2N3055 Power Supply

power supplytransistorsvoltage-regulator

Could someone explain how this circuit works and how the output is maintained at 12V?

schemstic http://www.radanpro.com/Radan2400/Napajanje/7812+2N3055.jpg

There is very little information on how the circuit works online. I know the 12V output from the 7812 is used to bias the power transistor, but how is the voltage at the emitter held at 12V too? Any help with this would be greatly appreciated.

Best Answer

In fact, the output is not held at exactly 12 volts. If the 7812 output is at 12 volts, then depending on the current levels at the load the output voltage will be less than 12 volts, and possibly a good deal less.

EDIT - As Russell McMahon pointed out, I was ignoring the diode in the linked circuit. I was also ignoring the fact that 7812s have a nominal output in the range of 11.4 to 12.6 volts. Combining the two suggests that the base voltage will be in the range of 12.1 to 13.3 volts. Apply this as appropriate to the following. In effect, the added diode is intended to compensate for the transistor base-emitter voltage drop. And it does, but not very well. END EDIT

With the base held at 12 volts, the voltage drop of the base-emitter junction will determine the exact output voltage. Since this is a silicon transistor, the usual number is a voltage drop of ~0.7 volts. However, that number really only applies at currents around 1 to 10 mA. For 3 amps you should expect a voltage drop in the range of 1.5 to 1.6 volts. The base current supplied by the 7812 will be multiplied by the gain ($h_fe$ or $\Beta$) of the 3055, so the 7812 does not have to provide an enormous amount of current, although at maximum current the gain will drop, so you might expect a gain of 20 or so, for a base current requirement from the 7812 of 150 mA.

For an open circuit (no load), you can expect about 12 volts from this circuit. At 3 amps, something in the vicinity of 10.5 volts, or maybe a bit lower. This is not a very good circuit (but it is simple and cheap).

The other thing you need to watch out for is power dissipation. At full output and a 24 volt supply, The 2N3055 will be dropping about 13-14 volts, and will be carrying most of the current (at least 95%). So the power dissipated will be about 3 amps times 14 volts, or roughly 40 watts. If you build this, you must provide a fairly beefy heat sink for the transistor. Also note that, due to the low gain at high currents, the 7812 will dissipate significant power. To provide 150 mA of base current at a supply voltage will dissipate .15 amps times 12 volts, or 1.8 watts. This is more than a bare 7812 can handle without going into thermal shutdown, so you will also need a heatsink here as well, although not nearly as big as on the 3055.