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Pass Transistors

pass transistor array with heat sink

One way of making your power supply produce more amps or current, is to use pass transistors. They are called a pass transistor because they enable amps to pass while the voltage is kept constant. A real common transistor to use is a 2N3055. It is good for an intermittent current or amp draw of 10 amps. A heavier one to use is a 2N3771. This one is good for 15 amps intermittent. Both of these are NPN type transistors. These are placed in a parallel circuit, if more that 10 amps surge, is required.

Some of the transistors that I have used: 2N3055, 2N3772, 2N3771. There are too many NPN type transistors, that can be used, to list here. I like and favor the NPN. None of these transistors can handle a constant load that is towards the nature of a 100% duty cycle. Meaning, on all the time, 100% of the time at full load or rating.

One would need to build the pass transistor array, so as to have a very super cooling effect to have the set up handle a constant load nearer to 100%. Most power supplies, in all honesty, can only do a heavy load for about 5 minutes on and 5 minutes off. That being a 50% duty cycle. It is not a linear type situation, though. Way more cooling is needed to have 10 or more minutes on and then off.

Heat is the biggest enemy of our power supplies. Keep them cool, and they last a lot longer.

The collector of the pass transistor is connected to the rectified and filtered positive of the first stage, right out of the transformer secondary. The base of the pass transistor is connected to the output of the voltage regulator. The emitter is the positive output of the power supply. When using more than one pass transistor, there needs to be a small amount of resistance put on either the base or the emitter of each pass transistor that is in parallel. This is to make each of the pass transistors do an equal amount of work. I choose to put a .1 OHM at 10 watt on the emitters. They can be most any low value resistor. I have used resistors under 1 ohm. Just as long as they are all the same value. They do, however, need to be at least 10 watt. When there is maximum current being drawn, they get very hot very fast !

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