Overvoltage - Page 1
Forum section: electronics / beginners

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 10:05:28 pm
content: Hi guys,I have on my desk a mini digital car camera device which is powered from a 5V USB connection. When my friend was installing the camera into his car, he decided it would wire it directly into his 12V fusebox, leaving out the important 5V transformer. My question is, where is the best place to start fault finding? I spliced into my 5V and ground from the USB and I think something might be shorting.Any advice would be appreciated. The following picture is low quality and only of the top side - just thought it would be nice to see what I am dealing with.Best regards,Andy Cooke

author: c4757p
author_rank: Super Contributor
timestamp: April 17, 2013, 10:11:14 pm
content: Unfortunately you might be screwed. A device designed to operate from USB isn't going to have much protection on the power input, and it clearly doesn't. Your friend likely stuck the 12V straight up every part on that board.Of course, if the rail appears to be shorted, you might be in luck - the short in one part could have prevented damage to others. In that case, track down the short and start by replacing that part. Of course, you really can't know whether it'll be a wild goose chase until you're nose deep in repairs - I'd just write it off, personally.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 10:22:30 pm
content: Hi c4757p,Thanks for getting back to me so quickly! It's rather funny that he bought the camera after seeing mine in operation - just neglected to look after it. The actual operation of the camera has not been affected as it has an internal 3.8V cell - although it is not possible to charge it at present, nor do any of the usual USB functions work (like data transfer).There is a rather large SMD fuse on the far right of the board - though I have not idea how to test it. Currently, when I plug the USB cable into the device, the voltage reading between the 5V and ground is 0V. Is this common, or a short?Best regards,Andy Cooke

author: c4757p
author_rank: Super Contributor
timestamp: April 17, 2013, 10:26:26 pm
content: Quote from: andrew_c on April 17, 2013, 10:22:30 pmThat sounds a bit easier to fix. Sounds like you may have just blown a voltage regulator or three.QuoteProbe its resistance. Should be less than 10 Ohms. I'm not seeing a fuse, though...QuoteDead short - but where exactly are you probing? That thing's probably going to have multiple voltage rails, some of which may be shut down when it's inactive.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 10:55:14 pm
content: Might need to have a neb around on the board for a VC then - though I don't think it'll have gotten that far? The fuse looking bit is in the top right of the previous photo (might need to scroll right above). Anyway, I measured 0.5ohms - nothing.I measured the voltage from the USB cable - I couldn't get any other readings on the mainboard after this.Best regards,Andy Cooke

author: c4757p
author_rank: Super Contributor
timestamp: April 17, 2013, 11:03:03 pm
content: Quote from: andrew_c on April 17, 2013, 10:55:14 pmYeah, I'm not seeing it. That board's got two "top rights", and all I see is some caps and inductors on one of them, and a ferrite bead, diode, transistor, couple caps and two connectors on the other.QuoteThen you're definitely shorted somewhere.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 11:20:23 pm
content: My apologies,This is where I was thinking..What is the best way to find a short then? lol.. Not looking forward to this challenge now - might just send it off to Dave for a Tear-Down-Tuesday. Nice thing is that the PCB isn't very densely populated over by the USB port; sure it'll be something like a transistor that's arcing the 5V to ground?Ideas on a postcard.Best regards,Andy Cooke

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 17, 2013, 11:31:56 pm
content: USB devices of that kind normally have some overvoltage protection, as the USB 5V power supplies might also fail and deliver more than 5V, but maximum 500mA, usually.This is done by a power zener diode (>5V6, or so), directly between GND/Vcc, near the USB connector.This seems to be the melf diode D15.The 12V overvoltage has overheated this diode, as no fuse is available, and the 500mA definitely will be exceeded, so the diodes internal Si crystal has melted and might have built a short afterwards.If you own a DMM, set it to diode test, and check if the diode shows a normal behaviour, or if it is low ohm in both directions.In latter case, the diode might be desoldered and afterwards replaced by a new one.The rest of the circuitry obviously has survived, because several switch mode power supplies can be identified on the top left, which normally beware the circuitry of overvoltage intrinsically.P.S.: FB7 is simply a ferrite bead for supressing external noise, not a fuse, and will probably not short anything.Frank

