15 May
2012
15 May
'12
12:43 a.m.
David Griffith wrote:
John Robertson wrote:
That's why good linear power supplies have an overvoltage crowbar
circuit. Output voltage over a threshold will trip an SCR or the like,
directly across the output, blowing the fuse. The load never sees a
voltage above the crowbar threshold. In a 5V TTL system, this might be
set at 6.5V, since the absolute maximum rating of most 5V TTL families
is 7V. (Unfortunately some newer 5V CMOS families have abs max at less
than 7V.) The MC3423 was often used for a highly integrated crowbar
controller, but it appears to be discontinued.
Of course, they don't put a crowbar circuit into cheap consumer crap,
since they're absolutely delighted if you have to replace it at some
point after the warranty has expired. They're playing the odds that the
failure isn't too likely to happen during the warranty period.
One advantage to using a switching supply to
replace the old linears
is that failure in linears can put the supply B+ through the
regulator transistor and this may let the smoke out of ICs, whereas
switching supplies simply shut down when they fail. Shut down is much
preferred to roasted electronics!
Ah, yes. This is why one can easily find replacement switching power
supplies for the Commodore 64 and friends. I've had a stock brick
blow out on me, but fortunately it didn't kill the computer. Others
in my cohort were not so lucky.