Modifications to 13.8V Output
(See circuit diagrams in the original articles for part-number references.)
These modifications come in three basic levels:
- Simply accept the existing +12V DC output, or tweak RV1 to increase it. To
reach 13.8V, you sometimes need to change R8 to the next-lower standard
value. This will give up to 10A (the existing rating of the +12V output)
with a voltage drop of less than 1V on full load.
Note: you will always need a load resistor on the 5V output - see the
- Go for higher current capability and better voltage regulation. This
1. Swapping D3 and D4 so that the 13.8V output uses the larger,
lower-resistance diode D3.
2. Modifying filter choke L2 (which can be a problem because there are no
other windings to reduce the core flux)
3. Modifying the fault detection circuits so that they monitor the new 13.8V
rail instead of +5V.
The weak point then is the secondary winding of T1, so the next level is...
- Rewinding the secondary of T1, which requires you to take
full responsibility for all the output and control circuits.
All of this can be done - the article by VK6APH
is an excellent example of a 'Level 3' mod - but letís also keep some perspective:
A typical 100W-class
transceiver requires 13.8V at 20A peak (260W), which is beyond the capabilities
of any PSU rated for a total output of only 200W. Even if a 300W PSU can be
modified to deliver 13.8V, it will be marginal - and may be stressed towards the
limits of reliability.
you seriously proposing to operate maybe £1000-worth of transceiver from a PSU
that wasnít designed for the purpose, that you have modified, and whose
circuit you donít quite understand? Does that seem like a sensible risk? It
doesnít to me.
The photograph in Part 2 shows a ĎLevel 1í modification of the PSU pictured in
- A mains rocker switch and Powerpole DC connectors are epoxied into
existing slots in the case
- A better mains input filter is bolted directly to the
case. This seems to be sufficient to kill all the output noise - nothing is
audible on an HF transceiver powered from this particular unit.
- Further options to reduce output noise would be to wind both output
leads on a ferrite toroid; ground the negative lead to the case
immediately behind the output socket; and bypass the positive lead to the
case, also immediately behind the output socket. I have not tried any of
- The metal-bodied 10W
load resistor for the +5V rail is bolted to the case, underneath the mains
filter. The case provides the heat-sink that these resistors need.
Note the widespread use of heat-shrink insulating sleeving in the
This PSU is reasonably safe when its
cover is replaced, but don't forget that it was originally designed for
use inside a PC case. Without this additional protection, it does not meet
modern safety standards because it's
possible to contact mains and other high voltages through the ventilation
slots... but the PSU will
overheat if those slots are blocked. TAKE CARE!