author: c4757p
author_rank: Super Contributor
timestamp: April 17, 2013, 11:41:39 pm
content: Yeah, that's just a ferrite bead. Bit of wire run through a piece of ferromagnetic compound to absorb high frequency noise. Basically an intentionally shitty inductor.Quote from: andrew_c on April 17, 2013, 11:20:23 pmTwo methods:1) Run current through it, and probe around for the voltage. The spot with the lowest voltage is the short. Advantage: Most multimeters are sensitive enough on the voltage mode to do this. Disadvantage: Requires a power supply capable of driving a dead short for a period of time without blowing up. Catch: Make sure the power supply is set to no more than 5V, so that if the short clears, you don't damage anything.2) Probe the resistance between power and ground. The spot with the lowest resistance is the short. Advantage: Much easier and not ham-fisted. Disadvantage: Requires a very precise ohmmeter (single digit milliohms), which I doubt you have.Though I suspect the short is in that diode right next to the USB port. You can just remove it to check, though you'll no longer have overvoltage protection until you replace it.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 11:45:35 pm
content: I love it when a plan comes togetherI am getting 0.4ohm in both directions across the diode. Can I substitute this for something like a 1n4001 which I have plenty of?Best regards,Andy Cooke

author: andrew_c
author_rank: Regular Contributor
timestamp: April 17, 2013, 11:48:37 pm
content: Also, the diode test is showing -0.001 in both directions?Best regards,Andy Cooke

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 17, 2013, 11:55:37 pm
content: Quote from: andrew_c on April 17, 2013, 11:45:35 pmIts defect, obviously, as expected.Well, it's a zener diode, not a normal rectifier diode! A 1N4001 is useless in this placeI propose a 1,3W, 6V2 zener, if it's available in that case.First desolder this device, without damaging the PCB or the copper pads.Afterwards, the device should work again from external USB power supply. But please, do not omit the Zener permamently.Frank

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 12:15:21 am
content: Quote from: Dr. Frank on April 17, 2013, 11:55:37 pmEh, it's just a repair. Case doesn't matter a rat's ass if you can get it on the pads.

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 12:20:57 am
content: Quote from: Dr. Frank on April 17, 2013, 11:55:37 pmHint: The usual method for desoldering SMD parts is with hot air, but I don't know if you have it. However, that diode will have good thermal conductivity end-to-end,andyou don't care if you damage the diode (it's already broken!). Add a bit of solder to one end (for a nice, liquid mass to hold the tip in), then just hold the iron there until the whole diode lifts off. Don't apply much pressure, or you could lift a pad if the adhesive softens. That will be lead-free solder, which requires a pretty high melting temperature, but you may be able to get it to melt slightly lower by mixing in some leaded solder.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 18, 2013, 12:42:16 am
content: You guys are fantastic! Well I wouldn't have thought about it for at least a year or more. I wasn't even sure what kind of diode it was - back to electronics school I go. I have removed the diode temporarily and all is working well!I will certainly see what I can find around (1,3W, 6V2 zener) there is plenty of room in there for the odd bulky component. I will hang around on here - see if I can help any less technically minded folk - though I'm sure I'm near the bottom of the pile.Thank you once again - first place I will come to next time I have a query.Best regards,Andy Cooke

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 12:46:24 am
content: Quote from: andrew_c on April 18, 2013, 12:42:16 amGood! I'm honestly a bit surprised that the little Zener held up against automotive DC (well, not really "held up" - it was blown short - but it still never stopped protecting the circuit), especially considering they are relying on the fuse that's supposed to be built into a USB port and don't have one on the board.

author: andrew_c
author_rank: Regular Contributor
timestamp: April 18, 2013, 12:49:05 am
content: In case anyone wants to see some crappy soldering.Best regards,Andy Cooke

author: andrew_c
author_rank: Regular Contributor
timestamp: April 18, 2013, 12:12:21 pm
content: What would we think to... ?http://www.ebay.co.uk/itm/271177853473Best regards,Andy Cooke