Can a transistor be destroyed by backfeeding ground or power to it? - Page 1
Forum section: electronics / beginners

author: Superkryptonite
author_rank: Contributor
timestamp: December 12, 2021, 04:19:49 am
content: Hi guys,I have a snowmobile I'm working on that I believe I blew a transistor in the ECU.This transistor gives ground to the coil side of a relay which controls the fuel pump. It no longer gives out the ground signal.I was testing the circuit with low power test light, I forget exactly what I did, but I think I had the test light connected to ground, and I touched the ground side of the relay coil (which is connected directly to the ECU) which wasn't working, and the relay clicked. By doing this, could I have permanently damaged the transistor inside the ECU?Also, I might have had my test light hooked up to power before doing the ground side, and touched it to the ground side of the relay coil (which is connected directly to the ECU). Could that have fried it?

author: bob91343
author_rank: Super Contributor
timestamp: December 12, 2021, 05:39:20 am
content: Your terminology is difficult to understand. However I think you simply shorted the transistor and verified that the relay closes. Having done that, the question arises whether the transistor is bad or there is no drive signal. You can verify that with a scope on the lead from the relay.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 12, 2021, 06:32:29 am
content: QuoteFrom your description, it sounds like the transistor inside the ECU was already faulty before you connected the test light. Putting the test light in parallel (via ground) with the MCU output transistor shouldn't cause any damage unless the MCU itself was not grounded but still connected to 12V.

author: fordem
author_rank: Regular Contributor
timestamp: December 12, 2021, 12:31:22 pm
content: The answer to your question is YES - it can be done.That said, reading the sequence of events, I don't think that you would have caused the failure - as long as you're connecting the power or ground via the test light, and not directly, and you're doing it to a circuit capable of handling the current of a relay - no, you're good.

author: Superkryptonite
author_rank: Contributor
timestamp: December 12, 2021, 02:34:39 pm
content: Quote from: Kim Christensen on December 12, 2021, 06:32:29 amThat's a possibility. And what if I had connected the test light to +12v and then touched the ground side of the coil connected to the ECU? Even though the ECU wasn't sending anything out.Quote from: fordem on December 12, 2021, 12:31:22 pmThanks! I'm really not sure what the hell happened. My test light is only about 250mA.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 12, 2021, 05:45:22 pm
content: QuoteIf the other side of the relay coil is normally connected to +12V, then you would effectively be bridging the relay coil with the test light. If the MCU was powered and not trying to turn the relay on, then I'm 100% sure nothing bad would happen. But, if the MCU was trying to turn the relay on, that would mean that the transistor inside would have to supply current for the relay AND the test light (An extra 250ma). Thatmightblow the transistor inside the MCU.

author: CaptDon
author_rank: Super Contributor
timestamp: December 12, 2021, 09:54:43 pm
content: Was the relay already not working and that is why you tested it?? If so the ECU was already bad. If you hooked up the test light to a circuit that was working and now doesn't work then you fixed it to death. If you shorted +12 directly to a circuit in the ECU which pulls to ground you may have over-currented the circuit and killed the transistor/mosfet. I would have designed in a protection circuit against the over current of a shorted relay but that would have cost 5 extra cents and we know where that story ends!!!!

author: Superkryptonite
author_rank: Contributor
timestamp: December 13, 2021, 01:53:39 am
content: Right. I'm not 100% sure. I just started testing and kind of forget.Here's a doosie for you guys:Multimeter, one lead to relay coil positive (battery constant), one lead to coil negative (grounded by ECU). Key on.Multimeter shows 12.3vChange multimeter to continuity test or resistance. One lead to coil negative (grounded by ECU), and other lead to a ground source (battery, chassis ground, etc).NO CONTIUNITY! How is that possible? To get 12.3v reading between the coil, there must be a ground?All powers and grounds to the ECU test out good using a 3A test light (current carrying capacity is good and in tact)

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 13, 2021, 03:12:08 am
content: QuoteThere is a small voltage across the transistor inside the MCU therefore a multimeter set to continuity test or resistance is not going to read correctly.Test for a voltage drop there instead. Watch it change as the MCU commands the relay to turn off/on. When the relay is off you should see around 12V between the MCU output and ground. When it commands the relay to turn on, the voltage should drop to something lower than 1 volt or so.

author: Superkryptonite
author_rank: Contributor
timestamp: December 13, 2021, 03:52:11 am
content: Quote from: Kim Christensen on December 13, 2021, 03:12:08 amOh okay I see. I always thought it was just a regular positive and negative line at the coil and you could measure it like two posts of a battery.

author: Steffalompen
author_rank: Contributor
timestamp: December 13, 2021, 03:09:42 pm
content: Quote from: Superkryptonite on December 13, 2021, 01:53:39 amSo the coil dropped the full available voltage and was thus energized while you was measuring across it.And still it didn't actuate?PS. (ergo the low side transistor is either on or shorted)If you have a multimeter then please throw that test light in a volcano

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 13, 2021, 07:20:35 pm
content: QuoteYou can do that by setting your multimeter to read voltage only.Trying to read continuity or resistance on a live circuit is a no-no.When your multimeter is in resistance measurement mode what it does is put a small current into the device you are testing. Then it measures the voltage produced by that current through the device and calculates the resistance using ohms law and displays that result. (Resistance = Voltage/Current)So if the device under test already has a current through it, and thus a voltage drop across it, putting an meter set to read ohms across it will add more of a voltage drop. But the meter doesn't "know" about the current already flowing and will thus give a false reading or show "out of range" (displays OL).