Accidentally put 12 volt on 5 volts output. Which component failed? - Page 1
Forum section: electronics / repair

author: bigjoncoop
author_rank: Regular Contributor
timestamp: May 13, 2019, 08:20:27 pm
content: On my 5.8 GHz video transmitter I accidentally put 12 volt on the 5 volt output. Now the video transmitter will not power on. Which component most likely failed?The diode on the input seems to be fine. I can trace continuity from the input pin all the way up board to the voltage regulator section. There is a 5-pin LDO next to the inductor.What is strange to me it is that at the bottom right-hand side of the board there is another 3 pin diode that has continuity between one of the pins and the 5 volt output pin on the connector and it seems fine when I tested it with a multimeter... now shouldn't that 3 pin diode have stopped any voltage from passing when I put the 12 volts on it in the wrong direction? I would have thought these boards have protection against accidentally inputting voltage on the output..When I apply voltage to the input pin absolutely nothing happens on the transmitter. The segmented LED doesn't light up there's no video output, nothing....If you have any ideas where I could start looking what to test please let me know. Thank you.PS. I am somewhat tech savvy but m just getting into component level stuff.... Thx again

author: helius
author_rank: Super Contributor
timestamp: May 13, 2019, 08:47:20 pm
content: I don't understand why that module needs a 5V output. Output to what?

author: EOC_Jason
author_rank: Contributor
timestamp: May 13, 2019, 09:20:42 pm
content: Quote from: bigjoncoop on May 13, 2019, 08:20:27 pmBreakdown voltage maybe?

author: DaJMasta
author_rank: Super Contributor
timestamp: May 13, 2019, 09:39:55 pm
content: Does it draw power when the voltage is applied? Do any parts heat up?Could be a diode or transistor that's shorted and would be drawing a lot of current and heating up.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 13, 2019, 10:11:06 pm
content: Quote from: helius on May 13, 2019, 08:47:20 pmThe 5V output is to provide power to a camera. It's actually very useful! You may not have any 5V source readily available on some craft.

author: bigjoncoop
author_rank: Regular Contributor
timestamp: May 13, 2019, 10:18:03 pm
content: Thanks guys for your quick response.Sorry I should explain what this is for. I fly fpv quadcopters. Don't do any racing just freestyle / tricksThe 5V output is there if you want it to power your camera. Which I don't use normally. I've installed so many of these video transmitters. I became too comfortable and accidentally put the input voltage on the 5 volt output. Twice in a row, on brand new transmitters...no nothing heats up that I can tell. Now there is two 3-pin shockey diodes. On one of them only one side works and on the other one it's completely shot. My plan is to replace them tomorrow. I just wanted to get some input, to see if those two diodes should be the only thing that got destroyed. Otherwise if it's going to be a whole mess of things I won't waste my time. They're only $20 a piece but they were brand-spanking-new never used. So if it could be a simple fix I would go for it. I will post another picture and circle the two diodes.Now another thought is to just power the transmitter module itself with 3.3 or 5 volts directly. That way I wouldn't need any of that circuitry around it. I don't need the LED readout, the button to change channels or the 5 volt output.... But then I run into the problem of I can't find a schematic or a pinout of the transmitter module to find out which pad is what, besides the ground pad. I still need to figure out which one is VCC, video in, and this protocol we use called smart audio. Smart audio allows us to control the frequency and channel and power output from our controllers / goggles which is why I don't need the LED readout or button. I have tried to continuity test from the connector which I know which pin is what to the transmitter module itself but I don't get any tones. Just different ohm readings which I don't know how to decipher.I think just powering the transmitter module itself would be the easiest thing to do in my situation if I could find a diagram or pinout.Again thank you for your quick response and any input that you may have would be greatly appreciated thank you.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 13, 2019, 10:34:55 pm
content: As others have mentioned, a catastrophic failure usually means shorted Silicon components.Leave the unit powered for a while, and with your finger try to find out which components are becoming warmer.Be aware that the warm component could be a symptom and not the cause

author: DaJMasta
author_rank: Super Contributor
timestamp: May 14, 2019, 01:01:11 am
content: If you have a multimeter, you can put the meter on a current range and put it in series with your power source (Like power source -> meter -> voltage input), if it's drawing much current, then there is probably a part that has shorted and likely something would warm up, but if there's no current flowing, the failure has made the bad part an open circuit. In either case, you may be able to trace around the board looking at datasheets to try to identify a part that should be one way (have a diode drop, have some resistance, be open circuit), but which obviously isn't (short or open when it obviously shouldn't be, generally). Could give you an idea which part is the issue.Also worth measuring the 5V output and any onboard rails (3.3V?), to see if the voltage regulators for those parts are at least working.