12 Volt 30 Amp PSU
Using a single 7812 IC voltage regulator and multiple outboard pass
transistors, this power supply can deliver output load currents of up to 30 amps. The
design is shown below:
Notes:
The input transformer is likely to be the most expensive part of the entire project. As an
alternative, a couple of 12 Volt car batteries could be used. The input voltage to the
regulator must be at least several volts higher than the output voltage (12V) so that the
regulator can maintain its output. If a transformer is used, then the rectifier diodes must
be capable of passing a very high peak forward current, typically 100amps or more. The 7812
IC will only pass 1 amp or less of the output current, the remainder being supplied by the
outboard pass transistors. As the circuit is designed to handle loads of up to 30 amps,
then six TIP2955 are wired in parallel to meet this demand. The dissipation in each power
transistor is one sixth of the total load, but adequate heat sinking is still required.
Maximum load current will generate maximum dissipation, so a very large heat sink is
required. 30 amps at 12 volts will generate at least 360 watts of heat, each transistor
will dissipate a 1/6th of the output current by Kirchoff's law and hence dissipate at least
60 watts of heat. In considering a heat sink, it may be a good idea to look for either a
fan or water cooled heat sink. In the event that the power transistors should fail, then
the regulator would have to supply full load current and would fail with catastrophic
results. A 1 amp fuse in the regulators output prevents a safeguard. The 400mohm load is
for test purposes only and should not be included in the final circuit. A simulated
performance is shown below:
As shown above, each transistor contributes around 4.5A to the total load
current. The base current is about 40 ma per transistor. A DC current gain of 45 or greater
is required for each power transistor in order to reach maximum output current.
