with the shaper pulse, which ensures that counters DD12...DD7 and trigger DD2 are reset. The decay of a negative polarity pulse with the help of a differentiating chain C5-R1.3 is converted into a short positive pulse that triggers a single vibrator. The pulse from the output of the single vibrator opens the electronic key DDXNUMX, allowing the passage of counting pulses from the reference frequency generator.

The main part of this frequency generator is a multivibrator on DD1.1, DD1.2 with quartz frequency stabilization [2]. Chips DD2 ... DD4 make up a line of frequency dividers by 10. Thus, the inputs of the multiplexer DD6.1 are fed with frequencies of 1 MHz, 100, 10 and 1 kHz. The multiplexer DD6.1 together with the trigger DD7 and the counter DD8 form a node for automatic selection of the measurement limit. When you press the SB1 button, the automatic limit selection circuit is reset by applying a logic "8" to the input R DD1 through the resistor R4.

The counter DD8 is set to zero, and the multiplexer DD6.1 supplies the input of the electronic key DD1.3 with a frequency of 1 MHz, which corresponds to the lowest measurement limit. In case of overflow of counters DD9...DD12, at the transfer output of DD12, a positive polarity pulse decays, which increases the state of the counter DD8 by one and writes a logical "7" from input D to the trigger DD0. This logical "0" triggers the shaper. By the negative pulse of the shaper, the counters DD9...DD12 are reset and the trigger DD7 is transferred to the logical state "1". As a result, the shaper pulse duration will be equal to the delay time. On the decline of this pulse, the single vibrator is restarted. Changing the state of DD8 will cause the frequency at the output of DD6.1 to be equal to 100 kHz, and this corresponds to an increase in the measurement limit by 10 times.

Microcircuits DD9...DD12 are ten-day counters with output to a seven-segment indicator. Vacuum luminescent indicators were used as indicators, which have low current consumption and better brightness characteristics compared to LED matrices. The DD6.2 multiplexer controls the decimal points of the indicators.

It is recommended to set up the device in the following order

1. Input R DD8 temporarily disconnect from the button SB1.

2. Connect a rectangular pulse generator with a frequency of 2 ... 3 Hz to the connection point R50 and R200. There are no special requirements for it, and it can be assembled according to any of the schemes given in [2, H].

3. As a model, connect a capacitor with a capacity of 0,5 ... .4 μF. It should be remembered that the accuracy of the meter depends only on the accuracy of the calibration.

4. Resistor R8 should achieve the closest possible correspondence between the readings of the device and the actual capacitance of the exemplary capacitor. After tuning, it is desirable to lock the R8 engine with paint.

Details

In the meter, microcircuits of the K176, K561, K1561, and also 564 series can be used. Resistors are of the MLT-0,125 type. Resistor R8 is better to use a multi-turn type SP5-1. As a calibration capacitor, the author used K71-5V 1 μF ± 1%. It should be noted that not all copies of the K176LA7 IC work stably in a quartz oscillator, therefore it is not recommended to use K1LA176 as a DD7.

As indicators, you can use, in addition to those indicated in the diagram, IVZ, IV8. If, however, liquid-crystal indicators are used, which will require a slight modification of the circuit [3, 4], the device can be powered by a single 9 V Krona battery.

Literature

1. Kurochkina L. A. Digital meter for the capacitance of oxide capacitors. - Radio, 1988, N8, pp. 50-52.
2. Shelestov IP Radio amateurs: useful schemes. Book. 2. - M.: "Solon", 1998.
3. Biryukov S. A. Digital devices based on CMOS integrated circuits. 2nd ed., revised. and additional - M.: Radio and communication, 1996.
4. Bystrov Yu. A. et al. Optoelectronic devices in amateur radio practice - M .: Radio and communication, 1995

Author: A. Uvarov; Publication: cxem.net

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