12V, 3A Power Supply

This 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 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.

Circuit diagram :

12V, 3A Power Supply-Circuit-Diagram

12V, 3A Power Supply Circuit Diagram

Normal 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 12.5cm2 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.

Author : D. Prabakaran - Copyright :EFY