Do you want a Dead pcDuino? - Page 1
Forum section: electronics / general

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 27, 2013, 09:16:00 pm
content: I've got a deadpcDuino. Do you want it Dave for a teardown?

author: c4757p
author_rank: Super Contributor
timestamp: March 27, 2013, 09:25:10 pm
content: Teardown? How, with a hot air station?

author: Alana
author_rank: Frequent Contributor
timestamp: March 28, 2013, 08:52:03 am
content: How it failed? Maybe Dave can have his troubleshooting and fix video with this?

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 28, 2013, 08:57:38 am
content: Was working one moment for about 24hours then stopped. Now its shorted across the power input. Measures around 30ohms. And takes 1 amp at 0.2v. I suspect its the axp powersupply chip. If I freeze it I can get 1amp at 1.1v.On the 3.3v side there is no short.

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 10:43:20 am
content: Just a guess - haven't seen the schematic - but you connected it backwards and blew the reverse polarity protection diode.

author: amyk
author_rank: Super Contributor
timestamp: March 28, 2013, 11:09:35 am
content: Just noticed the way the forum cut off the topic title made it look like "Do you want a Dead parrot"...

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 11:22:47 am
content: HA! Found the schematic and I see the bigfail...Quote from: johnnyfp on March 28, 2013, 08:57:38 amOne method of reverse protection is to put a diode antiparallel to the power input after a fuse or polyswitch, so that if the power is backwards, the diode conducts and trips the fuse. That's precisely what they've done, but the bastards fused it at 2A! The diode is kicking in but not enough to trip the fuse. (My guess is that you are powering it with a current-limited power supply.) Andthis, friends, is why you spend the extra couple cents on a P-channel MOSFET and do reverse protectioncorrectly. Anything that requires tripping a fuse belongs in the 1980s.Remove D2 (should be near the power input "USB" connector) and it should work fine again. Youshouldreplace it (MMSZ5233B - fairly large surface-mount 6V Zener providing both overvoltage and reverse polarity protection) to get your protection back, but the board will work without it as long as you don't do anything stupid that requires the protection again.Quote from: amyk on March 28, 2013, 11:09:35 amIt's not dead, it's pining for a new diode!

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 11:24:14 am
content: Schematic here.It's at the bottom of sheet 9. (Clearly the bastards can't do power input at all. Not only does the diode happily gulp power without tripping the fuse, but they used a diode that has no visible maximum current rating on any manufacturer's datasheet - though I'd bet money it's less than 2A,andthey've got some sort of polarized capacitorbeforethe fuse - so even if the fuse kicks in, you can still get fireworks if you hook it up backwards.)

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 28, 2013, 07:33:58 pm
content: I understand and would agree with you, except I didn't reverse polarity. It was working fine, powered up one moment. I was ssh'd onto it and listing a directory. Then suddenly my monitor switch it self off (yes I was at the time powering the pcduino from the monitor usb port) and the pcduino was dead. But I will investigate that diode. However, I was offering the board to Dave to do just that. To add to his gazillion of other projects to show off.I am been sent a replacement anyway so was not fussed in sending it.

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 08:04:51 pm
content: It is possible that the problem wasn't reverse polarity, but a bad diode. It's a 6V Zener, meant to provide overvoltage protection as well, and it's not unlikely for manufacturing variations to make that diode take some current at only 1V under its Zener voltage. 6.0V is only at the Zener current, 5mA for this diode. A Zener diode with that voltage is actually an avalanche diode, with a positive temperature coefficient, so the lack of current limiting causes it to go into thermal runaway.(Personally, I probably would have omitted the OVP. The power comes in via a USB port. You'd pretty much have to intentionally splice a power supply to a USB cable to get more than the specification-permitted 5.5V. And if I really wanted OVP, it'd be with some sort of voltage monitor circuit, not a Zener repurposed as a crowbar.)

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 28, 2013, 08:45:30 pm
content: No luck. I removed D2. Same as before.

author: c4757p
author_rank: Super Contributor
timestamp: March 30, 2013, 02:09:46 am
content: Hmm... Could be anything across the DC_5V rail, then.That could be U5 (the AXP209 that you suspected earlier), or a buttload of capacitors. Don't rule out the caps, I think it's equally likely to be a capacitor. The schematic doesn't say what kind of capacitor they are, but it does say that they're polarized, and all the common polarized caps tend to fail short-circuit. One more thing to check is the connectors - DC_5V goes to a bunch of connectors, some with fairly fine pin pitch, and it's not completely unusual to get some conductive crap (especially on an electronics workbench - solder balls!) stuck in a little connector.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 31, 2013, 02:58:49 am
content: Fascinating.I removed all the caps and U5 off the board and it's still shorted. I then removed the USB connectors and still the same. There is nothing left that I can see on the 5v rail.Could if be a failure in the multi layer PCB. The isolation between layers? I suppose without the PCB diagrams there's no real way to find out.The only thing left that I can think of that will find this is to connect up a car battery and let it burn. But this will in effect destroy it.

author: c4757p
author_rank: Super Contributor
timestamp: March 31, 2013, 03:16:04 am
content: Quote from: johnnyfp on March 31, 2013, 02:58:49 amQuoteTechnically? Yes. Realistically? No. Also, there aren't many things that connect to the DC_5V rail, so I doubt it's on a power plane or anything. Just a couple traces, probably on the top or bottom layers.QuoteI think the only thing left is an internal short in U5. U5 may well eject bits of silicon and plastic if you do this - be careful and wear eye protection. I'd probably just write it off as U5 at this point and chuck it in the bin - nothing else on the rail.P.S. Nothing good ever came out of "connect up to a car battery" unless the thing you are connecting is a car.They're kind of dangerous.

author: c4757p
author_rank: Super Contributor
timestamp: March 31, 2013, 03:20:23 am
content: By the way - an easier way to find a short like that, rather than popping shit off the board until it goes away, is to put a current through the short, then probe with a voltmeter. The short is where the smallest voltage is found. If you don't have a voltmeter sensitive enough, you could even whip up a little op-amp amplifier - we don't give a crap about precision and accuracy here, you just need it to show what's smaller than what. Just be careful generating the current - most (especially cheaper) power supplies don't like driving a dead short for too long and may blow themselves up. Stick a low value power resistor in series to move some of the power dissipation out of the supply's pass transistor.

author: Things
author_rank: Regular Contributor
timestamp: March 31, 2013, 11:13:43 am
content: Alternatively just shove enough current behind it, eventually the short will clear itself

author: amyk
author_rank: Super Contributor
timestamp: March 31, 2013, 11:24:14 am
content: After that saga with the LeCroy scope I think Dave probably isn't interested in tracking down another odd short...

author: johnnyfp
author_rank: Frequent Contributor
timestamp: March 31, 2013, 07:52:48 pm
content: Yeah I figured as much.But its been fun trying to track down the fault. Specially trying to understand how they put the board together. For a double side loaded board I don't understand how they kept the understand of the components on the board without any glue. Could they used flux or a glue that burnt away.When I put a double sided loaded board together I use a spot of epoxy, but then my components are not 402s or smaller.

author: sen
author_rank: Newbie
timestamp: March 31, 2013, 08:51:28 pm
content: Different kinds of solder with different melting points on either side of the board is how that's usually done, as far as I know.For my own boards I've always just made do with a hot air reflow station and being careful to minimize heat, haven't had anything fall off the bottom when doing the other side yet.