Cheap and quick power converter

In another page I offered the design of a 3A 12v->5v power converter using one of National's SimpleSwitcher packages. While this is a nice and very efficient way to obtain a lower voltage from a 12v automotive (or other) source it is far from the cheapest nor is it the simplest.

I had need recently of another 5v supply and turned to a simple design using only five parts; a 7805 positive voltage regulator, a TIP42C pass transistor, a resistor and two filter capacitors. This works admirably to power a low-current 5v device such as a USB hub that does NOT have large current devices attached. If you need lots of current you'll need to modify this to use a transistor that will pass more current and use a larger heatsink.

For this converter I used a small TO220 heatsink on the pass transistor, none is needed on the 7805. As a matter of fact I could have used a 78L05 low-power regulator; the amount of current going through the regulator is very small as the pass transistor carries most of the load.

The schematic above shows how simple the circuit is. The layout above that is the small board that I made to carry the parts. The board was left large so that it could sit nicely in an old project box that I had lying around from a previous project.

The TIP42C was left with enough spacing around it to fit the heatsink. All of the other parts are placed to keep things compact just in case I want to use a much smaller project box. The 12V input lead and 5V output lead are both strain relieved using some copper wire (a small cable tie would do).

I chose a 5 ohm resistor for R1 which should keep the maximum current through the TIP42 below it's maximum value even though I hope I'll never draw more than just over 2A from the supply.

This circuit can be used with any 78xx part to deliver 5, 6, 8, or 9 volts from a 12V source. Below is an 8v unit to power my Canon DSLR from 12v. The same circuit just 'bent' to put the 7808 and TIP42 side-by-side.

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