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:
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 bypassed by the outboard pass transistors. As the circuit is designed to handle loads of up to 30 amps, then six TIP2955 are pararelled to meet this demand. The dissipation in each power transistor is thus kept to a minimum but adequate heat sinking is required. Maximum load current will generate maximum dissipation, so a very generous heatsink will be required. Should the power transistors fail, then the regulator would have to supply full load current and would certainly be destroyed. A 1 amp fuse in the regulators output prevents this from happening. The 400mohm load is for test purposes only and should not be included in the final circuit. A simulated performance is shown below:
As can be seen above, each transistor contributes around 4.5A to the total load current. The base current is around 140mA. A dc current gain of about 45 or higher is needed for the maximum load requirement.
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