Attachment to the multimeter for measuring the capacitance of capacitors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Attachment to the multimeter for measuring the capacitance of capacitors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The journal "Radio" published articles [1, 2] with descriptions of capacitor capacitance meters. According to the author, the most successful device is described in the article [1]. They can measure the capacitance of capacitors without soldering them out of the board, which significantly speeds up and simplifies the repair and adjustment of electronic devices. On its basis, the proposed device was developed. During development, the task was to assemble a prefix for a multimeter or voltmeter using inexpensive and widely available components, easy to adjust and set up, capable of autonomous operation on batteries five days a week for eight hours a day.

In contrast to the prototype [1], the set-top box contains a stabilized boost voltage converter, a battery discharge control unit and automatic shutdown. The set-top box uses micro-consuming operational amplifiers. To establish and calibrate the prototype [1], it is necessary to select the appropriate capacitors. Setting up and calibrating the set-top box is much easier and more convenient with tuned resistors.

Attachment to the multimeter for measuring the capacitance of capacitors
Fig. 1

Rice. 2 (click to enlarge)

The scheme of the proposed attachment is shown in the figure. It is powered by a GB1 battery of three Ni-Cd or Ni-MH batteries. The battery is charged from an external power supply with an output voltage of 8 ... 12 V. The field-effect transistor VT1 stabilizes the charging current, the value of which is set by selecting the resistor R2. Control over the discharge of the battery to a voltage of 2,5 ... 2,9 V is carried out by a trigger on transistors VT4 and VT5. It turns off the set-top box, preventing overdischarging of the batteries. The R6VD5C3 circuit is designed to open the VT4 transistor when the set-top box is turned on with the SA1 switch, which is shown in the "Charging" position.

The step-up voltage converter contains a blocking generator based on transistors VT2 and VT3, transformer T1, capacitor C1, resistors R1 and R3, as well as positive (VD3C4) and negative (VD4C5) polarity rectifiers. The frequency of the converter is about 100 kHz, it is operable at an input voltage of 1,8 ... 5 V, and its output voltage is stabilized at ± (7 ± 0,5) V.

Main Specifications

Capacitance measurement limits, uF minimum ....... 0,001
maximum ......10000
Measurement error as a percentage of the limit, no more for a capacitance no more than 10 μF ....... 5
for a capacitance greater than 10 uF ....... 10
Supply voltage
minimum ....... 2,5
maximum......5
Consumed current, mA, not more ....... 13
Overall dimensions, mm ....... 65x75x35
Weight with battery, g......200

The principle of operation of the proposed attachment is the same as that of the prototype. The triangular-shaped pulse generator is assembled on the op-amp DA 1.1, DA2.2, DA2.4. Op-amp DA1.1 works as a comparator, from its output a rectangular signal is fed to the input of the integrator at the op-amp DA2.2, which converts rectangular voltage pulses into triangular ones. The generator frequency is determined by RC circuits (R23C8 - 1 kHz, R24C9 - 100 Hz, R25C10 - 10 Hz, R26C11 - 1 Hz), which are switched by the DD1 multiplexer. The resistors of these circuits are tuned, they set the required generation frequency. In the feedback circuit of the generator, there is an inverter on the op-amp DA2.4, which provides a self-oscillating mode. A voltage follower is assembled on the op-amp DA2.3. From its output, a triangular-shaped voltage with an amplitude of 50 mV is applied to the tested capacitor C*. Diodes VD21 and VD22 - protective. A differentiator is assembled on the op-amp DA3. Resistor R42 limits the current if the capacitor being tested is broken.

Using the SA2 switch, the multiplexers DD6 and DD17 are controlled through the diodes VD1-VD2. In the positions from the 1st to the 5th switch SA2, the channels from X1 to X5 of the DD2 multiplexer are switched, providing measurement in the range from 1 nF to 10 μF, and the X1 channel is open at the DD1 multiplexer, thereby ensuring the operation of the generator at a frequency of 1 kHz. In positions 6 to 8 SA2, channels are switched from X2 to X4 of the DD1 multiplexer, this gives a measurement of capacitance values from 100 to 10000 microfarads at frequencies of 100, 10 and 1 Hz, and the DD2 multiplexer remains open channel X5.

