ELECTRONICS FOR YOU

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OCTOBER 2001

C I R C U I T

I D E A S

S.C. DWIVEDI

12V, 3A POWER SUPPLY

D. PRABAKARAN

mA (which can be increased to several
amperes with additional pass transistors),
output short-circuit protection, and lower
input voltage.

A low power dissipation is achieved

by driving external series-pass transistor
2N4241 (T1) from pin 2 of CA3085. Nor-
mal output pin 8 is returned to ground
via diodes D3 and D4 to ensure error am-
plification operation in the linear region.
Ripple rejection is approximately 50 dB
on no load and 35 dB on full load.

A 2x2x2.5cm aluminium heat sink fas-

tened onto a 1.5mm blackened aluminium
sheet of 12.5cm

2

area on 2N4241 helps

the circuit in dissipating heat without ex-
ceeding maximum device ratings.

CA3085 can dissipate up to 650mW

power in free air, without any heat sink.
AFCO-make C-05-4 heat sink is suitable
for this IC. An improper heat sink may
cause device junction temperature to ex-
ceed the limit, resulting in progressive
deterioration of the device.

T

his circuit provides a 12V regu-

lated power supply with output
current up to 3 amperes. It is spe-

cially designed for use with 2m handheld
rigs with linear power amplifier and CB

portable QRP rigs.

The circuit uses monolithic IC CA3085

voltage regulator in 8-lead TO-5 package.
Its salient features include good load and
line regulation, output current up to 100