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. 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:
Calculations:
This circuit is a fine example of
Kirchoff's current and voltage laws. To summarise, the sum of the currents
entering a junction, must equal the current leaving the junction, and the
voltages around a loop must equal zero. For example, in the diagram above, the
input voltage is 24 volts. 4 volts is dropped across R7 and 20 volts across the
regulator input, 24 -4 -20 =0. At the output :- the total load current is 30
amps, the regulator supplies 0.866 A and the 6 transistors 4.855 Amp each , 30 =
6 * 4.855 + 0.866. Each power transistor contributes around 4.86 A to the load.
The base current is about 138 mA per transistor. A DC current gain of 35 at a
collector current of 6 amp is required. This is well within the limits of the
TIP2955. Resistors R1 to R6 are included for stability and prevent current
swamping as the manufacturing tolerances of dc current gain will be different
for each transistor. Resistor R7 is 100 ohms and develops 4 Volts with maximun
load. Power dissipation is hence (4^2)/200 or about 160 mW. I recommend using a
0.5 Watt resistor for R7. The input current to the regulator is fed via the
emitter resistor and base emitter junctions of the power transistors. Once again
using Kirchoff's current laws, the 871 mA regulator input current is derived
from the base chain and the 40.3 mA flowing through the 100 Ohm resistor. 871.18
= 40.3 + 830. 88. The current from the regulator itself cannot be greater than
the input current. As can be seen the regulator only draws about 5 mA and should
run cold.