Capacitor capacitance meter with self-calibration. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Capacitor capacitance meter with self-calibration. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In amateur radio practice, the task of measuring the capacitances of capacitors often arises. Industrial multimeters with capacitance measurement are quite expensive and not available to everyone. The design of a simple meter, assembled on three microcircuits, is proposed, which allows measuring the capacitance of capacitors, as well as the frequency of a periodic signal, and can be used as generators with digital frequency control.

Schematic diagram of the meter is shown in the figure.

(click to enlarge)

The device contains a microcontroller DD1 type PIC16.84A, a digital liquid crystal indicator from AON DD2 type K0-4V2 (or similar) and a timer DA1 NE555 (KR1006VI1). The capacitance meter is based on the dependence of the output frequency of the RC generator on the DA1 timer on the capacitance value of the capacitor Cx:

fout≈1/kRCx,

where K is some constant. Since this dependence is inversely proportional, the microcontroller calculates the reciprocal of y=1/fout.

To improve the accuracy of measurements and ease of adjustment, self-calibration is used according to pre-selected known values of the capacitances of reference capacitors (C3, C5, C6). The meter has three capacitance measurement ranges: 1st - 10 .... 1000 pF, 2nd - 470 ... 4700 pF, 3rd - 4700 pF ... 0,47 μF. In the first range for self-calibration, a reference capacitor C5 with a capacity of 470 pF is used, in the second C6 with a capacity of 2000 pF, and in the third - C3 with a capacity of 15 nF. Before installing these capacitors in the device, they must be selected according to the value of the nominal capacitance using some kind of capacitance meter.

It is desirable to choose capacitors with minimum values of temperature coefficients of capacitance. To increase the linearity of the meter in the timer, a current source is used on the transistor VT1, the current value of which in different ranges is set using resistors R9R11. Switching is carried out by the microcontroller DD1 by switching ports B0-B7 from input to output. The choice of measurement ranges and the mode of operation of the meter is carried out by switches S1 and S2, according to the table:

Capacitor capacitance meter with self-calibration

Self-calibration is carried out periodically, approximately once a minute. At this moment, the value of the capacitance of the reference capacitor is displayed on the LCD indicator. In all three ranges, the capacitance value is displayed in picofarads, and the letters "R." are displayed in the left corner of the indicator. In the frequency counter and generator mode, these letters are not displayed. During self-calibration, instead of Cx, one of the reference capacitors is automatically connected to the circuit and its capacitance is measured. In this case, the value of the correction code N0 is calculated, which is stored in the RAM of the microcontroller Nout=N0Nin.

In parallel with the measured capacitor Cx, a capacitor C4 of small capacity is connected. This is done to reach a more linear section of measurements, and the capacitance value C4 is automatically subtracted in DD1 from the value of the measured capacitance Cx. When measuring small capacitances (up to 100 pF) in the first subrange, it is possible to display non-zero readings on the indicator when the measured capacitor Cx is turned off. This uncompensated capacitance value (in units of picofarads) must be subtracted from the measured CX value.

In the oscillator mode, a variable resistor R1 is connected to the current source VT13, by adjusting which the frequency of free oscillations fgen is set. If the switch S3 is turned off, then the microprocessor measures the generation frequency. At the output fgen, the signal is pulsed. If you need a signal with the same pulse and pause duration, then the output fgen must be applied to a frequency divider (for example, on a K561IE10 microcircuit). A sawtooth signal can be taken from output "a". If the output "a" is applied to the input of the integrator on a logic element, then you can get a signal close in shape to a sinusoidal, the frequency range of the generator is from tens of hertz to 20 ... 30 kHz. To expand the range towards low frequencies, you can increase the value of the resistor R13.

Button S4 - with normally closed contacts. It is used to reset the microcontroller by breaking the power circuits when switching from one measurement range to another. The button must be held down for a few seconds. Quartz Q1 - any at 1 MHz. The device can use the MOS timer type 7555.

You can program the microcontroller yourself according to the method described in [1]. When measuring the frequency of external oscillations, switch S3 must be turned off.

References:

Denisov A. Frequency meter on the PIC16.84 processor//Radiohobby. - 2000. - No. 1. - S.4243.

Author: A.M. Savolyuk

See other articles Section Measuring technology.

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