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wireties
10-02-2012, 16:58
Anyone ever had Q1 on the sidecar PCB go bad (as in part of the chip is blown off)? We are not sure what caused the problem, it was in a working robot but the board could be several years old. I ordered new parts from Digikey (they are cheap) and am hoping a quick solder job will bring the board back to life.

TIA
EricVanWyk
10-02-2012, 17:29
Could you check what revision the board is? It should be in silkscreen near the DB37 connector.

The most likely cause is a two stage fault:
First, the gate becomes damaged or disconnected, typically by ESD. Later revisions have better protection for this.
Then a large current is applied. Since the FET is damaged it drops a lot more voltage than normal, which drops a lot more waste power. Typically the FET will just burn itself out, leaving a small hole and a large smell. Rarely, a sudden surge current (typically a short) can cause it to pass away with more interesting fanfare.

I wouldn't bother trying to repair the board, the remaining components have likely been stressed past the point of reliability.
DonRotolo
10-02-2012, 22:25
It a repaired board legal for use in competition?
EricVanWyk
10-02-2012, 23:58
Yes, but I would not personally trust a board that has experienced the failure mode described to be reliable enough for competition. It could be worth repairing for a practice bot though.
wireties
11-02-2012, 02:21
Yes, but I would not personally trust a board that has experienced the failure mode described to be reliable enough for competition. It could be worth repairing for a practice bot though.

If it still works, why not? How would you propose verifying a brand new sidecar is reliable? And if this board passes the same criteria, why not use it?
DonRotolo
11-02-2012, 11:18
EEs use the term "walking wounded" to describe circuits that have a much higher probability for failure than nominal. Eric really knows these sidecars well, and I'd defer to his expertise on this one.

Of course, if you don't HAVE another one, you use what you have!
EricVanWyk
11-02-2012, 11:58
It is entirely a value judgement, I am only giving an opinion based on what known modes could have occurred to create the symptoms you described. You have better access to the board, and can therefore make a better judgement than I can.

From my viewpoint, I am assuming that this board has already experienced two damaging events: ESD and reversed input polarity. Both of these can stress the rest of the components on the board, which is something I personally don't want to deal with in competition. It may work perfectly, but it is forever less reliable than a board that hasn't been stressed in these ways.

Unfortunately, I don't know of a way to test for these stresses other than to wait for them to result in further failures.
wireties
11-02-2012, 12:14
EEs use the term "walking wounded" to describe circuits that have a much higher probability for failure than nominal. Eric really knows these sidecars well, and I'd defer to his expertise on this one.

Of course, if you don't HAVE another one, you use what you have!

Thanks for the feedback!. Agreed, if we had a new sidecar - toss this one. But in theory that transistor was supposed to protect the rest of the circuit. I am an EE also, it seems a $2 transistor and 10 minutes to replace is worth a try. Thus back to the original query - anyone ever had this transistor blow, replaced it and got the sidecar working again?
EricVanWyk
11-02-2012, 13:08
I am an EE also

My apologies, I didn't realize.

I have only repaired boards with this fault that I have intentionally broken on the bench.

I would verify the stability of the power supplies, unloaded and at 2Amps. I would also manually check each PWM and Relay channel on the bench before handing it back to the students.

Can you check what revision number the board is?
wireties
11-02-2012, 19:28
[quote=EricVanWyk;112429
Can you check what revision number the board is?[/quote]

Thanks for the info Eric - the silk screen says rev 8 from 6/25/2009
Joe Ross
12-02-2012, 10:31
In my experience, whenever a piece of power circuitry blows, it also damages the pwb underneath.

Of course Eric used to work with medical devices, and I work aerospace, so we tend to be more conservative then most.
wireties
12-02-2012, 14:52
In my experience, whenever a piece of power circuitry blows, it also damages the pwb underneath.

Of course Eric used to work with medical devices, and I work aerospace, so we tend to be more conservative then most.

Agreed, we have limited resources - just trying to be thrifty. I would certainly never reuse such a device in a professional context.