[personal] Re: Altair 680 power supply...

Chris Elmquist chrise at pobox.com
Tue Jan 19 07:38:52 CST 2010

On Tuesday (01/19/2010 at 12:44PM -0000), Mike Hatch wrote:
> ----- Original Message ----- From: "Dave McGuire" <mcguire at neurotica.com>
> To: "General Discussion: On-Topic and Off-Topic Posts"  
> <cctalk at classiccmp.org>
> Sent: Tuesday, January 19, 2010 8:34 AM
> Subject: [personal] Re: Altair 680 power supply...
>> On Jan 18, 2010, at 5:00 PM, Chris Elmquist wrote:
>>> Is it fair (as in rational, smart, non-stupid) to put multiple 7805 in
>>> parallel?  Can't say I've ever seen that done.  Only the more typical
>>> design of adding a pass transistor to carry more current.
>>   What about balancing problems?  They won't be identical from part  to 
>> part..
> What's the current ?, 78S05 will give 2A

The discussion was about a way to get more current from a 7805 design
that uses a bypass resistor to cheat the regulator.   A number of TO-220
packaged options were suggested but I was curious if one could get there
with a different sort of cheat--  since plain old 7805 are so cheap and
plentiful (still).

> 7805 cannot be directly paralleled due to the small difference in 
> regulation points part to part. You would need a small value series 
> resistor per part to load share.

Right.  I saw one other suggestion that in addition to the series resistor
you would also bring them into a Schottky diode summing network which
presumably keeps them from back driving each other.

In any case, you need enough other stuff to accomplish this that
retrofitting with an LM323, 78S05 or any of the drop-in switchers is
probably a much better option.


Chris Elmquist

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