Also note that, due to the low gain at high currents, the 7812 will dissipate significant power. If you build this, you must provide a fairly beefy heat sink for the transistor. So the power dissipated will be about 3 amps times 14 volts, or roughly 40 watts. 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%). The other thing you need to watch out for is power dissipation. This is not a very good circuit (but it is simple and cheap). At 3 amps, something in the vicinity of 10.5 volts, or maybe a bit lower. 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.įor an open circuit (no load), you can expect about 12 volts from this circuit. For 3 amps you should expect a voltage drop in the range of 1.5 to 1.6 volts. However, that number really only applies at currents around 1 to 10 mA. Since this is a silicon transistor, the usual number is a voltage drop of ~0.7 volts. With the base held at 12 volts, the voltage drop of the base-emitter junction will determine the exact output voltage. In effect, the added diode is intended to compensate for the transistor base-emitter voltage drop. Apply this as appropriate to the following. Combining the two suggests that the base voltage will be in the range of 12.1 to 13.3 volts. I was also ignoring the fact that 7812s have a nominal output in the range of 11.4 to 12.6 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.ĮDIT - As Russell McMahon pointed out, I was ignoring the diode in the linked circuit. In fact, the output is not held at exactly 12 volts.
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