# Help with variable power supply

Discussion in 'General Electronics Chat' started by Radioflyer, Jul 25, 2013.

Jul 25, 2013
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Please help if you can, I need to build a variable voltage DC power supply to power a device requiring 4-10v and draws about .1-.5A. I've looked at all kinds of circuit options (lm317, nte953, darlington pair, 2n3055 NPN, rheostats, etc) and I think i'm just confusing myself the further I get into it. I don't understand the possibility of using the LM317 and a potentionmeter when the power I need supplied is far greater than the 1/2 watt max a common 5k pot can handle... 5v @ 200mA, P=5v(.5A), I have to be missing something with my understanding of pots and voltage dividers... a 1/2watt max of power doesn't seem like much to provide to a device, which is the typical rating of the pots I've seen in the said circuits. If its easier, can somebody please refer me to an alternate method to build my required power supply? I would really like to make sense of this all.

2. ### ian field AAC Fanatic!

Oct 27, 2012
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1,156
The LM317 is pretty much a 1.25V 3-terminal regulator - you raise the reference pin above ground by a certain amount to get the output voltage you want. In a sense, the regulator regulates its own bias as the voltage divider (or pot) is wired from the regulator output to ground.

You can use a fixed ratio to set the voltage, or use a pot to make it variable - the current in the reference pin is so small that for most purposes its effect on the divider ratio can be ignored.

Jul 18, 2013
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+1 on the LM317, should give you what you need.
Max.

4. ### PackratKing Well-Known Member

Jul 13, 2008
843
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You may find a larger wirewound ceramic rheostat -- Ohmite -- will be better able to handle the wattage... follow the diagram provided with the LM317...

I built a 0 - 12 v variable supply, using a 10-turn 2 watt " Clarostat" as the adjustable element... still in service after 25+ years