On 2013 Mar 13, at 4:01 PM, Josh Dersch wrote:
Hey all --
This'll probably rank as a "duh" moment for most of you but I
thought I'd
share (mostly because it amused me):
I was trying to figure out why the unregulated 8VDC line of my
SWTPC 6800's
power supply kept dipping so suddenly under moderate load (and why
it would
keep falling and falling as long as I left it running that way...)
The overall behavior was:
- Power up with minimal load (just the motherboard w/it's onboard
regulator): 8V line was fine at about 8.7V.
- Add up to two RAM cards and the 8V was still at around 8.3 or so.
- Add a third card and 8V drops to 6.5V and continues to drop
steadily over
time.
This had me scratching my head. Then I happened to notice that the
fuse
(F1 on the 12V supply board) was glowing a dull orange and getting
brighter.
Well, turns out that someone (me!) had installed the wrong fuse --
it was a
2.5A fuse where a 10A was required (F2 takes the 2.5A, btw).
Apparently
when fuses start to give out they draw a lot of current -- lesson
learned
:).
Fuse replaced and all is happy. So, the lesson is: check your fuses
carefully.
Interesting fault mode, but I wouldn't express that as the fuse
"drawing a lot of current". That fuse should be the one in the +8V
line and it looks like what was happening is your cards were drawing
current just short of blowing the fuse. As the fuse filament heated
up it's resistance increased, so more of the +8 supply voltage was
dropping across the fuse, reducing the +8V output level. (Overall
current should actually be going down).
Yay, another SWTPC 6800 (I have one too).