View Full Version : Chip Death?: Shorted output to NEG
I was about to do the output trimming. I did something stupid: I used alligator clips on the board, on the output pads. I applied power and was about to test. Then, I believe the alligator clips dug through the coating on the PCB, shorting output to whatever bus is at the edge of the board. Ground? Input voltage?
The fast-blow 500mA fuse blew immediately. I removed the alligator clips, and now new fuses blow immediately.
Nothing looks damaged on the board.
Specific questions for the good people here: What did I short to? Could it have killed something? And what, most likely? (Probably the power stage ...)
Any input would be most welcome!
-- Inspecting the board, it looks like have I shorted OUT to NEGATIVE in.
what's negative in? the power supply is only positive and ground. I guess you mean it shorted output to power ground?
This is a bridged amplifier, so shorting one of the outputs to ground shorts one half of the bridge. There is over-current protection that is supposed to protect against this sort of thing, but there is very little information on how it works in the datasheet. Maybe it's not damaged, but possibly it is still shorted. When you shorted the tracks with the alligator clip they may have welded together at that point. Measure the resistance between the output that was shorted and ground....
One thing to look out for is the two mounting holes at the output end of the PCB, they are connected to ground.
If you use a metal screw and/or washer, then it's possible that it could short with the pins/pads of R50 or C28, as they are very close. I had previously this intermittent problem where I had a washer that was just touching R50 causing a short. The only way I worked out what the problem was, was the smoke that started coming out of R50! I use nylon screws in that position now.
Actually, due to the compactness of this board there is potential for accidental short circuits in quite a few places.
Excellent info! Thanks.
quote:Originally posted by col_s
what's negative in? the power supply is only positive and ground. I guess you mean it shorted output to power ground.
.. and yes, that would be correct.
I got some funny resistance readings, around 10-45 Ohm, from out to ground. I see INF resistance over C81 and 0 over C82. I don't exactly see how these caps could have blown, I could be measuring badly, it might be somewhere else etc. etc., but if it is the case that I blew the C82 cap to a short, what type of capacitor is this? If I need to replace, I can get "1000PF 63V G0805 VS SMD X7R" locally. Would that do?
... and for completeness, I must state that I really have to clean that board. Shouldn't have plugged it in, there's quite a bit of flux on it.
If you have 0 resistance over c82 then that means there is a short between that output and ground, but it's probably not c82 (I don't think ceramic caps fail shorted). It could be anywhere from inside the output chip all the way through to the output pads. The output that is shorted is the one that connects to C28 so it is on that half of the board.
First make doubly sure that you really are measuring 0ohms across C82. If there is a short you should also see 0ohms across C25 and C22. If there is and you can't see any obvious shorts along the path from the output chip to that output (check this thoroughly first), then you could try carefully removing the inductor that is on that side, ie. the inductor between C25 and C101. Now you can narrow down whether the short is on the chip side or on the output side. With the inductor removed, try to measure across C22, if it is 0ohms then the short is in the chip or very close to it. If it's not, then the short is somewhere near the output pads. It helps to look at the schematic although some of the component numbering is incorrect.
Again, pretty awesome info. Thanks much.
Hey, a slightly off-topic question perhaps: I sorta envy your troubleshooting skills, col_s, do you have an electronics education? In any case, can you recommend books / tutorials / etc.?
I did a little bit of electrical engineering (not my main field) back when i was at university and learned the basics, but most of my recent electronics skills i have taught myself. I guess like many of us this was not from any particular course or anything, just by doing stuff like building this kits like these and more importantly trying to understand how things work, especially when the break :-)
A large part of my job has been to solve complex problems so i'm sort of used to that sort of stuff. As far as general problem solving, i've been trained in Kepner Tregoe and a few other things like that, which helps.
OK, I was measuring the C82 resistance wrong, the output filter now gives me resistance readings that look OK.
I measure 0 Ohms between the power rails :/ When I measure across the power input pads, I see 0 Ohms. I'm thinking the output chip is dead ... can you guys think of a good way to measure this without removing the output stage chip? I don't mind removing other components, I just don't think I can get the output stage off without destroying it ...
I was installing my 2nd dual AMP11 amplifier....
Putting everything into an enclosure...
Everything was fine... It was playing fine...
Until I try to straighten the speaker output wire and power it back on....
I see smoke coming out of from one of the AMP11 module..., Only one speaker has sound coming out..
I turned it off and after careful inspection, I see a little trace of wire shorting the speaker output.... ZERO resistance across the output...
I haven't had chance to look further.... I try to turn power back on... but no sound ....
There is no visible damage on the surface of both chips.....
Any suggestions on where I can start looking at ??
My Amp11 is sitting on a shelf still, so I don't have any info :/
The amp is short circuit protected on the output, so that should cause such damage. Is the output still zero ohms after you removed the little trace of wire?
------X-max. can be several meters on any driver, too bad it can be done only once...------
I measured the resistance across the output and its around 0.9 ohm...
Yeah, that's not good. When the output is shorted I believe that the output chip will cycle in an out of fault state about once a second as it tries to auto-recover. I guess if it is left with the outputs shorted for a significant amount of time that this continual trying would put a lot of stress on the chip, especially if it was not adequately heatsinked.
This won't help you now, but you can increase the amount of time that the output chip stays in the fault state by adding a larger capacitor to ground from the fault pin (it is 100nF on amp11). This reduces the stress on the chip during a fault, and so makes the amp more robust. It's described in ST Microelectronics app note AN1994 (the output chip is a ST505), along with how the overcurrent protection works. I think a longer delay would be better, 1 second seems a bit too quick for me.
Link to AN1994: http://www.st.com/stonline/products/literature/an/10606.htm
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