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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Capacitor capacitance meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology This device is based on the device previously described in our journal [1]. Unlike most of these devices, it is interesting in that it is possible to check the health and capacitance of capacitors without dismantling them from the board. In operation, the proposed meter is very convenient and has sufficient accuracy. Anyone who repairs household or industrial radio equipment knows that it is convenient to check the health of capacitors without dismantling them. However, many capacitor capacitance meters do not provide such an opportunity. True, one such construction was described in [2]. It has a small measuring range, a non-linear scale with a countdown, which reduces accuracy. When designing a new meter, the task of creating a device with a wide range, a linear scale and a direct reading was solved so that it could be used as a laboratory one. In addition, the device must be diagnostic, i.e., capable of checking capacitors shunted by p-n junctions of semiconductor devices and resistor resistances. The principle of operation of the device is as follows. A triangular-shaped voltage is applied to the input of the differentiator, in which the tested capacitor is used as a differentiating capacitor. At the same time, a meander is obtained at its output with an amplitude proportional to the capacitance of this capacitor. Next, the detector selects the amplitude value of the meander and outputs a constant voltage to the measuring head. The amplitude of the measuring voltage on the probes of the device is approximately 50 mV, which is not enough to open the p-n junctions of semiconductor devices, so they do not have their shunting effect. The device has two switches. "Scale" limit switch with five positions: 10 µF, 1 µF, 0,1 µF, 0,01 µF, 1000 pF. The "Multiplier" switch (X1000, x10, x10, x1) changes the measurement frequency. Thus, the device has eight capacitance measurement subranges from 10 μF to 000 pF, which is practically sufficient in most cases. The triangular oscillation generator is assembled on the op-amp of the DA1.1, DA1.2, DA1.4 microcircuit (Fig. 1). One of them, DA1.1, operates in the comparator mode and generates a rectangular signal, which is fed to the input of the DA1.2 integrator. The integrator converts square waves to triangular. The frequency of the generator is determined by the elements R4, C1 - C4. In the feedback circuit of the generator, there is an inverter on the op-amp DA1.4, which provides a self-oscillating mode. Switch SA1 can set one of the measurement frequencies (multiplier): 1 Hz (X1000), 10Hz (x10), 10Hz (x10), 1 kHz (X1).
Op-amp DA2.1 is a voltage follower, at its output a triangular-shaped signal with an amplitude of about 50 mV, which is used to create a measuring current through the tested capacitor Cx. Since the capacitance of the capacitor is measured in the board, there may be residual voltage on it, therefore, in order to prevent damage to the meter, two anti-parallel bridge diodes VD1 are connected in parallel to its probes. Op-amp DA2.2 works as a differentiator and acts as a current-voltage converter. Its output voltage: Uout=(Rl2...R16) IBX=(Rl2...Rl6)Cx-dU/dt. For example, when measuring a capacitance of 100 uF at a frequency of 100 Hz, it turns out: Iin = Cx dU / dt = 100-100MB / 5MC = 2MA, Uout = R16 lBX = 1 kOhm mA = 2 V. Elements R11, C5 - C9 are necessary for the stable operation of the differentiator. Capacitors eliminate oscillatory processes at the meander fronts, which make it impossible to accurately measure its amplitude. As a result, a square wave with smooth fronts and an amplitude proportional to the measured capacitance is obtained at the DA2.2 output. Resistor R11 also limits the input current when the probes are closed or when the capacitor is broken. For the input circuit of the meter, the following inequality must be satisfied: (3...5)CxR1<1/(2f). If this inequality is not met, then in half a period the current IBX does not reach a steady value, and the meander does not reach the corresponding amplitude, and an error occurs in the measurement. For example, in the meter described in [1], when measuring a capacitance of 1000 μF at a frequency of 1 Hz, the time constant is defined as Cx R25 \u10d 910OO uF - 0,91 Ohm \uXNUMXd XNUMX s. Half of the oscillation period T / 2 is only 0,5 s, therefore, on this scale, the measurements will turn out to be noticeably nonlinear. The synchronous detector consists of a key on a field-effect transistor VT1, a key control unit on an op-amp DA1.3 and a storage capacitor C10. Op-amp DA1.2 issues a control signal to the key VT1 during the positive half-wave of the meander, when its amplitude is set. Capacitor C10 stores the DC voltage emitted by the detector. From the capacitor C10, the voltage carrying information about the value of capacitance Cx is fed through the DA2.3 repeater to the RA1 microammeter. Capacitors C11, C12 - smoothing. From the engine of the variable calibration resistor R22, voltage is removed to a digital voltmeter with a measurement limit of 2 V. The power supply (Fig. 2) produces bipolar voltages of ±9 V. The reference voltages form thermally stable zener diodes VD5, VD6. Resistors R25, R26 set the required output voltage. Structurally, the power source is combined with the measuring part of the device on a common circuit board.
