Increasing Regulator Current
Although the 78xx series of voltage regulators
are available with different current outputs, you can boost
the available current output with this
circuit. A power transistor is used to supply extra current to the load
the regulator, maintaining a constant
voltage. Currents up to 650mA will flow through the regulator, above
this value and the power transistor will
start to conduct, supplying the extra current to the load. This should
be on an adequate heat sink as it is likely
to get rather hot. Suppose you use a 12v regulator, 7812. The
input voltage should be a few volts higher
to allow for voltage drops. Assume 20 volts. Lets also assume
that the load will draw 5amps. The power
dissipation in the transistor will be Vce * Ic or (20-12)*8=40watt.
It may keep you warm in the Winter, but
you will need a large heatsink with good thermal dissipation.
If you want to increase the output current
with a negative regulator, such as the 79xx series, then the circuit
is similar, but an NPN type power transistor
is used instead.