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From: jhs@disunix.UUCP
Newsgroups: net.micro.atari16
Subject: Re: Atari 520ST power supply failure
Message-ID: <8605221723.AA21666@mitre-bedford.ARPA>
Date: Thu, 22-May-86 13:23:42 EDT
Article-I.D.: mitre-be.8605221723.AA21666
Posted: Thu May 22 13:23:42 1986
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One point that may clear up some confusion.  When you hang too much load on
a circuit like the MMU, this does at least three things:

1.  It causes a DROP in the voltage at the load.
2.  It causes more current to flow through the source device, even though
    each individual load device may draw a slightly reduced current.
3.  It causes an INCREASE in the voltage drop ACROSS COMPONENTS INSIDE THE
    MMU, and thus an increase in power dissipation there.

What is especially damaging is when the driving circuit (MMU in this case) is
so overworked that it goes out of "saturated mode", in which the voltage drop
inside the switching transistors is very low, and into a "linear" mode in
which substantial voltages can develop, say, across base-collector junctions.
Substantial may mean 2 or 3 volts instead of the normal 0.2 or so.  Multiply
this by a 2x increase in current, and you can see why something might fail.

I haven't investigated the ST upgrade problem, and in fact don't own an ST
yet, but I suspect this is what is going on.

-John Sangster
jhs@mitre-bedford.arpa