Capacitance meter of ionistors and high-capacitance capacitors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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There are several ways to measure the capacitance of capacitors, but not all of them are suitable for measuring capacitances greater than a few hundred microfarads. Particularly big problems arise when measuring ionistors, the capacitance of which can reach 10 F or even more. Meanwhile, there is a relatively simple and, by the way, long-known method based on measuring the charging time of a capacitor from a voltage source through a resistor of known resistance. As you know, if you connect a capacitor with a capacitance C through a resistor with a resistance R to a voltage source U (Fig. 1), the capacitor will start charging and the voltage across it (U_С) will increase exponentially:

U_C = U(1 - e^-t/(RC)),

where e is the base of the natural logarithm (e ≈ 2,718); t - time; RC is the so-called time constant of an RC circuit, which is independent of voltage. At the time when t = t_RC\uXNUMXd RC, the voltage across the capacitor will be equal to U_C = U(1 - e^-1) ≈ U(1 - 0,367) ≈ 0,633U. Therefore, by measuring the time interval from the beginning of the charging of the capacitor to the moment when the voltage across it reaches the value of 0,633U, it is possible, using a simple calculation, to determine the capacitance of the measured capacitor С = t_RC/R. If the resistance of the resistor is "round", for example 10 kOhm, all calculations can be easily done in the mind. For example, for the specified resistor, the capacitor charging time to 0,633U was 46 s, then the capacitance of the measured capacitor C_х = 46 / 10⁴ = 46 mF = 4600 uF. Thus, in this case, the conversion factor is K = 100 µF/s. For a resistor R = 1 kOhm, the measurement time will decrease by a factor of 10, and the conversion factor K = 1000 μF / s.

Rice. 1. Dependence U_C from t

According to this principle, the proposed meter works. You can make it in the form of a prefix to a computer or other electronic device with a built-in stopwatch, for example, to an electronic (electronic-mechanical) clock or a cell phone. Particularly noteworthy is the relative ease of implementation of this method and the absence of the need for calibration using reference capacitors (a digital voltmeter is sufficient). In addition, the voltage can also be any (within reasonable limits), the main thing is that it does not change during the measurement. It may take several minutes to measure the capacitance of ionistors, in combination with a measurement error of several percent, this is quite acceptable for amateur radio practice.

It should be noted that the measurement error is affected by leakage currents and series resistance (ESR) of capacitors and capacitors. For example, the ESR of some types of ionistors can reach 30 ohms, and if you charge such an ionistor through a 100 ohm resistor, the measurement error can be tens of percent. Therefore, the resistance of the resistor through which the capacitor is charged must be at least 1 kOhm.

Readers are invited to the measuring attachment to the electronic-mechanical clock. The scheme of the device is shown in fig. 2. It is powered by a battery built into the watch (1,5 V), and the watch itself can also be used for its intended purpose. In the initial state, the supply voltage is supplied to the microcircuit, and the clock operates in the normal mode. When the set-top box is connected, the contacts of the XS1 socket open, the clock stops and the supply voltage is supplied to the set-top box. It contains a boost stabilized voltage converter on the DA1 chip, a comparator on the op-amp DA2, an electronic key on the VT1 transistor and a light indicator on the HL1 LED.

Rice. 2. Diagram of the device (click to enlarge)

After applying the supply voltage to the attachment, the transistor VT1 is closed and the voltage converter is de-energized. To measure the capacitance of a capacitor or an ionistor, it is first discharged and then connected with respect to the polarity to the terminals XS2, XS3 and briefly press the button SB1 "Start". The clock is supplied with a supply voltage, and they will start counting the time, at the same time the voltage converter starts to work, a voltage of 3,3 V appears at its output and the HL1 LED turns on. Since the measured capacitor is discharged, the voltage at the inverting input of the op-amp DA2 is less than at the non-inverting one and the output will be 2 ... 2,2 V. The transistor VT1 will open, and after releasing the SB1 button, the voltage will continue to flow to the voltage converter and to the clock, which continue counting down the charging time. The choice of the output voltage of the converter (3,3 V) is due to the fact that in this case the capacitor will be charged to the voltage U_C \u3,3d 0,633 2,088 \u2d XNUMX V, therefore, using the attachment, you can measure the capacitance of supercapacitors and capacitors with a rated voltage of XNUMX V or more.

As soon as the capacitor is charged to the specified voltage, a voltage close to zero will appear at the output of the op-amp DA2, the transistor VT1 will close, the clock and the voltage converter will be de-energized and the LED will turn off - the measurement process is completed. It remains to read the clock and determine the capacity, taking into account the conversion factor set by switch SA1. For the convenience of measurements, the clock is preliminarily set to the origin. To re-measure the same capacitor, you must first discharge it by pressing the SB2 "Discharge" button for several tens of seconds. To discharge an ionistor and an oxide capacitor with a capacity of more than several thousand microfarads, this must be done several times.

The adjustment begins with checking the performance of the voltage converter and setting the switching threshold of the op-amp. To do this, the terminals of the collector and emitter of the transistor VT1 are temporarily shorted with a wire jumper, the terminals XS2 and XS3 are connected to each other and a voltage of 1,5 V is supplied from an adjustable power supply. When changing the position of the switch SA1 and reducing the supply voltage to 1,2 V, the output voltage of the converter should not change by more than a few percent. In the switch position SA1 "100" a variable (preferably multi-turn) resistor with a resistance of 2 kOhm is connected to the terminals XS3, XS33. Converter output voltage U_п measured with a digital voltmeter with a resolution of at least three decimal places. A variable resistor is set on the terminals XS2, XS3 voltage U \u0,633d XNUMX U_п. Then, by controlling the voltage at the output of the op-amp, the engine of the construction resistor R5 is set to a position in which the slightest change in its position leads to the switching of the op-amp. So the switching error due to the bias voltage of the op-amp will be compensated. After removing the jumper between the collector and emitter of the transistor and the variable resistor, the prefix is ​​\uXNUMXb\uXNUMXbready for operation.

The console uses resistors and capacitors for surface mounting. Fixed resistors RN1-12 and capacitor C1 (K10-17v) - size 1206, trimmer resistor - PVZ3A (POZ3A), PVA3A (RVG3A), capacitor C2 - tantalum size A or B. To improve the measurement accuracy, resistors R3 and R4 should be selected with deviation from the nominal value is not more than 0,5%. You can use any low-power transistor with a base current transfer coefficient (h_21Э) not less than 100. LED - increased brightness of green or red glow with a housing diameter of 3 or 5 mm. The inductor is wound on an annular magnetic circuit with a diameter of 6 mm from the CFL transformer and contains 6 ... 7 turns of PEV-2 0,3 wire. Switch - small-sized sliding PD9-1 (SPDT), B3001, B3037, buttons - any small-sized with self-return, clips XS2, XS3 - "crocodile".

Rice. 3. PCB drawing

Rice. 4. Layout of elements

Rice. 5. Appearance of the device

Most of the parts are placed on a single-sided fiberglass printed circuit board, the drawing of which is shown in Fig. 3, and the layout of the elements - in fig. 4. The buttons are fixed on the top cover of the case, holes for the LED and the switch slide are made in it. Holes are made for wires in the front and rear walls of the case. Watches - any electronic-mechanical, in the case of which you can install a nest. Their refinement is minimal - you need to cut the printed conductor coming from the "+" battery to the clock chip and install the XS1 socket (jack for connecting stereo headphones). The appearance of the device is shown in fig. 5.

Author: I. Nechaev

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