Variable power supply

[plcboy]

HI EVERYONE
Great site and this is my first post.
I have a DC power supply 12 volts dc capable of giving of 2.5 A.
I like to convert this unit into variable voltage p.s. If I connect a pot at the output then is it acceptable. Thanks for the help.

 

[Ian Rogers]

Hi, Welcome to the forum..
No! not really.. If you use a pot then you will not achieve a constant voltage, what would work on one application would not work on another.

You need a vary the output via a power transistor.. If you don't need much current ie < 1 amp use can use a linear reg LM317 this can be used as a variable output.

Ian

 

[plcboy]

Thanks IAN for the response.Do you mean that I should connect the output voltage to LM317 and then connect it to a pot.Thanks

 

[crutschow]

Thanks IAN for the response.Do you mean that I should connect the output voltage to LM317 and then connect it to a pot.Thanks

If you look at the data sheet for the LM317 you will see how to hook it up. It also shows how to add a power transistor to increase the current output. But you will have to heatsink the LM317 and the transistor since, at worst-case, they will be dissipating 25W.

The disadvantage of the LM317 is that it requires about a 3V drop to operate so your maximum output voltage will be about 9V. If you want more voltage than that you can use a low dropout type regulator such as an LT1084 which can provide 5A with a dropout voltage of about a volt, giving a maximum output of about 11V.

Also note that the minimum output voltage of these devices is 1.2V, but that is not a problem for most applications.

 

[Ian Rogers]

Sorry about that I should have linked in the datasheet....As far as the LM317 is concerned, I have a similar setup but I have a switch the will give 12v straight out the supply in one position then the variable voltage in the other... I hardly ever require more than an amp for my circuits... The LT1084 will be a better choice as Carl suggested.

Ian

 

[JimB]

So far nobody has asked what kind of PSU the existing PSU actually is.

Is it:

1/ just as simple transformer, rectifier, capacitor type?

2/ a fixed voltage regulated PSU of some type?

If it is the first type, then yes an LM317 or whatever would be the way to go.

If it is the second type, then it may be possible to make a simple modification to make it variable voltage.

JimB

 

[plcboy]

HI JimB
yes it is 2 the power supply is a fixed voltage regulated PSU with output of 12.5V, 2.5A. Really what I want to do is to have variable voltage from 0 to 12v.
So I am not sure yet if I can utilize LM317 at the output of mentioned power supply since the output voltage is regulated. Thanks a lot and still waiting for responses.

 

[KeepItSimpleStupid]

What's the general application? Replace adapters of radios or a true general purpose power supply. I have a really nice PS for that purpose. The secondary voltage is selected at the same time as the output voltage, so the dissipation is reduced. This **broken link removed** is a real nice supply for battery substitutiion.

 

[JimB]

HI JimB
yes it is 2 the power supply is a fixed voltage regulated PSU with output of 12.5V, 2.5A. Really what I want to do is to have variable voltage from 0 to 12v.
So I am not sure yet if I can utilize LM317 at the output of mentioned power supply since the output voltage is regulated. Thanks a lot and still waiting for responses.

OK, in that case what is the PSU, make, model, schematic if you have one, photo of the insides and outsides?
JimB

 

[sanjubluerock3]

hi i have attached the circuit diagram with pcb layout it gives u voltage range from 1.2v to 30V DC

 

[plcboy]

HI
Thanking you all for your help and support, I will get back to you on this subject.

 

[plcboy]

Hi Sanju and thanks for the response.Have made it and does really work based on what you have mentioned. Thanks a lot.
plcboy

 

[sanjubluerock3]

that's good....and you can use any junk smps as case

 

[plcboy]

HI
To be honest with you I did not understand your message however I hope I did not disrespect you and If I did I do apologize.Thanks