I just brought my new Steady-Volt 24Amps, 13.8Volts power supply
that cost several thousand bucks at RadioCom somewhere there in
Pasay. I was excited and at the same time curious on what is inside
of the unit. When I came home, I did not bother to check
the output voltage if it was correct but I did open the
unit to check the inside. To my surprise, there was not even one IC
present in the regulator board of the supply. Further checks revealed
that regulation was accomplished using an ordinary zener diode,
resistors,capacitors and few transistors. To my contentment, I tried
to trace their original circuit to find out how it works and the
results are:
1. Zener diode was used to provide regulation. It is known that
zener diode is not capable of sustaining good regulation
specially on higher current.
2. The power transistors were wired in series
pass configuration and is installed after the regulator. This design
flaw is the main reason why the supply could not maintain
regulation when the load draws high current.
3. The OVP circuit (over voltage protect) is continuously on (relay)
while the power supply unit is idle. With this condition, the relay
coil might easily burned and will not sustain long hours of continuous
operation.
Several research on the internet leads me to the data sheet of LM317
from National Semiconductor which have the example applications of
their product. One example of LM317 circuit looks promising and can be easily
be replaced with a 7812 regulator to eliminate the use of biasing resistors.
The regulator circuit employs a PNP transistor that drive the bases of the
current hungry pass transistors. The 7812 regulator IC was wired as
wrap-around thus,it can able to maintain good regulation even if the load
draws more current. Prototype followed after I have secured all the
needed parts on the circuit. Final testing of the modified unit showed a
very stable output voltage at a varying load currents. My modified power
supply was able to power my IC-2200H at 65 watts and to the IC-720 HF
transceiver 100 watts on SSB. ---73 de hevir
