C-tester. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In an amateur radio workshop, next to various measuring instruments, the "C-tester" (ST) can take a modest, but quite legitimate place for measuring the electrical capacitance of "microfarad" capacitors. It is not often necessary to measure the capacitance of such capacitors. Therefore, together with ST, it is supposed to use external devices: a stopwatch or a clock with a second hand and, in some cases, a multi-limit milliammeter (tester). This achieves the utmost simplicity, small dimensions and low cost of the ST. Assembled according to the scheme shown in Fig. 1, it does not require adjustment, calibration, selection of parts and will provide a relative measurement error of no more than ± 10% (excluding the error of external devices) in the range of 5 ... 10000 μF. Such a measurement error for these capacitors is acceptable in most practical cases. If necessary, it can be significantly reduced.

Fig.1. Schematic diagram of the C-tester

In the ST scheme, the principle of indirect determination of the electrical capacitance of a capacitor is implemented by the time of its discharge from the initial voltage to some final voltage, which is in a fixed relation to the initial one. With an initial voltage equal to E, the voltage across the capacitor U during its discharge obeys the equation: U \uXNUMXd E e ^-t/RC, (1) whence C = t/R * 1/(/nE - /nU), (2)

Let's accept: t = RC. (3)

Substituting the value of t from (3) into formula (1), we obtain: U = E / e, (4) that is, subject to the condition of formula (4), the capacity from (3) is determined as follows: C = t / R. (5)

Thus, according to formula (5), with an initial voltage equal to E and a final voltage calculated according to formula (4), the value of the measured capacitance is directly proportional to the time t. Let's take the resistance of the resistor R equal to 1 MΩ. Then the capacitance of the capacitor in accordance with formula (5) will be determined by C = t 10 ^-6 (F) = t (µF), (6) i.e. the capacitance of the capacitor C in microfarads is numerically equal to the time of its discharge t in seconds. The ST provides three ranges of capacitance measurement with ten-day multipliers x1, x10, x100 and discharge resistors with resistances of 1 MΩ, 100 kΩ, 10 kΩ, respectively. With this in mind, formula (6) will look like C = tn, (7) where: C - capacitance, μF; n is the range multiplier (1, 10, or 100).

ST is arranged and works as follows. The measured capacitor is connected to the "Cx" terminals (observe the polarity for polar capacitors). The capacitor with one of its outputs through a chain of normally closed contacts of the buttons SB1, SB2, SB3, marked "x1", "x10", and "x100", the resistor R4, which limits the charging current of the capacitor, and the power switch SA1 is connected to the power source G1. The other output of the capacitor is connected to the common wire through the terminals "lut" and "case", closed by a jumper (the jumper is not shown in Fig. 1). When the power is turned on with the SA1 toggle switch, the capacitor is charged to the supply voltage. This is the initial voltage.

The operational amplifier DA1 is connected according to the voltage comparator circuit. Its inverting input is connected to the measured capacitor, and the non-inverting input is connected to the voltage divider R5, R6, at the division point of which a voltage is set equal to U \u2,718d E / e, where E is the power supply voltage, V; e is the base of the natural logarithm (e=1). This is the end voltage. In the initial state, with a fully charged capacitor, the voltage at the output of the comparator is low, the transistor VT1 is closed, and the HL1 LED is off. By pressing and holding any of the buttons (SB2, SB3 or SB1), the measured capacitor is connected to the corresponding resistor R2, R3 or R5, and its discharge begins. When the voltage across the capacitor becomes equal to the voltage of the R6-R6 divider, the comparator switches, the voltage at its output is set to about 1 V, the transistor VT1 opens, and the HL1 LED lights up. The time t in seconds is measured from the moment the button is pressed to the moment the LED lights up. Now you can release the button. The capacitor through the chain of normally closed contacts of the buttons SB2, SB3, SB4 and the resistor RXNUMX will charge again, and the LED will go out.

