In-Circuit Transistor Tester

2016-01-23 22:19  
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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

PARTS LIST
R1470kΩ
R2470Ω
R3470Ω
C10.47μF (Electrolytic)
D11N4148
D21N4148
D31N4148
D41N4148
D5RED LED
D6GREEN LED
IC1NE556

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.TransistorLED D5LED D6
1Good NPNOffFlashing
2Good PNPFlashingOff
3Open Circuit NPN of PNPFlashingFlashing
4Short Circuit NPN or PNPOffOff

NE556 Dual timer IC

NE556 Dual timer IC



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