[XCSSA] Blowing Fuses and Unsure Why

[X-otic Computer Systems of San Antonio]

On Friday 09 January 2009 11:23:16 am X-otic Computer Systems of San Antonio 
wrote:
[...]
> I think I get what you're saying, but that's for if the fuse on the
> left of my xformer blows right? Can I use this setup if one of my
> fuses blows to the right of the xformer? The only fuse that's blowing 
> is on the 9V rail - the 5V works just fine before and after the fuse
> blows on the other side.

Depends on how much current it's drawing.  You can decrease the wattage of the 
bulb until it stops blowing the low voltage fuses... (300W, 150, 100, 60, 40, 
etc) then you can work safely.

But I would disconnect caps, regulators and components until you either stop 
blowing fuses or the light bulb goes out.  

Another, safer way you could also do this passively, with the power shut off  
and disconnecting the transformer by just watching the resistance of the 9V 
circuit with a DMM Ohm meter (after you disconnect the transformer).


> Now, if I understand you correctly, I can hook this up so that the
> bulb lights up iff there is a short? 

Yes.  Or more specifically.. the closer you get to drawing 3Amps on the AC 
side (with a 300W bulb), the brighter the bulb gets (the ampergace will not 
be GREATER than 3Amps because the bulb limits the AC current to three amps 
(pre transformer amps) because the bulb's wattage (300) will limit it to 
300W/110v=2.7A.  If you want to limit it to around 0.9A, then switch to a 
100W bulb.  Get it?


> I'm a little fuzzy on that so I 
> don't suppose you have a picture? I actually have extra light bulbs,
> so this might work out well!

meh.. I googled for some.. but it's not an off the shelf lab/bench device.  
Just old timers who do a lot of bench work build them.  Let me render an 
ascii schematic for you:
               _ bulb  switch
-AC-hot-------/ \------o/ o ---o AC hot 
-------------------------------o AC gnd
-------------------------------o AC neutral

(switch to mono space fonts to view that)

In your case.. I would try the passive DMM/short detection method first (have 
to disconnect the xformer from the board).  Then if that doesn't help yo 
ufind it.. do the light bulb thing.

Tweeks