29 Oct
2006
29 Oct
'06
5:56 p.m.
While repairing a floppy drive with a shorted a bypass cap, I noticed
that the manufacturer had very thoughtfully inserted a picofuse in
the DC supply line. Predictably, it was the 10 ohm resistor in
series with the voltage regulator that turned to toast. The fuse is
none the worse for wear.
You were lucky, considering that resistors are cheaper and easier to get
than picofuses...
This brought to mind an old friend who earned her
living as a field
engineer for DEC. At one point, she mentioned that resistor- and
transistor-protected fuses were a regular pattern for DEC equipment.
This would be back in the early 70's, BTW.
Did her observation reflect reality? Are there any other
manufacturers that routinely engaged in this practice?
It's certainly the case for transistors, which fail very quickly when
overloaded (much more quickly than even 'FF' fuses). It's a well-known
fact to those that repair SMPUS that the chopper will fail long before
the fuse does (Actually, I beleive the fuse is there to prevent the whole
thing catching fire, not to protect components in the supply).
-tony