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How to make power supply variable?

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MitParmar

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I'm 1st year engineering student. I made following power supply for 1st year projects. I have to use this for future projects.

DualReg PowerSupply | Flickr - Photo Sharing!

This is the schematic of my power supply project. It gives fix 15v, -15v and 30v DC as output. I want variable output for future projects of next semester. So, please suggest me how to make it variable???

Can I use variable resistor?? Where it should be placed???

Thanks...
 

MitParmar

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Thanks, but I already made this circuit. Is it possible to make output variable in my circuit??
 

Embedded_Geek

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You see properly in the circuit diagram. R2 is a variable so you can vary the output with that. See the circuit diagram given in the website mentioned above in my previous post.
 

guitargore

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Thanks, but I already made this circuit. Is it possible to make output variable in my circuit??

The simple answer is that it is not practical to convert this circuit into a variable supply.

The LM78XX series regulators are fixed output regulators with no externally accessible control loop. You would need to put an adjustable regulator on the output them in order to make a variable supply and in that case you might as well just remove your current regulators and replace them with a variable design.

The LM317 mentioned above would fit your needs nicely. The datasheet, http://www.national.com/ds/LM/LM117.pdf, gives a very good explanation of the theory of operation in the application hints on pages 9-10.
 

BradtheRad

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A fixed regulator can be tricked into producing a variable range of voltages.

The schematics I've seen involve connecting a pot (or resistor network of some kind) at pin 3, the one that would normally go to ground.

From there, the pot configuration is different depending on what you want.
 
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