e99ch08.pdf

rugged PSU

for ham radio transceivers

008

Tr1

400mA

D11

D12

IC1

LM338

D14

D13

1N4001

15V

2x 9V / 80VA

13V

4x 1N4002

Amplimo 31011

TIC126

LM338

18V

Th1

D 2

D 3

BC557

TIC

126

S1a

BC547

D10

S1b

D 6

1N4002

4700

100n

10 µ

63V

100 µ

63V

4 µ 7

63V

adj.

1N4002

994078 - 11

Design: N.S. Harisankar VU3NSH

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, overcurrent, 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 trans-

ceivers.

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 decou-

pling and also increases the ripple rejection from 60 dB to about

86 dB. If junction R1-R2 is not grounded by switch S1A, transis-

tor T2 starts to conduct, causing the regulator to switch to zener

diode D7 for its reference voltage (13 V). The PSU output volt-

age 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 use-

ful to control the RF power level of modern VHF/UHF hand-

helds.

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 dras-

tic 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’ indi-

cators 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 regu-

lator 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).

240V

ELEKTOR

50Hz

No. 994078

(994078-1)

F = 400 mA

Elektor Electronics

12/99

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