Power Supply Woes

Thread Starter


Joined Nov 16, 2010
Hello all, I've been working on a 28v 3a power supply for an old computer I have, but I've hit a standstill with my circuit. I have a transformer with 24v 4a secondaries, which is rectified with an 8a standard bridge and 3300uf worth of filter caps. This brings the clean DC up to about 34v. All working well. Next I have a basic lm317 circuit with a pot to select between 4 and 30v, also working, I can plug a 12v fan or bulb in at the appropriate pot settings with no problems. To boost the current to 3a I have a 2n3055 pass transistor mounted on a heatsink, with the base pin tied to the 317's output and the collector tied to 34v DC. When I put my computer fan on the emitter to test, it 'jitters' back and forth, as the voltage is spiking all over the place (between 12 and 0). All power everywhere else seems good, don't understand. It worked for a few mins when I first build it then after recycling it did the same. I've gone through two transistors now... Does anyone have any ideas? It is modeled around dozens of like circuits online, makes no sense.


(sorry if this is the wrong forum)
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Joined Jul 17, 2007
The forum you put the thread in is just fine.

What is missing is the exact schematic on how your circuit is configured at the moment.

Simply saying that "It is modeled around dozens of like circuits online" means it could be in any of dozens of configurations.

The more complete your schematic is, the better the chances of you receiving a useful answer.

You will find that we are pretty consistent around here; if the first post does not include a schematic, the first reply will ask for one. Without a schematic, all we can do is make random guesses; and that wastes everyone's time.

It sounds like you are using the 2N3055 as a voltage follower. It's output will go all over, as only the base will have regulation.

You need to have the 2N3055's output in the control loop of the voltage regulator.

See National Semiconductor's datasheet for the LM117/LM317:
Look at the schematic on the top of page 17, "High Current Adjustable Regulator"

You will also need a diode from Vout to Vin, cathode towards Vin. I don't have time to draw it up at the moment.
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