wrote in message
oups.com...
Hi all,

You may recall me mentioning I was thinking about adapting a 24V PSU
for use at 13.8V for powering up mobile HF rigs and stuff. I had some
spare time yesterday and took this thing to bits for a better look
inside. It turns out the mains transformer has several secondary
tapping points. I discontected the DC-stab/reg side of the supply
circuitry and reconnected the secondary tapping points to lower voltage
tappings.
Now I measure 16.8V after the transformer's output has been rectified
and applied across the *big* electrolytic. This is under no-load
conditions. Now, is that 3V enough 'headroom' to use up in regulation
and stabilisation for 13.8V final output if I throw the old 28V
reg/stab away and build something from scratch? There are 6 pass
transistors plus a driver (2n3055) already mounted which I plan to use
in the adapted version, so basically I'll just junk the existing
control board and re-use everything else. Any observations/tips?
Thanks,
P.


Paul -

An ideal secondary voltage is about 5 more volts than you want to regulate.
Let's say that it is 13.8 volt finished.
Then and ideal secondary voltage would be 18 or 19 Volts AC on the
secondary.

Using 6 pass transistors (2N3055) - The 2N3055 is good for an intermittent
current or amp draw of 10 amps.
A heavier one that is often used for 13.8 VDC supplies is the 2N3771.
This one is good for 15 amps intermittent. Both of these are NPN type
transistors.
These are placed in a parallel circuit, if more that 10 amps surge, is
required.

Heat is the biggest enemy of our power supplies.
Keep them cool, use well sized heat sinks and they last a lot longer!!

You will find that the LM723 is the most used regulator IC for commercial
linear supplies -
it has been around since the 1970s !!

Here are some web site articles and references for both your design and
building.

David Metz, WA0AUQ Classic web article (November 1996)
Regulated Power Supply Construction
OR What is Inside your Astron?
http://www.seits.org/features/pwrsup.htm

Metz's schematic
http://www.seits.org/features/pwrsup.gif

Astron Power Supply schematics
See how the "best of breed" US commercial unit is built
http://www.repeater-builder.com/astr...ron-index.html

KBT Linear Regulated Supplies
Specification sheets on pass transistors, ICs, DIY, How to, FAQ
Index for multiple knowledge pages is gray bar on left column of main page
http://www.kbt-dc-supplies.com/index.php

Best of luck with the project !

w9gb