Either you underestimated the temperature rise of this 15 amp transistor or you just happen to prefer bulky parts over the TO-220 plastic cases with metal tabs and are hoping for the best. So yes I would say there is something wrong with this picture. It would be like using jumper cables to drive 10 Watt speakers. Now considering you got this really big low thermal impedance transistor, does it make sense to mount it on a really "high" resistance small chunk of extruded aluminum metal. No heat sink gives you 50 to 70'C/Watt junction to ambient thermal resistance.Ĭonsidering your heatsink may only increase the "effective" surface area of the TO-3 by eyeball, a factor of 4, I would estimate your thermal resistance is around 12'C/Watt. This pushes the reference voltage of the 7812 regulator up by 0.7V giving roughly 12.7V at the output.The advantage of the TO-3 case is <1.6'C/Watt thermal resistance vs the TO-220 of <1.8'C/Watt. If you add a silicon diode from the GND pin to your circuit ground, there is a voltage drop of 0.7 ish volts across the diode when conducting. If the voltage at the load were to go up the transistor would limit the current as Vbe would drop (turning the transistor off - well slightly more off) which would cause the voltage to drop back down.īy using a 7812, it regulates to 12V at Vout relative to its GND pin. If the voltage across the load were to drop the transistor would have to compensate and pull it back up to maintain 0.7V Vbe. So if you apply 12.7V to the base, you should get about 0.7V less at the emitter - so 12V at the emitter in this case.Īs long as there is a load attached, and there is sufficient voltage at the input of the regulator, the transistor will always be driven on, so there will be 0.7V Vbe. When an NPN Bipolar transistor is on, there is roughly 0.7V to 1V Vbe (voltage at base relative to emitter). 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. To provide 150 mA of base current at a supply voltage will dissipate. 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.
How to install 2n3055 transistor full#
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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