The device uses variable resistors of the SPZ-22 type (R21, R22, R25, R26). Fixed resistors R12 - R16 - type C2-36 or C2-14 with a tolerance of ± 1%. The resistance R16 is obtained by connecting several selected resistors in series. Other types of resistors R12 - R16 can also be used, but they must be selected using a digital ohmmeter (multimeter). The remaining fixed resistors are any with a dissipation power of 0,125 watts. Capacitor C10 - K53-1A, capacitors C11 - C16 - K50-16. Capacitors C1, C2 - K73-17 or other metal-film, C3, C4 - KM-5, KM-6 or other ceramic capacitors with TKE not worse than M750, they must also be selected with an error of no more than 1%. The rest of the capacitors - any. Switches SA1, SA2 - P2G-3 5P2N. It is permissible to use the transistor KP303 (VT1) with the letter indices A, B, C, F, I in the design. Transistors VT2, VT3 of voltage stabilizers can be replaced by other low-power silicon transistors of the corresponding structure. Instead of OU K1401UD4, you can use K1401UD2A, but then at the limit of "1000 pF" an error may occur due to the offset of the differentiator input created by the input current DA2.2 to R16. The power transformer T1 has an overall power of 1 W. It is acceptable to use a transformer with two secondary windings of 12 V each, but then two rectifier bridges are needed. An oscilloscope is required to set up and debug the device. It's a good idea to have a frequency meter to check the frequencies of the triangular oscillator. Exemplary capacitors will also be needed. The device begins to be adjusted by setting the voltages to +9 V and -9 V using resistors R25, R26. After that, the operation of the triangular oscillation generator is checked (oscillograms 1, 2, 3, 4 in Fig. 3). In the presence of a frequency meter, the frequency of the generator is measured at different positions of the SA1 switch. It is acceptable if the frequencies differ from the values of 1 Hz, 10 Hz, 100 Hz, 1 kHz, but they should differ exactly 10 times from each other, since the correct readings of the device on different scales depend on this. If the generator frequencies are not a multiple of ten, then the required accuracy (with an error of 1%) is achieved by selecting capacitors connected in parallel with capacitors C1 - C4. If the capacitances of capacitors C1 - C4 are selected with the required accuracy, you can do without measuring frequencies.
Next, check the operation of the OS DA1.3 (oscillograms 5, 6). After that, the measurement limit is set to "10 μF", the multiplier is set to the "x1" position and an exemplary capacitor with a capacity of 10 μF is connected. At the output of the differentiator, there should be rectangular, but with tightened, smoothed fronts, oscillations with an amplitude of about 2 V (oscillogram 7). The resistor R21 sets the readings of the device - the deviation of the arrow to the full scale. A digital voltmeter (at a limit of 2 V) is connected to sockets XS3, XS4 and a reading of 22 mV is set with resistor R1000. If capacitors C1 - C4 and resistors R12 - R16 are precisely matched, then the readings of the device will be multiples on other scales, which can be checked using reference capacitors. Capacitance measurement of a capacitor soldered into a board with other elements is usually quite accurate within 0,1 - 10 microfarads, except when the capacitor is shunted with a low-resistance resistive circuit. Since its equivalent resistance depends on the frequency Хс = 000/ωС, in order to reduce the shunting effect of other elements of the device, it is necessary to increase the measurement frequency with a decrease in the capacitance of the measured capacitors. If, when measuring capacitors with a capacity of 1 microfarads, 10 microfarads, 000 microfarads, 1000 microfarads, respectively, the frequencies of 100 Hz, 10 Hz, 1 Hz, 10 kHz are used, then the shunting effect of the resistors will affect the reading of the device with a 100 Ohm resistor connected in parallel (an error of about 1%) or less. When measuring capacitors with a capacity of 300 and 4 microfarad at a frequency of 0,1 kHz, an error of 1% will be due to the influence of a resistor connected in parallel, already with a resistance of 1 and 4 kOhm, respectively. At the limits of 0,01 μF and 1000 pF, it is advisable to check the capacitors with the shunt circuits turned off, since the measuring current is small (2 μA, 200 nA). It is worth recalling, however, that the reliability of small capacitors is noticeably higher due to the design and higher allowable voltage. Sometimes, for example, when measuring some capacitors with an oxide dielectric (K50-6, etc.) with a capacitance from 1 microfarad to 10 microfarads at a frequency of 1 kHz, an error appears, apparently associated with the intrinsic inductance of the capacitor and losses in its dielectric ; instrument readings are smaller. Therefore, it is advisable to make measurements at a lower frequency (for example, in our case at a frequency of 100 Hz), although in this case the shunting properties of the parallel resistors will already affect their higher resistance. Literature
Author: V. Vasiliev, Naberezhnye Chelny
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