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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Capacitance meter and EPS of oxide capacitors - attachment to the multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The author continues the topic of measuring the parameters of oxide capacitors using an attachment to the popular 83x series multimeters. As in previous developments, the set-top box is powered by the internal ADC stabilizer of the multimeter. Measurement of ESR (ESR) and capacitance of oxide capacitors can be carried out without desoldering them from the board. Articles [1,2] describe an attachment that measures the ESR of oxide capacitors. It would be much more convenient if she also measured their capacitance. A diagram of such a prefix is shown in Fig. 1.
Main Specifications
The prefix consists of two meters: EPS and capacitance. The type of measurement is selected by switch SA2. In the "ESR" position, the ESR of the capacitor connected to the "Cx" sockets (XS1, XS2) is measured, and in the "C" position, the capacitance. The circuit design of the EPS meter, as already mentioned above, is taken from [1, 2], there is also a description of the operation and adjustment. Switch SA2 (section SA2.2) was added to disconnect the XS2 socket from the common wire when measuring capacitance and the connection of the drain and source terminals of the transistor VT3 was changed to eliminate the shunting effect of its internal diode on the accuracy of its measurement. Reducing the capacitance of the capacitor C6 to 0,22 microns reduced the settling time to 4 s. The influence of the voltage across the capacitor C9 on the accuracy of the EPS measurement is excluded by reducing the resistance of the resistor R3. The capacitance meter was assembled according to a well-known scheme published back in 1983 by the British magazine "Wireless World", and in Russian translation - in 1984 by the magazine "Radio" [3]. The low output voltage (3 V) and the low load capacity of the ADC stabilizer of the multimeter required the use of low-voltage op amps DA1-DA3 Rail-to-Rail and a current consumption of not more than 45 μA in the capacitance meter [4]. The supply voltage -3 V, necessary for the operation of the meter, was obtained from a voltage converter with a high efficiency on the DA4 microcircuit, included according to a typical circuit. The function generator, assembled on the op amp DA1.1, DA1.2, DA2.1, generates bipolar rectangular pulse signals at the output of the comparator on the op amp DA1.1 and triangular at the output of the integrator on the op amp DA2.1, shown respectively in Fig. 2, a and b. The node on DA1.2 is an inverter that provides positive feedback. The capacitance measurement limit, depending on the frequency of the generator (50, 5 or 0,5 Hz), is selected by switch SA1. The amplitude of the triangular signals at the output of the integrator is given by the ratio of the resistances of the resistors R1 and R4 of the comparator. It is equal to 2 V.
These signals, the amplitude of which is reduced by a resistive voltage divider R10R11 to 50 mV, are fed to a buffer amplifier with a unity voltage transfer coefficient, assembled on an op-amp DA2.2. The signal from its output is fed to the measured capacitor Cх, one output of which is connected to the socket XS1. With such an amplitude of this signal, in most cases it is possible to carry out measurements without soldering the capacitor from the board. Socket XS2, to which the other output of the measured capacitor is connected, is connected through a resistor R17 to the inverting input of the op-amp DA3.2. When a capacitor is connected, this op amp and resistor R18 form a differentiator, at the output of which bipolar trapezoidal pulses appear (Fig. 2, c). The maximum input current of the differentiator, equal to the output current of the buffer amplifier, is limited by the same resistor R18 (R17 A synchronous detector is assembled on a field-effect transistor VT4 with an insulated gate. The use here of a field effect transistor with a pn junction, as in [3], is impossible due to the low supply voltage. The comparator on the op-amp DA3.1 and the field-effect transistor VT1 control the state of the synchronous detector. Consider its operation from the moment the capacitor C is connectedх. With the appearance of a rectangular pulse of negative polarity at the output of the comparator on the op-amp DA1.1 (Fig. 2, a), the transistor VT1 opens and the +3 V supply voltage is supplied to the non-inverting input of the comparator assembled on the op-amp DA3.1. At its output, a voltage of about +3 V appears and is maintained (Fig. 2, d), so the VT4 transistor is closed. This state of the comparator and transistor VT4 is also maintained with a positive polarity of the triangular-shaped pulse coming from the output of the function generator to the non-inverting input DA3.1 through the resistor R12. When the polarity of the triangular pulse changes, when the voltage begins to change linearly from 0 to -2 V (Fig. 2, b), the transistor VT1 is already closed (the voltage at its gate is + 3 V) and the output of the comparator from the input negative pulse is set and held for the time tH3M, the voltage is about -3 V (Fig. 2d). Transistor VT4 of the synchronous detector opens. By this moment, the trapezoidal pulse of positive polarity at the