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.
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
|Open Circuit NPN of PNP
|Short Circuit NPN or PNP
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