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Re: [WTB] TED 8360 and PLA for Commodore 16/116 [message #210407 is a reply to message #210405] Wed, 30 October 2013 12:38 Go to previous messageGo to previous message
Ray Carlsen is currently offline  Ray Carlsen
Messages: 75
Registered: March 2012
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On 10/30/2013 12:14 AM, Computer Nerd Kev wrote:
> On 29 Oct 2013, George wrote:
>
>> SbM says...
>>
>>> 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.

> Hmm... that's a cheat I haven't seen before either.
<snip>

I have. As a TV repair tech in the tube days, I saw it in the low
voltage PS of portable TV's so they could use a smaller and cheaper
regulator. The heat that would normally be thrown off by a big heat sink
was passed to the resistor. That got hot instead but wasn't prone to
failure as a regulator (IC or power transistor) would be.
Single chip regulators like the 7805 have something called
fold-back current limiting, which means if the load becomes excessive (a
short downstream), the voltage will drop to limit current flow and
prevent damage. That's how they legally got away with no fuses in the
+5V line inside the C64 "brick". However, if the regulator shorts input
to output, all bets are off. That's what kills chips in the C64 because
that voltage goes upwards of 11 volts at failure! Chips rated for 5
volts are "eaten" very quickly. A fuse wouldn't protect against that
kind of failure anyway but it does prevent the overload from causing a
fire. For that, most transformers have a heat sensitive thermal cutout
built into them. It opens if the transformer gets too hot. That's why
you never hear of a C64 PS, as bad as they are, from starting a fire.

Ray
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