Transistor voltage regulator
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M$2 Answers
The 2SC2335 is a very poor choice due to low gain and low power. The gain drops substantially above 2 amps, and is just above 10 below 2 amps. It is intended for high voltage applications.
Use a high current transistor like 2SC5200. It has gain of almost 100 at 5 amps, so the base current will be about 60 mA. Power dissipation will be about 20 watts, so a good size heat sink will be required.
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M$
http://focus.ti.com/docs/prod/folders/print/tps54620.html
Or you can get a free sample from ti of a premade module like this:
http://focus.ti.com/docs/prod/folders/print/ptn78020w.html
I would suggest getting the through hole version and read the data sheet to set the correct output voltage. Make sure that the computer power supply can source enough current. If you need help feel free to ask.
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M$
I chosen 2sc2335 because I had it at hand. Thank you for your answer I'll use a 2SC5200 and a 150R 1W resistor to base.