Asteroids PCB - 5V Resistance

baloo70

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I load tested the power supply and it seems to work fine, but when I connect the power supply to the PCB the 5V rail measures 1.8V. I disconnected the power supply and measured the resistance between 5V and ground on the PCB and it was 50 Ohms. I think this is a little low. Can someone verify what a properly working PCB should read (resistance) from 5V to ground? Thanks!
 
easier said than done...

Nearly every component on the board has a leg going to both 5v and ground.
Brute force method is to cut the 5v trace for each row until you find the problem area.

I'll be very curious how you find it........
 
Or put a 5V supply on the rail that can handle more current and find the hot chip, or possibly the one that self destructs... only partially kidding here. In this case where there's not a dead short, but something that's dragging the supply down... I'd probably actually try it. A switcher w/ 15-20A on the 5V should do it.

DogP
 
Or put a 5V supply on the rail that can handle more current and find the hot chip, or possibly the one that self destructs... only partially kidding here. In this case where there's not a dead short, but something that's dragging the supply down... I'd probably actually try it. A switcher w/ 15-20A on the 5V should do it.

DogP

Will this melt a track?
 
It could burn up a trace, depending on the current draw and the size of the trace.

If you have an ESR meter you can use it to find the short circuit. An ESR meter is a high frequency milliohmmeter. Translation: It measures very low resistances - down to .01 ohms using high frequency signals. Find where the resistance is lowest on the board and you'll find the short.
 
Will this melt a track?

It's very unlikely, since it's only dragging the 5V line down to 1.8V... so it's a very weak short. Since it's dragging it down that much, there should be one chip that's getting hotter than the others right now too.

And if it burns a trace, then there's a clue of where to look... I can repair traces much faster than I can track down shorted chips ;) .

BTW, how did you load test the supply? Are you sure the supply just isn't weak (it's pretty common for supplies to fail under load)? 50 ohms is still pretty high... on a 5V line, that's only 0.1A. Of course DC resistance between rails doesn't really give you an idea about total current draw (since the chips aren't active and drawing current), but I wouldn't necessarily think 50 ohms between rails was a problem.

DogP
 
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