|Re: [WTB] TED 8360 and PLA for Commodore 16/116 [message #210393 is a reply to message #210385]
||Mon, 28 October 2013 19:31
Registered: February 2013
> As regards the C16/116, I guess any good-quality
> regulated 9VDC power supply should be OK, right?
My C16 came with a pretty strange power supply setup inside.
Of course it has a 7805 regulator to produce +5VDC from the
+9VDC input supply. But in parallel with the regulator
circuit there's a 20-ohm, 5-watt resistor. So part of the
input supply goes directly from the 9V input to the output
of the regulator. I assume the resistor is chosen so that
by itself it produces a bit less current and voltage than
the computer's minimum requirements, and the regulator just
provides a small amount of additional power to keep the
output at a regulated 5V.
This setup greatly reduces the load on the regulator, which
is good, but it does mean that if the regulator fails, some
amount of unregulated current will still flow through the
resistor. It also means that if there's a spike on the 9V
input, it may get through the resistor whereas it wouldn't
make it past the regulator.
I've never seen this setup used anywhere else. It seems a
bit risky, but it means the resistor dissipates most of the
power consumed in dropping the voltage from 9V to 5V, so you
don't have to have a massive heat sink on the regulator,
which is probably why they did it that way.