# Variable Power Supply

Discussion in 'Power Electronics' started by Stoneww, Apr 18, 2017.

1. ### Stoneww

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2
Apr 18, 2017
Hey I want to make a Variable Power supply but I'm not sure if it will work. My idea is to have a few batteries in series and then use a potential divider with a variable resistor to change the out put voltage.

Can anyone tell me if this would work? Thanks

I'm also thinking if I can re-arrange the formula to find specific values of VR1 (variable resistor) for a demanded output voltage, I've tried my self but couldn't do it.

Last edited by a moderator: Apr 18, 2017
2. ### Tha fios agaibh

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729
Aug 11, 2014
Not really. It's not practical for most uses.
You won't be able to put much of a load on it.

Once you add a load its resistance is paralleled R1 and the voltage will change .

Look into voltage follower and regulator circuits.

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3. ### davennModerator

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Sep 5, 2009
hi Stoneww
welcome to EP

^^ I agree with this ^^
It will be pretty much useless for anything practical

Use a proper variable voltage regulator like a LM317

that will handle up to around 1 to 1.2A if the IC is well heatsunk
use the TO3 case type for a solid 1.5 A capability

Use a TO3 cased LM388 with a good heatsink for up to 5A

Dave

Last edited: Apr 18, 2017
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4. ### Arpan Sarkar

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Apr 9, 2017
As the last two posters said....
Your idea is to have a few batteries in series and then use a potential divider with a variable resistor to change the out put voltage- You have made a potential divider circuit but this is not a good idea to control the out put voltage .
without any load the output should be as you want but with load :-
1.output voltage will change
2.if you put hi power load, resistors will burned up
Just use variable linear regulator or any type of other variable voltage regulator circuit and don't forget to choose them as per your output requirement.

Last edited by a moderator: Apr 18, 2017
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5. ### Stoneww

42
2
Apr 18, 2017
Thanks a lot , I'm just getting into electronics at home and I don't have much money so I'm trying to build my own stuff to save money but also gain experience. And would the circuit you provided work with a transformed 230v AC that has been full bridge rectified?

6. ### davennModerator

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Sep 5, 2009
yes, but change C1 to around 2200uF

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7. ### Tha fios agaibh

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Aug 11, 2014
Another idea for someone on a budget is to use an old computer power supply. They are filtered and regulated with several common voltage outputs. You could add Daves lm317 circuit or similar down stream.

8. ### bob monsen

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Jan 18, 2010
Hold on there pardner. When you say 'transformed 230v AC', if you use a bridge rectifier on the mains, you will end up with a bucketload of voltage, attached to a giant current source. Enough to light your hair on fire pretty quickly. Attach that to your little circuit, and it'll smoke, taking your house with it.

If you are using a transformer to convert the voltage down to, say, 15VAC, then it'll be OK to connect to a bridge rectifier. Use a small fuse in the mains path so if you short it out, you don't catch on fire.

Also, remember that mains voltage is very dangerous. You could instead use an old 12V wall brick from a discarded laptop, for example, which would be much safer than direct connect to the mains.

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9. ### davennModerator

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Sep 5, 2009
he said transformed mains implying a transformer down to a lower voltage before the bridge