A simple and easy to use small capacitor tester

1. Circuit working principle

The circuit of a simple small capacitor tester is shown in the figure below. It uses the light-emitting diode to determine whether the small capacitor is good or bad.

The transistors VT1 and VT2 in the figure below form a Darlington compound amplifier tube, which has a very high magnification factor. When the red and black test leads of the tester touch the two ends of the small capacitor, a charging current flows through the capacitor to be tested. Although this charging current is very small, it is amplified by the high-magnification Darlington tubes VT1 and VT2 to drive the light-emitting tube LED to light up. If the LED does not light up, it means that the capacitor has no charging ability, indicating that the internal wire is broken and damaged.

2. Component selection and production

VT1 can use 9014 type silicon NPN transistor, B≥200, VT2 can use 9013 type silicon NPN transistor, B≥100, both tubes require the penetration current to be as small as possible. LED uses red light emitting diode, it is better if high brightness light emitting diode can be used.

Rl is a RTX-1/8W carbon film resistor, the power supply G uses two No. 5 batteries, and the red and black test leads can be the test leads used in ordinary multimeters.

The right picture is the printed circuit board of the tester. The size of the printed circuit board is 25mm×20mm. This tester does not consume power when not in use, so the tester does not need to be equipped with a power switch.

When using, just touch the red and black test leads to the two ends of the capacitor to be tested at the same time. The LED should flash once and then go out, indicating that the capacitor is intact.

If the LED does not flash, it means that the internal wire of the capacitor is broken and damaged; if the brightness of the LED flashes and then only dims instead of going out, it means that there is leakage inside the capacitor; if the LED is always on, it means that the capacitor is short-circuited and damaged.

Since this tester has a high test sensitivity, it is suitable for testing various small capacitors with a capacity below 0.1μF and above tens of pF. When testing, do not touch the metal parts of the red and black test leads at the same time, otherwise the addition of human body resistance will make the LED always emit weak light, making you mistakenly believe that the capacitor has leakage.

In addition to judging the quality of small capacitors, this tester can also be used as a continuity meter based on the light emission of LEDs to check the continuity of circuits and to judge the quality of PN junctions in diodes and transistors.