“The majority of electronics enthusiasts have this experience. Although the medium and high-end digital multimeters have capacitance test gears, the measurement range is generally only 1pF~20μF, which often cannot meet the needs of users and brings inconvenience to capacitance measurement. The three-digit Display capacitance test table introduced in this circuit adopts four integrated circuits, the circuit is simple, easy to make, the digital display is intuitive, and the precision is high. The measurement range can reach 1nF~104μF. Especially suitable for hobbyists and electrical maintenance personnel to make and use.
The majority of electronics enthusiasts have this experience. Although the medium and high-end digital multimeters have capacitance test gears, the measurement range is generally only 1pF~20μF, which often cannot meet the needs of users and brings inconvenience to capacitance measurement. The three-digit display capacitance test table introduced in this circuit adopts four integrated circuits, the circuit is simple, easy to make, the digital display is intuitive, and the precision is high. The measurement range can reach 1nF~104μF. Especially suitable for hobbyists and electrical maintenance personnel to make and use.
1. The working principle of the circuit
The circuit principle is shown in Figure 2.
Figure 2 Circuit diagram of three-digit digital display capacitor test table
The capacitance meter circuit is composed of a reference pulse generator, a capacitance time converter to be measured, a gate controller, a decoder and a display.
The capacity-time converter of the capacitor to be measured converts the capacity of the measured capacitor into a monostable time td proportional to its capacity value. The reference pulse generator produces standard period count pulses. The opening time of the gate controller is the monostable time td. During the td time, the cycle count pulse is sent to the back counter through the gate for counting, and the decoder drives the display to display the value after decoding. The period T of the count pulse multiplied by the count value N displayed on the display is the monostable time td. Since td is proportional to the capacity of the capacitor being measured, the capacity of the capacitor being measured is also known.
In Figure 2, the integrated circuit IC1B resistors R7~R9 and capacitor C3 constitute a reference pulse generator (essentially an astable multivibrator), and the output pulse signal period T is related to R7~R9 and C3, and is fixed at C3 In the case of 11μs, 1.1ms and 11ms, three pulse signals with periods of 11μs, 1.1ms and 11ms can be obtained through the different selection of R7, R8 and R9 by the range switch K1b.
IC1A, IC2, R1~R6, button AN and C1 constitute the capacitor capacity time converter to be tested (essentially a monostable circuit). Press AN once, IC2B pin 10 will generate a negative narrow pulse to trigger IC1A, and its 5 pin will output a single high level signal. R3~R6 and the capacitor to be measured CX are monostable components, and the monostable time td=1.1 (R3~R6) CX.
IC4, IC2C, C5, C6, R10 constitute gate controller and counter, IC4 is CD4553, its pin 12 is counting pulse input terminal, pin 10 is counting enable terminal, CD4553 executes counting when the potential is low, and pin 13 is counting clearing terminal, the rising edge is valid. When AN is pressed, the 13-pin of IC4 gets a rising pulse, the counter is cleared, and the 4-pin of IC2C outputs a monostable low-level signal and adds it to the 10-pin of IC4, so IC4 counts the reference input from its 12-pin Pulses are counted. When the monostable time is over, IC4 pin 10 becomes high level, IC4 stops counting, and finally IC4 transfers the units, tens, and hundreds of the counting result from its pins 9, 7, 6 through time-sharing transmission. Pin and pin 5 output the corresponding BCD code cyclically.
IC3 constitutes a decoder driver, which decodes the BCD code sent by IC4 into a decimal digital pen segment code, and directly drives the seven-segment digital tube after being limited by R11~R17. The 15-pin, 1-pin and 2-pin of the integrated circuit CD4553 are the digital selection output terminals. The selection pulses of R18~R20 are sent to the bases of the transistors T1~T3 to make them turn on in turn. The cooperation of these two parts of the circuit completes the three-digit decimal system. Digital Display.
The function of C7 is to generate a rising pulse on R10 when the power is turned on, which automatically clears the counter.
2. Component selection
In the circuit, IC1 selects NE556; IC2 selects CD4001; IC3 selects CD4543; IC4 selects CD4553. Seven-segment digital tube can choose three-character common cathode digital tube. T1~T3 select 8550 (or other PNP transistors). C1 should not be greater than 0.01μF, and C3 uses small metallized capacitors. R3~R9 select 1/8W metal film resistors for use. There are no special requirements for other components, and you can select them according to the circuit label.
3. Production and debugging methods
After the whole circuit is installed, it can be installed in a plastic box, and the digital tube and the range switch can be installed on the panel. When making and debugging, the key is to call out three standard pulse signals of 11μs, 1.1ms and 11ms. When debugging, you need to use an oscilloscope to adjust the resistance of the three resistors R7, R8 and R9 respectively. The three pulse signals can be easily obtained. The resistance values of R7, R8 and R9 in the circuit are experimental data for reference only. The rest of the circuit does not need to be debugged. As long as a good device is selected, the installation is correct, and the corresponding magnification is marked at the range switch, an economical, practical, accurate and reliable digital capacitance meter can be obtained.
4. How to use
When testing capacitors, the value obtained by multiplying the count result by the magnification of the range used is the capacity of the capacitor under test. For example, when the reference pulse period is 1.1ms and the timing resistance is 10K, the range magnification is 0.1μF. If a capacitor with a nominal capacity of 4.7μF is measured, and the result is 49 after pressing AN, the capacity of the capacitor is 49×0.1μF=4.9μF.
It should be noted that when using the 1pF~999pF range, due to the influence of the distributed capacitance, the measurement result minus the distributed capacitance value is the accurate value of the measured capacitance. You can measure the distributed capacitance value of the capacitance meter in this way, set the range in the 1pF~999pF range, press the AN button without connecting the measured capacitance, and the measured count result is the distributed capacitance value of this block. Through experiments, the value is generally about 10pF.
The attached table lists the composition relationship of each range.