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Re: [Amps] PC power supplies in series

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Subject: Re: [Amps] PC power supplies in series
From: "David Lisney" <>
Date: Sat, 3 Jun 2006 23:02:09 +0100
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Although I have not tried PC power supplies in series I have in the past 
used a pair of 5v 40a switchmodes in series, I modified both to give about 
7v out in order to end up with approximately 13.8v out....

I also modified the crowbar circuits in them to trip at about 7.5v out....

The results were good... and bad....

Running a relatively low power packet station 24/7 the electricity 
consumption was noticeably less than the linear power supply that had 
previously powered it... this surprised me as although the linear supply had 
24v on the reservoir capacitors hence about 10v dropped in the pass 
transistors and a kind of implied efficiency of about 60% the load was just 
a TNC and a converted Storno PMR turned down to about 5w output. I doubt my 
station used much more than about 2a at 13.8v on transmit...

The switchmode ran stone cold, and the tiny room the radio gear was in 
seemed cooler too when I started to use it. (I was a youngster at the time 
living in digs)....

The bad news with my switchmode arrangement was it's behaviour when driving 
a bigger load such as a 160w 2m amplifier... as the load went off the power 
supply suffered overshoot and crowbarred... the nasty fix was a massive 
electrolytic across the output... this seemed to tame the overshoot a bit 
and stopped it crowbarring....

Like many projects I actually assembled this creation in a nice case with 
loads of output terminals......  it looked ok and worked fine for a long 
time.... then like many other ham projects I murdered it with stupidity... 
I am embarrassed to admit it involved "zapping" defective Nicad cells... (a 
long tradition in British ham radio).

I think this highlights a point made by another poster, although these 
supplies were current limited they get a little confused when run into 
current limit when wired in series, a supply can shutdown and then be 
presented with voltage backwards across it's output,
there may be merit in the diode across each one for this very reason.....

FWIW some unmodified PC supplies are now rated for pretty high current on 
their 12v rails... one I used years ago had a 15a 12v rail (very handy for 
running peltier heat pumps)

To the people that emailed me regarding the transistor tetrode bias supply I 
apologise for not yet uploading the diagram or pictures, I will soon.

Regards David

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