In-Circuit Transistor Tester

in-circuit transistor tester

In Circuit Transistor Tester Schematic

Here is a circuit that can indicate the condition of a transistor by using two LEDs. A good NPN transistor conducts during the positive half cycle (pulses are generated by 556 dual timer) and D5 is off while D6 flashes. With a good PNP transistor D6 is off and D5 Flashes.

In Circuit Transistor Tester Schematic

R1 470kΩ
R2 470Ω
R3 470Ω
C1 0.47µF (Electrolytic)
D1 1N4148
D2 1N4148
D3 1N4148
D4 1N4148
IC1 NE556

Both LEDs flash if the transistor is open and will be off if it is shorted.

The tester checks the transistor even if in-circuit resistance across the transistor is as low as 50 ohms; below this the glow becomes dim. Diodes in the ensure that only low saturation voltage of a good transistor turns the LED off.

Condition of LEDs
S.No. Transistor LED D5 LED D6
1 Good NPN Off Flashing
2 Good PNP Flashing Off
3 Open Circuit NPN of PNP Flashing Flashing
4 Short Circuit NPN or PNP Off Off

NE556 Dual timer IC

NE556 Dual timer IC

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