Un-stucking a Stuck Switching Transistor - Page 1
Forum section: electronics / beginners

author: 108CAM
author_rank: Contributor
timestamp: October 17, 2022, 04:22:20 am
content: I have a flasher relay with a stuck switching transistor, A "TIP 42A" to be exact and was wondering if there's any tricks that can be used to get the transistor unstuck so that I don't have to replace it.All I know is that I have a switching transistor that doesn't want to switch and is not creating the pulses required to flash the light as it remains on as if it were connected directly to a power source.Keep in mind that I prefer answers to be explained in simple terms as I don't know the meaning of many technical terms that lots of people use.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2022, 05:01:35 am
content: When a transistor is "stuck" permanently in the "ON" state it's either a shorted transistor needing replacement, or something else is "stuck" (i.e. quite likely blown) and permanently pulls current from the transistor's base terminal (this being a PNP).So if you don't want to replace the transistor you must find a different culpritIf the rest of the circuit works right and drives the transistor correctly, the transistor needs replacement.What's the whole circuit like and what can you see happening in it?

author: 108CAM
author_rank: Contributor
timestamp: October 17, 2022, 05:13:37 am
content: Quote from: magic on October 17, 2022, 05:01:35 amThe light just stays on but a tiny bit of activity in the transistor is still happening because the light dims slightly at the time it should turn off.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2022, 05:31:43 am
content: It may very well be a defective transistor, "leaking" some amount of current even when it's supposed to be turned off.Or it could be something else driving the transistor incorrectly. If you don't want to just replace the tranny and see if it helps, definitely more details about the circuit will be needed for anyone to speculate about possible other problemsA schematic or pictures of the board at least.

author: MikeK
author_rank: Super Contributor
timestamp: October 17, 2022, 11:57:36 am
content: Transistors are not mechanical devices. One that isn't functioning properly gets replaced.

author: 108CAM
author_rank: Contributor
timestamp: October 17, 2022, 11:02:39 pm
content: Thanks for the replies. Now I need to work out which of the 2 transistors has gone bad so I can replace it but I'm fairly sure I know which one it is as a multimeter test showed continuity across all 3 legs no matter how I had the probes positioned while the other one showed continuity across only 2 legs. (Those 2 legs being where the internal switch stopped when the power was removed)

author: james_s
author_rank: Super Contributor
timestamp: October 18, 2022, 12:53:27 am
content: That's a pretty obvious symptom that the transistor is bad, remove it from the circuit and check it again just to be certain that the fault isn't something else, but I'd expect it's a bad transistor. Why are you not wanting to replace it? TIP42 is cheap and readily available, and it's the part I would expect to fail most often, for example if the output gets shorted due to a bulb blowing or getting broken.