hams on classiccmp

Tony Duell ard at p850ug1.demon.co.uk
Mon Jan 19 14:02:03 CST 2009

> > That depends, a lot, on the PSU design. Certainly SMPSUs can do some
> > very odd things if a capacitor goes open-circuit or just high ESR. How
> > much damage that does to the rest of the PSU or worse the rest of the
> > machine depends on the design.
> Do you think that the designers of this stuff have learned which approaches 
> they might want to stay away from?  :-)

Alas my expeerience suggests the reverse. Back when computers were 
expensive, and chips were expensive, the manufacutrers took the trouble 
to put all sorts of protectinon in their machines to protect said 
expensive decices in the event of a failure. Now they don't bother.

A triviel example, but it's almost a classic computer. As is well-known 
when you have a multiplexed LED display, the instantaneous current 
through each LED is much higher than the sort of current that you'd pass 
continuously to have the display at a suitable rightness. High enough 
that if the scanning fails for any reason, the display LEDs will be burnt 

In the HP98x0 series of machines, there are monostables triggered from 
the display stroe signal so that if the processor side of things 
malfucntions, the display will be blanked. It's impossible for one 
digit/column to be stuck on.

They did a similar thing in the 59309 digital clock, but that's more 
reasonalbe since the scanning cna be derrived from an external 
oscillator. If you switch it to 'Ext' with no oscillator connected, then 
the display could be damaged without this protection circuit.

But how many modern devices have such prtection circuits fitted? Darn it, 
many modern PSUs don't even have crowbar circuits.


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