Multimeter with small capacitor test circuit

It is difficult to test the quality of small capacitors below 0.01uF with an ordinary multimeter. If a first-level crystal transistor amplification circuit is installed, the charging/discharging phenomenon of the small capacitor can be observed, thereby judging the quality of the capacitor.

1. Circuit working principle

The following figure is a circuit diagram of a multimeter equipped with a small capacitor tester. In the figure, Sl is a small double-pole double-throw toggle switch. When the switch is in position ①, the transistor VT does not work and does not affect the use of the various gears of the multimeter. When it is necessary to measure a small capacitor, the switch can be turned to position ②, and the multimeter is set to the R×lkΩ gear. When the red and black test pens touch the two ends of the small capacitor, the charging current of the small capacitor is amplified by the transistor VT, which can cause the pointer of the multimeter to have a large deflection. If the pointer of the multimeter does not deflect, it means that the internal wire of the capacitor is broken and damaged.

2. Component selection and production

It is best to use a silicon NPN transistor with a larger magnification, such as 9014 type, with B≥200 and the penetration current requirement as small as possible. Sl is a 2×2 small toggle switch.

The entire circuit can be installed inside the plastic shell of the multimeter, and the switch S1 is fixed in the appropriate position of the plastic shell. Normally, turn the switch to position ①, and when you need to test a small capacitor, turn the switch to position ②. Of course, the additional circuit can also be made in a small plastic box and connected to the multimeter when in use. When testing a small capacitor, the multimeter pointer should deflect and then return to its original position. Swap the red and black test leads and touch the two ends of the capacitor. The pointer of the meter should have a larger deflection angle than last time, and then return to its original position. If the pointer does not deflect, it means that the internal wire of the capacitor is broken and damaged; if the pointer cannot return to its original position after deflection, it means that the capacitor has leakage.

Based on the deflection angle of the pointer, the capacitance can be roughly estimated.

This tester can be used to determine the quality of small capacitors with a capacity of more than tens of pF.