Rugged PSU For Ham Radio Transceivers


This rugged power supply
is based on the popular LM338 3-pin voltage regulator. The LM338 is
capable of supplying 5 A over an output voltage range of 1.2 V to 32 V
with all standard protections like overload, thermal shutdown,
over-current, internal limit, etc., built in. In this power supply, some
extra protections have been added to make it particularly suitable for
use with low to medium-power portable and mobile VHF/UHF
(ham) and 27 MHz transceivers. Diodes D4 and D5 provide a discharge
path for capacitors C1 and C2. Diode D8 protects the supply against
reverse polarity being applied to the output terminals. Capacitor C1
assists in RF decoupling and also increases the ripple rejection from 60
dB to about 86 dB.

Circuit

Circuit diagram

If junction R1-R2 is not grounded by switch S1A, transistor T2
starts to conduct, causing the regulator to switch to zener diode D7 for
its reference voltage (13 V). The PSU output voltage will then be 12.3 V. Normally, T2 will be off, however, and the PSU output voltage is then about 8.8 V. The high/low switch is useful to control the RF power level of modern VHF/UHF
handhelds. Transistor T1, a p-n-p type BC557, acts as a blown-fuse
sensor. When fuse F1 melts, T1 starts to conduct, causing LED D6 to light. If, for whatever reason, the PSU output voltage exceeds about 15 V, thyristor THR1 is triggered (typically in less than a microsecond).

Such a high-speed ‘crowbar’ may look like a drastic measure, but
remember that this kind of protection is required by digital ICs that
will not stand much overvoltage. The crowbar, when actuated, will
faithfully destroy fuse F1 rather than allow the PSU to destroy expensive ICs. The two LEDs
on the S1B contacts not only act as ‘high/low’ indicators but also as
power-on indicators which are turned off when the mains voltage drops
below about 160 V. If you envisage ‘heavy-duty’ use of the PSU,
then voltage regulator IC1 should be mounted on as large a heatsink as
you can get. The minimum we’d say is an SK129 heatsink from Fischer (Dau
Components).

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