C I R C U I T I D E A S
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
D. PRABAKARAN
by driving external series-pass transistor
2N4241 (T1) from pin 2 of CA3085. Nor-
his circuit provides a 12V regu- portable QRP rigs.
mal output pin 8 is returned to ground
lated power supply with output The circuit uses monolithic IC CA3085
via diodes D3 and D4 to ensure error am-
Tcurrent up to 3 amperes. It is spe- voltage regulator in 8-lead TO-5 package.
plification operation in the linear region.
cially designed for use with 2m handheld Its salient features include good load and
Ripple rejection is approximately 50 dB
rigs with linear power amplifier and CB line regulation, output current up to 100
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 12.5cm2 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.
Z'
ELECTRONICS FOR YOU Z' OCTOBER 2001
Z'
Z'
Z'
S.C. DWIVEDI