"Lawrence Statton N1GAK/XE1" wrote in message
-did-not-set--mail-host-address--so-shoot-me...
writes:
Hi all,

Just bought this power supply ... 24V ...It's a heafty beast ...
... if I can drop the voltage down to 13.8 whilst retaining about
20 amps for the rig's TX current draw. ...Any pointers, please.


What you're saying is: You want to build a 20A 13.8V power supply with
OVP. That it happens to take 24Vdc instead of 120Vac is trivial.

Your options are -- design a nice switching converter (which based on
the tenor of your question is beyond your mettle at the moment) or
build an enormous linear regulator and dissipate a couple hundred
watts making a very nice place for the cat to nap Best to set that
24V supply on the shelf, hope some day to find a good use for it, and
either buy or build a 13.8V supply from scratch.


OR...OR...how about ... Unwind about half of the transformer secondary
and fiddle with the regulator (appropriately). You should maintain the
current level it originally had. The potential (sorry for the pun) for a
fault causing a disastrous overvoltage is less as well.
However be aware of the overhead voltage-- that is the difference in filter
output voltage relative to 13.8, so you have enough room for the regulators,
the ripple and mains dips. I did something like this with a 24V 5A supply
long ago to get 5V 25A (5V. TTL logic was big then). I actually rewound the
secondary with bigger wire to get the higher current capacity. ( also a
slight tweek on the regulator topology for improved overhead). It had the
common 723 regulator topology, so changing voltage was simply changing a
zener and I must have beefed up the series pass transistors.
Of course not knowing what your regulator topology is, this could be hard.
I also have a 15 amp., Kepco, rack-mount linear hernia generator. Then,
recently, a $99 MFJ 25 amp, hold-it-in-one-hand switcher. The ham fund had
it and it took a lot less time than the re-wind.

73, Steve, K9DCI