From the output of the operational amplifier DA3, pulses whose amplitude is proportional to the measured capacitance Cx are fed to a synchronous detector assembled on a field-effect transistor VT6 with a control unit on the op-amp DA1.2. From the capacitor-C7 through the decoupling voltage follower to the op-amp DA2.1, a voltage, also proportional to C *, is supplied to a voltmeter or multimeter, which must be in the voltage measurement mode of at least 1 V. The capacitance of the capacitor 07 must be at least 100 μF, otherwise measurement limit of 10000 uF and a generator frequency of 1 Hz, the voltmeter readings will be unstable.

On the limits of 1 nF and 0,01 μF, it is advisable to disconnect the tested capacitor from the shunt circuits.

The conclusions about their influence on the accuracy of capacitance measurements, formulated in [1] for the prototype, are also valid for the attachment.

Taking into account that the operational amplifiers in the set-top box process a signal with a frequency of no more than 1 kHz, a 1401UDZ chip was used, containing four micro-consuming op-amps. It is permissible to replace it with 1463UD4 or four single 140UD12. You should pay attention to the fact that the amplitude of oscillations at the output of the generator is the same at all frequencies (1, 10, 100 and 1000 Hz). Otherwise, reduce the resistance of resistors R11 and R18, controlling the current through them so that it does not exceed 0,2 mA.

The attachment uses trimming resistors SPZ-19 with a tolerance of ±10%. Fixed resistors - C2-33, with a tolerance of ± 5%. Oxide capacitors - K53-18. Capacitors C9-C11 - K73-17 or other metal-film capacitors, capacitor C8 - KM5a or KM56, with TKE not worse than MPO or PZZ. It is possible to use surface-mounted elements - resistors R1-12, R1-16, capacitors K53-68, K10-50 or their imported analogues.

Transformer T1 is wound on a magnetic circuit of size Sh4x4 made of 2000NM ferrite with a PEV-2 wire with a diameter of 0,15 mm. Winding I contains 15 turns, windings II and III - 35 turns each.

Op-amp DA3 was selected from the 140UD14 series due to the low current consumption and high input resistance. At the measurement limit of 1 nF, the influence of its input resistance is corrected by increasing the resistance of the resistor R43 from 10 (as in the prototype) to 12 MΩ. Compensation for the influence of the parasitic capacitance of the attachment and probes (setting the zero output voltage of the attachment at this measurement limit) is carried out by resistor R35. A non-standard frequency correction of the op-amp DA3 with a capacitor C18 is applied, which eliminates parasitic oscillations, since the differentiator is prone to self-excitation.

Setting up the set-top box begins with setting the generation frequency at each limit with trimming resistors R23-R26. Then connect an exemplary capacitor with a capacity of 10 microfarads or a little less. The trimmer resistor R16 sets the output voltage in volts, equal to one tenth of the capacitance of the exemplary capacitor in microfarads. Next, the prefix is similarly calibrated with trimming resistors R37-R40 at smaller measurement limits for other exemplary capacitors. The exemplary voltage source - LED AL102VM (HL1) can be replaced by AL307VM or a chain of several series-connected silicon diodes from the KD522 series. If necessary, the reference voltage is adjusted by selecting the resistance of the resistor R8 within ± 30%. If this is not enough, change the number of diodes in the circuit. The cut-off voltage is set within 2,5 ... 2,9 V.

The charging current of the batteries is selected by the resistor R2. In the copy of the author, the current is set to 26 mA. If necessary, replace the field effect transistor KP302V (VT1) with a more powerful KP903V.

Literature

Vasiliev V. Capacitor capacitance meter. - Radio, 1998, No. 4, p. 36, 37; 2000, no. 7, p. 50.
Kuchin S. A device for measuring capacitance. - Radio, 1993, No. 6, p. 21-23.

Author: A. Suchinsky

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