How to make a capacitance tester?

This is a more complicated circuit, but when you make it, you have a tester that can be used to measure the largest capacitance in the radio. Capacitors are not like resistors where the resistance is marked on the case. The capacity of many capacitors is not marked on the caseElectric Contacters For Pressure Gauge. The capacity of some old capacitors was wiped off during use. This meter can measure the capacity of the capacitor, which is a good helper for electrical technicians, veteran radio enthusiasts and radio enthusiasts.
The common 555 timer is the core component of this circuit. Its function is to charge the measured capacitance (Cx) to a fixed voltage, and then discharge the measured capacitance through the loop. The ammeter measures the current flowing through the 47 Ω resistor.

Because the 555 timer repeats this process several times per second, the pointer of the ammeter remains stable.
The current will vary depending on the capacitance of the capacitor, that is, the pointer of the ammeter will be deflected according to the capacitance of the capacitor under test, that is, the ratio of the capacitance to the line current is linear, just like a multimeter used to measure voltage and current .
This meter has five gears, from 100pF to 1 μF, which can be switched by a two-pole five-gear switch. In addition, the switch × 10 is used to measure a larger capacity capacitance. The current is divided into two channels for measurement. 100pF, 1000pF, 0.01 μF, 0. The capacitance display of 1 μ F or more is more accurate.
The accuracy of the components is not perfect. Even the best device. Generally, 9 resistors are also considered to have an error of 2%. If there is no OA47 diode. It can be replaced by OA91 or OA95 germanium diodes. Put this circuit into a plastic box, which is about the same size as a multimeter, but a bit deeper is better. With financial permission, the measuring instrument of the tester should be as large as possible, because it determines the accuracy of the measurement display. The range of the ammeter you bought is O ~ 50 microamperes, but the scale should be deflection from 0 to 100 (that is, the original scale 10, 20, 30, 40, 50 is replaced by 20, 40, 60, 80, 100), you can Use white correction fluid or small pieces of paper to modify.
The components are mounted on a printed circuit board, and the 555 integrated block is connected with a socket for easy replacement when needed. The connection line should be as short as possible, otherwise the stray capacitance of the line will reduce the accuracy of the measurement.
A capacitor of known capacity can be used to calibrate the tester, and a capacitor of known capacity can be found. Test several times. If it is not displayed correctly, it is very simple. Just buy a few capacitors of the same capacity and choose the one that is closest to the nominal capacity for calibration. Then use a few capacitors closest to the nominal capacity to accurately correct each gear.
When calibrating, disconnect × 1O (that is, both switches are disconnected and not used), a nominal capacitor is connected to the loop, the switch is set to a proper range, and the 47k Ω trimming resistor is adjusted. Until the capacity of the capacitor is accurately read. Then close the switch. At this time, the reading will change. Adjust the 1Ok Ω trimming resistor to make the two readings equal. For example, if you measure a 0. 01 μ F nominal capacitance, in O. The reading on the 1 μF file is 10, and the reading at this time should be 20.
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