output of the differentiator already has the most flat top, and the value of its amplitude, as is known, is proportional to the measured capacitance Cх. With the advent of the next rectangular pulse of negative polarity at the output of the op-amp DA1.1, the process is repeated. The detected parts of the trapezoidal pulses from the detector output (Fig. 2c, e) through the resistor R19 are fed to the capacitor C9, which is rapidly charged to their amplitude value (Fig. 2f). The resistor limits the charging current. From capacitor C9 constant voltage proportional to capacitance Cх, through the divider formed by the resistance of the resistor R16 and the input resistance of the multimeter (1 MΩ), enters the "VΩmA" input for measurement. The prefix is assembled on a board made of fiberglass laminated on both sides. The PCB drawing is shown in fig. 3, and the location of the elements on it - in Fig. 4. Photographs of the assembled console are shown in fig. 5. Single pin XP1 "NPNc" - suitable from the connector. Pins XP2 "VΩmA" and XP3 "COM" - from failed test probes for the multimeter. Input sockets XS1, XS2 - screw terminal block 350-02-021-12 of DINKLE series 350. Switches SA1, SA2 - sliding series MSS, MS, IS, for example, MSS-23D19 (MS-23D18) and MSS-22D18 (MS-22D16), respectively. Capacitors C2, C3 - imported film output for a voltage of 63 V. All other capacitors are for surface mounting. Capacitors C1, C4-C7 - ceramic size 1206, C8 - 0808, C9-C11 - tantalum B. All resistors - size 1206. BSS84 transistors are interchangeable with IRLML6302, and IRLML2402 with FDV303N. For other replacement, it should be taken into account that the threshold voltage, open channel resistance and input capacitance (Ciss) transistors must be the same as those being replaced. The IRLML6346 transistor is described in the article [1]. Op-amp AD8442AR will be replaced, for example, with LMV358IDR. In the case of such a replacement, the capacitance of the capacitors C2-C4 must be increased several times (for example, 1, 0,1 and 0,01 μF, respectively), and the resistance of the resistor R5 should be reduced by the same amount. It is also possible to use domestic OU KF1446UD4A, but the current consumed by the prefix will increase by 1 mA.
The conclusions of the protective diodes VD3, VD4, the DA4 chip and the SA2 switch in places where there are pads for them on both sides of the printed circuit board are soldered on both sides. Pins XP1 - XP3 are soldered in the same way, and XP2, XP3 are fixed by soldering in the first place, and then a hole is drilled "in place" and the pin XP1 is soldered. A piece of tinned wire is inserted into the hole near the lower resistor R11 on the output board and soldered on both sides. Before mounting, pin 7 of the DA4 chip should be bent or shortened. When working with a prefix, the switch of the type of work of the multimeter is set to the position of measuring direct voltage at the limit of 200 mV. Before calibration, the set-top box is first connected to an independent 3 V power supply and the current consumption is measured, which should not exceed 3 mA, and then connected to a multimeter. Next, set the switch SA2 to position "C" (lower according to the diagram in Fig. 1) and connect an oxide capacitor with a known measured capacitance to the sockets XS1, XS2. Switch SA1 is set to the appropriate limit and resistor R5 achieves the desired readings on the indicator. If the switch is in the middle position, the readings should be multiplied by 10, in the upper one according to the scheme - by 100. To reduce the measurement error, the capacitance of capacitors C2-C4 must be selected at each limit. The board provides contact pads for installing additional ceramic capacitors of size 0805. Please note that to facilitate the adjustment, the resistor R5 on the board is made up of two resistors connected in series (in Fig. 4 they are designated R5' and R5''). Calibration of the EPS meter is described in the article [1]. If resistors R14, R15 fail to set zero readings with closed sockets "Cx" [5], and this is possible when installing a transistor VT3 with a low throughput capacitance and the final resistance of the closed contacts of the SA2.2 switch section, you should connect the gate-drain terminals of the transistor in parallel a ceramic capacitor with a capacity of several tens of picofarads and repeat the adjustment. The printed circuit board for the size 0805 capacitor has contact pads. On fig. 6 shows a prefix with a multimeter when measuring a capacitor with a nominal capacity of 3300 microfarads.
With frequent use of the attachment, the contacts of the SA2 switch may be subject to wear. The instability of the resistance of the closed contacts of section SA2.2 will lead to an increase in the measurement error of the ESR. In this case, it is advisable to use a switching field-effect transistor, similar to IRLML2.2 (VT6346), with an open channel resistance of not more than 2 Ohm, instead of SA0,05 mechanical contacts. The source terminal of the transistor is connected to a common wire, the drain is connected to the source terminal of the transistor VT2, the gate is connected to terminal 14 DD1. The PCB file in Sprint LayOut 5.0 format can be downloaded from ftp://ftp.radio.ru/pub/2015/01/ESR-C-meter.zip. Literature
Author: S. Glibin
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