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.
| PARTS LIST | |
| R1 | 470kΩ |
| R2 | 470Ω |
| R3 | 470Ω |
| C1 | 0.47µF (Electrolytic) |
| D1 | 1N4148 |
| D2 | 1N4148 |
| D3 | 1N4148 |
| D4 | 1N4148 |
| D5 | RED LED |
| D6 | GREEN LED |
| 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
Please send your ideas, which are very important for our success…
#1 by Jose Hernandz on April 15th, 2012
this circuit work good, but I replace ci from .47uf to 470uf 16 v and I get better rate on the led flashing……..Thanks
#2 by jose hernandez on April 15th, 2012
this circuit work fine Ijust conect pin 9 topin 2and 6 and change c1 to 470uf 16v for bether flashes.
#3 by Mamade Ibrahim Joomun on March 15th, 2012
I have assembled one and it works perfectly. Just connect pin9 to pin 6 and 2.
This circuit will not identify Collector and Emitter. If you reverse them, the circuit will still display OK.
#4 by Mamade Ibrahim Joomun on February 5th, 2012
Hi Abe,
On the schematic, it has clearly shown that pin 9 should not link to pins 2 & 6.
Have you assembled one?
Thank and best regards,
#5 by Abe on September 6th, 2011
Hi every one:
This circuit works, the only problem was that
pin 9 has to be connected to pin 6 and 2.
#6 by Michel on April 21st, 2011
This is not working at all; what is the error? Schematic or parts?
#7 by Robert on November 6th, 2010
Have very carefully built this too but no joy at all.
Suspect I need to ground unused inputs or something but am not sure, also suspect that voltage rating on electrolytic may be important as it powered up lighting both LEDs dimly after some minutes with a 0.47uF 50V cap which was the lowest value I had on hand.
Anyone actually had it going and happy with it? If so what did you do with unused inputs exactly.
Just want to build a basic transistor checker that works reliably
#8 by forex robot on May 31st, 2010
found your site on del.icio.us today and really liked it.. i bookmarked it and will be back to check it out some more later
#9 by leo on May 17th, 2010
will this In Circuit Transistor Tester work I have built it but no go…