Possible LCD display damage from reversed battery polarity - Page 1
Forum section: electronics / repair

author: van-c
author_rank: Regular Contributor
timestamp: March 27, 2015, 05:14:27 am
content: I recently purchased a 7 inch FPV display to use when flying RC quad-copters. The display works perfectly with the included 12V wall adapter; however, when flying, I will usually be far away from AC power. So, I plan to use a 3-cell LiPo battery pack to power it.But when I tested the display using a LiPo battery, I inadvertently reversed the polarity and there was a loud “pop” (but no smoke or burning smell.) Since I had not yet installed a connector on the battery end of the power wire, I was able to immediately disconnect the battery.Despite my fear I had ruined a brand new display, I decided to try it again anyway using the wall adapter. It powered right up and seemed to work perfectly. But I am concerned that I may have damaged an electrolytic cap based on the pop I heard. I’ve done a thorough “sniff test” around vents in the case and don’t detect any burnt smell whatsoever.My question is how likely is it that a cap (or other component) is seriously damaged despite the fact that the display appears to work just fine? Removing the rear cover involves loosening a few screws, but it also involves peeling off a length of Velcro strip used to secure a sun shield. I just hate to weaken the Velcro adhesive if not really necessary to open the case to inspect for visible damage.Any advice?—Van

author: electr_peter
author_rank: Unknown
timestamp: March 27, 2015, 08:29:18 am
content: Do a measurement of DMM for resistance and diode check in both polarities. Probably a cap or reversed polarity diode has blown. It may not be that bad because reverse polarity diode won't be needed with fixed correct battery polarity (and hey, it works). Capacitor is not that critical as battery provides stable voltage.If you fear damaging the case, leave it as is and use it till problems occur.

author: wraper
author_rank: Unknown
timestamp: March 27, 2015, 02:02:35 pm
content: Check if there are any tantalum capacitors on the power rail. They can not stand any significant reverse voltage at all. If there are electrolytic capacitors, they could explode too.

author: van-c
author_rank: Regular Contributor
timestamp: March 27, 2015, 04:27:09 pm
content: Quote from: electr_peter on March 27, 2015, 08:29:18 amWith my Fluke 87v in Ohms mode, the reading across the power input is about 1Meg - 500K both polarities, but highly variable, indicating capacitance. When I checked it in diode mode, the DMM indicated about 3V in one polarity and 0V in the other, suggesting a diode. Would a damaged diode behave this way (3V drop rather than ~6V or is such an in-circuit reading typical?)Yeah, the pop did sound like a Tant giving up the ghost, but I didn't think those were still used very much. A small electrolytic is visible through a vent opening in the case. Its top side is not puffed, but I cannot see if there is any damage on the sides.--Van

author: van-c
author_rank: Regular Contributor
timestamp: March 27, 2015, 07:43:55 pm
content: I decided to go ahead and open up the case and I'm glad I did. As shown in the photos, a 220 uF electrolytic had vented (at the bottom seal). Replaced it and all now seems well.Thanks for your help.--Van