High-Power Car Battery Eliminator


To operate car audio (or
video) system from household 230V AC mains supply, you need a DC
adaptor. DC adapters available in the market are generally costly and
supply an unregulated DC. To overcome these problems, an economical and
reliable circuit of a high-power, regulated DC adaptor using reasonably
low number of components is presented here. Transformer X1 steps down
230V AC mains supply to around 30V AC, which is then rectified by a
bridge rectifier comprising 5406 rectifier diodes D1 through D4. The
rectified pulsating DC is smoothed by two 4700µF filter capacitors C1
and C2.

Circuit

Circuit diagram

The next part of the circuit is a series-transistor regulator
circuit realised using high-power transistor 2N3773 (T1). Fixed-base
reference for the transistor is taken from the output pin of 3-pin
regulator IC1 (LM 7806). The normal output of IC1 is raised to about
13.8 volts by suitably biasing its common terminal by components ZD1 and
LED1. This simple arrangement provides good, stable voltage reference
at a low cost. LED1 also works as an output indicator. Finally, a
crowbar-type protection circuit is added. If the output voltage exceeds
15V due to some reason such as component failure, the SCR fires because of the breakdown of zener ZD2.

Once SCR fires, it presents a
short-circuit across the unregulated DC supply, resulting in the blowing
of fuse F1 instantly. This offers guaranteed protection to the
equipment connected and to the circuit itself. This circuit can be
assembled using a small general-purpose PCB. A
good-quality heat-sink is required for transistor T1. Enclose the
complete circuit in a ready made big adapter cabinet as shown in the
figure.


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