Regarding Dave's tale of woe with the 5V rail short in EEVBlog #530, I have the following to add:
It is indeed possible to find a short by supplying it with a crap load of current and letting the short blow out. Often this will result in the physical destruction of a component, but at least you will have found the culprit. In the trade we call this procedure "tune for maximum smoke". The dangers are that you can vaporise PCB tracks in the board if you're not careful.
Sometimes you can get away with a safer approach. If you supply the board with a smaller amount of current, but enough to cause some heat dissipation in the short circuit, you can have a careful feel around for suspiciously warm components. The warm component is often the culprit. Careful though, you can burn yourself - as I have done many times. OUCH!!! B*stard, that's the culprit!!!
At work we see this kind of problem quite regular. And a short on the supply rail can often be the trickiest to pin down. Especially when you've got a bunch of BGAs on the board - nothing worse than that. So I've ordered in an AIM i-prober 520, which is a non-invasive current probe for measuring current flow in PCB tracks. I haven't had time to try it yet, but check it out:
http://www.aimtti.com/go/iprober/index.htm?gclid=CJihscbAp60CFUIMfAodZHMjoAI also wanted to take this opportunity to share a similar tale of woe with the forum members.
The other day we had a problem at work with a board that was drawing excess current. It wasn't a dead short, and curiously the board was otherwise fully functional, but it was drawing about double the expected current. We didn't have any good boards around us, so we had to troubleshoot it on its own. We were suspicious of the CPU because it was getting warm, so we lopped it out and then the current consumption problem went away. Of course, we weren't fully satisfied with this because with the CPU removed the board was basically not doing anything, so it was no guarantee that the CPU was really at fault. The easiest thing to do was change it and see what went on. We did that, and the problem was
still there!!!
We couldn't tune for maximum smoke because we weren't dealing with a dead short, so we began removing components from the supply rail one by one. Eventually we got all the way down to pretty much nothing on the board. Only the CPU was left.
What the hell is going on? Well, we powered the CPU up off the board and what do you know, it was drawing excess current! Tried another 10 components from stock, and they were all doing the same. We only had one batch code of component in stock, but we went through them all and only found 2 that were good. The two that were good had the same batch code printed on them, but I noticed that the packaging was slightly different! The good ones had a pin 1 indent, and the bad ones did not. How can this be, if they were the same batch?
It turned out that purchasing had got them from the grey market. Bar-stewards!!! They were almost certainly counterfeit. I lopped the top off one of them and nothing looked untoward, but nothing else explains the problem to me. I still don't know how the boards were otherwise fully functional though. Bit of a weird one. But the problem was definitely the CPU in the end.
Brian H.