cctalk Digest, Vol 16, Issue 2

[CRC]

On 1 Dec 2004 09:43:56, Paul Koning <pkoning at equallogic.com> commented:

>>>>>> "Jules" == Jules Richardson <julesrichardsonuk at yahoo.co.uk> 
>>>>>> writes:
>
>  Jules> I've got a board here with a short between ground and the +5V
>  Jules> rail (actually not quite dead - I'm getting around 10 ohms
>  Jules> between the rails)
>
>  Jules> Any useful tips for finding the fault? It's a large board,
>  Jules> multi-layer, lots of silicon on it unfortunately :-(
>
> 10 ohms means about 0.5 amps -- sounds like a perfectly respectable
> amount of current for a board with "lots of silicon" to draw.
> So... are you sure there is a short, rather than just circuitry?
>
>       paul

Unless there are passives (like terminators) between +5V and GND, 10 
ohms is a little low. 0.5 amps represents a minimum current that the 
board will pull. An ohmmeter will not normally cause the silicon to 
conduct and hence be counted in the total resistance. Some chips do 
have internal dividers, but the they are nomally in the 10K + range.

I normally use a current limited supply at the required voltage and set 
the limit so that it starts to limit. I then use a 5 digit voltmeter to 
trace where the current is going. This is done by placing one probe on 
the +5V input and then measuring the drop as you trace out the 
distribution path. The path drawing the current will continually drop 
while non-drawing paths will remain at a constant voltage drop.

On multi-layer boards you measure drops to the Vcc pins of the bugs and 
map the drop.

	CRC