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.

This is probably a stupid idea for some reason that's beyond me right
now, but how about losing the extra volts with a series of diode drops?
Provided the diodes were high forward current types with adequate heat
sinking. I know it wouldn't be as stable as a proper design, but would
it work well enough to run a mobile 100W rig?

wrote in message
oups.com...
This is probably a stupid idea for some reason that's beyond me right
now, but how about losing the extra volts with a series of diode drops?
Provided the diodes were high forward current types with adequate heat
sinking. I know it wouldn't be as stable as a proper design, but would
it work well enough to run a mobile 100W rig?

I think you'll end up with lousy regulation and
a ripple problem.

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.

I would cobble together a simple, say lm317 regulator to drive your series
pass driver and see what you come up with under some small loads. You may
be ok with that big transformer. The power supply I built uses 4 2n3055 in
parralel like what you have. Have fun.. JTT

James Thompson wrote:
I would cobble together a simple, say lm317 regulator to drive your series
pass driver and see what you come up with under some small loads. You may
be ok with that big transformer. The power supply I built uses 4 2n3055 in
parralel like what you have. Have fun.. JTT
===========================
There as an exellent design in RadCom Oct 2001 ,pages 34-35 by OZ1XB ,
with a LM317 , TIP32 (PNP) and 2xBUV20 (NPN) ,with the latter
replacable by 4x2N3055 or whatever you have .
Simple,rugged ,reliable design.
It incorporates sensing lines ,ensuring the full set voltage under all
load conditions at the consuming apparatus.

Frank GM0CSZ / KN6WH

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

Sounds like a good start

Don't forget the 6 x Emitter ballast resistors (each should drop 100mV
at full load), so maybe 10 milli ohms each (short length of electric
heater element ?).

May need a fan to keep the heatsink cool.

You probably also need to mount the driver transistor on a heatsink.

Regulation using an LM723 or similar. There should be plenty of
detailed designs out there - also see the Nat Semi application notes on
the subject.

See http://www.national.com/pf/LM/LM723.html

Regards,

Mark.