Digital capacitance meter.

This device is an analog device, converted into a more compact version of the battery-powered. He has 8 years used to repair TVs and showed itself at its best. The instrument used in CMOS chips that have many «gathering dust» in the old stocks. This, and the use of the LCD display IZHTS5-4 / 8 allowed to bring the device current consumption to 10 mA and nourish it from the battery «Krona». Unit dimensions (with a few tricks) allow to place it in the body of the meter «D-830» or the like.

On DA1 and DA2 chips assembled converter «capacity-time» (PSUs). It implemented the conversion of the measured capacitance of the capacitor C1 during pulse oscillation. The period of the pulse is given by: T = 2C1-R3lnppJ (1)

This formula shows that the stability of the measurement depends on the stability of the measurement resistors R1 … R3 and can be very high. Flicker consistently generates pulses when the capacitance value C1 of nF units to tens of thousands of microfarads. Voltage naS1 has a triangular shape and amplitude determined by the formula: R1 = R2’i0 IT.

The resistors can be chosen so that Uc will be no more than 0.1 V, which allows you to measure capacitance without vypaivaya them from the scheme, as the period of the signal will not affect the p-n junction diodes, transistors and ICs, C1 connected in parallel. Shelter DA1.1 creates «artificial ground» (mid-point) for the analog part, by the divider R1-R2. On OU DA1.2 DA2, or make your own converter. Use as DA2 chip KR544UD2 possible to reduce the error of the instrument in the measurement of small containers. To protect DA2 when connecting charged capacitor elements introduced VD3, VD4, R4, and diodes chosen with significant single valid pulse current and resistor — capacity of not less than 0.5 watts. On the water you DA2 6 pulses with a period proportional to the measured capacitance of the capacitor Cx, enter the control unit.

The control unit is built on chips DD1 … DD4. Impulses from the PSU via an inverter DD3.1 come to the count input (C) D-trigger DD2.2. With the input of the other flip-flop circuit receives second pulses. The logic of the connection and the triggers to each other such that the inverse output DD2.2 present low level with a duration equal to the period of PSUs (counting time) and high duration of about 1 second (time display).

With direct output (O 1) of the elements of CIO, R15 short pulse resets the counter to zero at the beginning of each measurement period. ‘

On-chip crystal oscillator DD1 assembled exemplary frequency which is supplied to the output DD3.4 6 from the output buffer (output 12). Element 2 or NOR DD3.4 transmits pulses of exemplary frequency 32768 Hz input to the counter only during the counting time. Second pulse from terminal 5 at the counter input trigger DD2.1, but with one shot pulses output frequency of 63 Hz for the indicator.

LCD display does not allow supplying a constant voltage, so this device is supplied to the indicator variable voltage frequency 63Tts, and the inclusion of segments is carried out by phase, ie, if a segment of a signal is of the same phase as that of the overall output of the indicator, the segment is extinguished, if in antiphase — segment included.

To control commas are elements of the chip DD4. On one of the inputs of elements DD4.2, DD4.3, DD4.4 signal is 63 Hz (in opposition to a common indicator). Each element when applied to the other input of the logical «0» repeats the output pulses (indicated by a comma), and when applying a «1» — inverts (comma repaid). DD4.2 controls point 3rd level (big-endian), which normally vklyuchena.Na element DD4.1 implemented RS-FF, the output of which is set to «1» by applying a pin 5 short positive pulse through the elements C8, R10 , VD5 the start of each measurement interval. When the meter is full, the negative differential output its MSB via the inverter DD3.2 and differentiating chain C9-R12 goes to the pin 6 DD4.1 and transfers its output to «0». If on-site DD4 chip is used more high-speed series for the right job DD4.1, you may have to reduce the resistance R12 for shortening the pulse on pin 6.

In the case of the establishment to pin 5 DD4.1 «0» through the element DD4.4 included a comma LSB, indicating overflow.

Elements DD4.4, VD6, R14 form a low battery indicator. When the voltage drops below 7 volts at terminal 12 DD4.4 is set low and «lit» commas 1st and 2nd bits, thereby indicating a low battery. DD3.3 element acts as a buffer inverter.

On-chip DD5 … DD8 made pulse counter with output to LCD display. When applied to the output 6 of the pulse counter 63 Hz frequency of the same phase as in the indicator present at the outputs pulses with a phase that depends on the switching segment so that the display is visible to the appropriate digit.

Almost all parts of the device are placed on a one-sided printed circuit board of the foil 1 mm thick fiberglass. The indicator is set on top of chips K176IE4 on stocks, which are made from the power outlet for 40-pin chip with a pitch of 2.5 mm. The socket is cut lengthwise into 2 parts (obtained two narrow single-row pad) and each shortened to 17 contacts. Conclusions indicator molded into the shape of the letter «T» with the distance between bends 35 mm. When installing the board should first solder jumpers and discrete elements and then the chip and pads for the indicator. Jumpers are made of tinned wire diameter of 0.3 … 0.5 mm.

All resistors except R4, — MLT-0.125. Capacitors used compact. Zener diode can be taken on imported voltage stabilizing 3.3V Diodes VD1, VD2, VD5 — any of a series of KD521, KD522, VD3.VD4 — any of a series of HER10x … HER20x. From domestic KD212 fit, but there may be problems with their installation because of the large size and thickness of the findings. The crystal can be used on a desktop or a faulty wristwatch. Chip DA1, in the case of its absence, you can replace almost any dual op amp imported, for example, LM358 (but need to change the drawing board or chip set-point wiring). DA2 can be replaced by KR544UD1, KR140UD6 with a slight increase in error. DD1 is quite possible to be replaced by K176IE12 (with a change in the drawing board). Chip K176TM2 replaced without changing the picture on K176TM1 or corresponding IC 561 series. The indicator can be replaced by IZHTS21-4 / 7.

With proper installation of the unit practically does not need adjusting. You only need to choose the resistors R3, R5, R7 with an accuracy better than 1% (R7 can be composed of resistors 1 ohm and 3.3 ohms in parallel).

The device does not provide switching of measurement limits, however, if necessary, measure the capacitance to 10,000 microfarads point wiring can be installed one meter and switch. To do this, remove the jumper that connects the output element DD3.4 4 and 4 th output chip DD5, and between these points switch S2 connected counter DD9. The second group of contacts S2 feeding «1» to output 9 DD4.2 disables indication separated 3rd bit (PCB provided for this contact, designated «x»). It should be noted that the measurement of vessels over 10OO uF readings becomes very convenient because of the visibility of the «run» readings between accounts, although it is possible to read the testimony.

In principle, the device for measuring the capacitance, rather than an equivalent series resistance of the capacitors (ESR).

However, with increasing ESR readings are sharply reduced (about 2 times when ESR = 10 … 150M). This feature allows the device to successfully use it to repair the radio: just married capacitors having a capacity on the testimony of the device by more than 2 times lower than the nominal value, regardless of the cause of low readings.

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