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Matchbox Capacitor Tester

Checking of an electrolytic capacitor without an LCR meter is not free from hassles. Here is a simple solution for checking the serviceability of a capacitor.

Circuit and working

Fig. 1 shows the circuit diagram of a simple capacitor tester which can be housed in a small matchbox. It is basically an oscillator circuit comprising two transistors, BC558 (T1) and BC548 (T2), and a few other components. The frequency of oscillation, which is in audio range, depends on value of the capacitor under test. The tester can check the serviceability of capacitors in the range of 1μF to 2200μF.

Fig. 1: Circuit diagram of the capacitor tester

Construction and testing

An actual-size, single-side PCB for the capacitor tester is shown in Fig. 2 and its component layout in Fig. 3.

Fig. 2: An actual-size PCB layout for the tester circuit

Fig. 3: Component layout of the PCB

Connector CON1 in the circuit is for inserting the capacitor under test. Four-pin berg strip female connector is used here to accommodate different sizes of capacitors. LSI is a small speaker used as headphone (shown in Fig. 4). Assembled PCB of the circuit can be enclosed in a matchbox as shown in Fig. 5.

Fig. 4: Earpiece of a headphone

Fig. 5: Capacitor tester in matchbox

After wiring, do verify that voltages are as per Test Points table.

For testing the capacitor, insert it in CON1. If the capacitor is good, a fire-siren like sound is produced. A continuous beep is produced if the capacitor is short. 

Author: Kaushik Hazarika