When measuring capacitance, the choice of one or another button is arbitrary and is determined only by the convenience of timing. The measurement can be started from any button, but not earlier than 10 s from the moment the power is turned on or the previously pressed button is released. This time is necessary for reliable charging of the measured capacitor. After the measurement, before disconnecting the capacitor from the "Cx" terminals, turn off the power with the "ON" toggle switch. In this case, the capacitor will be discharged through the closed contacts of the SA1 toggle switch, the resistor R4 and the jumper on the "lyt" terminals. When measuring the capacitance of oxide (electrolytic) capacitors, sometimes it is necessary to take into account their leakage current Iut, which can introduce a significant error into the measurement result (the result will be lower than the true value). Correcting the situation will allow the introduction of the coefficient Kut, depending on the capacitor lyt and the selected range of n. As applied to ST, taking into account the leakage current of the capacitor, formula (7) looks like this: С = tn Kut, (8) where: С is the capacitance of the capacitor, μF; Kut - correction factor Kut = 1 + (Iut / nE), n - range multiplier (1, 10 or 100); Iut - leakage current, μA; E - power supply voltage, V.

The power supply voltage is approximately equal to 9 V. Then Kut = 1 + (Iut / n9).

The Kut coefficient is easy to calculate using this formula, but it is easier to use the graph of its dependence on the leakage current Iyt, shown in Fig. 2.

Ris.2.

The capacitor leakage current is measured with a milliammeter connected to the "Iyt" terminals instead of a jumper. Connecting the milliammeter should be done with the power off. When the power switch is turned on, the capacitor charging current at the first moment can reach 20 mA, and then drops to a certain value determined by the capacitor leakage. In steady state, the leakage current can range from fractions of a microampere to 20 mA (for a broken capacitor). This must be taken into account when setting the milliammeter measurement limit at the time of power-on. When measuring the leakage current of electrolytic capacitors, they should be kept energized for some time (train) until the current value is established. During this time, the capacitor is not only charged, but also "formed", changing its capacitance.

The types of parts used can be any. Resistors R1, R2, R3, R5, R6 must have a resistance tolerance of no more than ±5%. The K140UD8 chip can be replaced with a K140UD6 or K140UD12 chip (including the pinout). On the CT panel are installed: toggle switch SA1, buttons SB1, SB2, SB3, terminals "Cx", "Iut" and LED HL1. The CT is powered by a 9 V battery, consuming a current of 6 mA.

If you want to reduce the measurement error, you should install resistors R1, R2, R3 with resistances that are as close as possible to the values ​​\u5b\u6bspecified in the diagram. It is also necessary to select the resistances of the resistors R5 and R6 so that the condition R1,72 / RXNUMX = XNUMX is observed.

It can reduce measurement error by 3%. And you can do so. Connect an adjustable constant voltage source to the "Сх+" and "Housing" terminals, observing the polarity, set its output to a voltage equal to the measured battery voltage multiplied by a factor of 0,368. For example, at E = 9,21 V, the voltage at the "Cx" terminals must be set equal to U = 9,21 * 0,368 = 3,39 (V). The buttons do not need to be pressed, the terminals "Cx-" and "Iyt" must be free. ST turns on. In this case, if the LED is on, a variable resistor with a resistance of 6 kOhm is switched on in series with the resistor R1, and by adjusting it, the threshold is found at which the LED lights up and goes out. If the LED is off, then the above steps must be done by including a variable resistor in series with resistor R5. The resistance of the variable resistor is measured and a fixed resistor with the same resistance is added. With this method of selection, the technological bias of the input voltages of the operational amplifier DA1 will be compensated, which is also a source of error, although a small one.

The method of measuring time t directly determines the accuracy of capacitance measurement. To measure time, you can use a stopwatch, a second hand of a clock, a flashing dot on a digital clock display, or you can, if you do not need greater accuracy, simply count seconds.

A decrease in the measured capacitance of a capacitor in relation to its nominal value may be due to an increased leakage current. If the LED does not go out when the power switch is turned on, the measured capacitor is either shorted or has a very large leak. When, after pressing the "x1" button, the LED lights up without delay, the capacitor is either open or has lost its capacitance. In any case, it is possible to draw a conclusion about the suitability of the capacitor.

The capacitance measurement range given at the beginning of the article is conditional. In principle, it is not limited to these figures and can be expanded in both directions without any changes in the circuit. Only the range of time measurement by an external instrument will be extended. It is possible that the measurement error of small capacities will increase due to the difficulty of measuring small time intervals.

Literature

1. F.E. Evdokimov. Theoretical foundations of electrical engineering. 5th ed. - M.: Higher. school, 1981.

Author: V. Gusarov, Minsk; Publication: radioradar.net

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