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DC Variable supply design help

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Full Member level 3
Jan 19, 2010
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Hi all

I'm trying to make a simple Variable DC power supply, and simulating it in Proteus and stuck on the value of a resistor R8 in the schematic,

Specs are 1.2V - 35 V supply and appx. 4mA - 3A,

may be the problem is due to the orignal components not found in Proteus, so I need your help.

Origibal Q1 is MJ4502 and D1 and D2 are 1N457

schematic and isis files attached


  • variable supply.pdf
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  • variable
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Dear jami
At first step , you can use another parts .
At second step , is it important to you , making your aim , with this circuit , or you can change it with better circuits ?
If you want design better type , i can help you .
Best Wishes

Ofcourse I'll go with the better design. can you please post some schematics for lab/professional use power supplies

Well , what do you want accurately ? Switching power supply or linear power supply ? and what current and voltage do you need at your out put and describe the specifications that you need on your power supply , thus i can post many power supply circuits that i have designed them , or i can tell you the way of their design .
Good luck

It is not so simple, specialy low voltage and high current output. I'll check in my documentation stock.


Basically I need a variable power supply for experimental purposes, means I need it to feed circuits being examined or under development ,
my preferences are as following

1- minimum voltage as possible upto 30-50V
2- minimum current in mA upto 3-5A or more
3- economical and efficent/ ripple free
4- fault or high current protection/ all possible protections

In conclusion I need an efficient variable supply for home Lab/Professional purposes

Also if you can help me in providing with schematic of Solar Charge control, it will be greatfull

Best regars

---------- Post added at 12:24 ---------- Previous post was at 12:22 ----------

One more thing,Supply should have analogue ouput current and voltage meters/ simple digital meters

Hello jami
I have many many circuits about power supplies .( because i love them !!)
Try below circuit :
Linear power supply.JPG
You can control the output voltage from 0 volt ( approx )( in fact from 20mv) up to 45 volt as well as . and the out put current is about 0 up to 4.99 amperes .
Good luck

here is the simulation model of your supply, but I failed to control output voltage and current.


  • Linear supply.PDF
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  • Linear
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hello jami
I think your simulator has some problem , because i simulated it with PSpice before posting for you .
By the way . i think the problem is here : you forgot to add a 5 volt power supply to the input of op amp . you just used a potentiometer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Good luck

first i used supply on inputs of op-amps, but output remains same, and if we use a supply on their inputs then how can we vary the output voltage and current

Daer Jami
The out put voltage formula is : vref (1+R5/R6) . and if you try to change the vref or R5 , thus the out put voltage will change . sp it means that should change the vref (V2) . do you know how ?!
Ok , now see below figures , that i changed the out put voltage in them , please :

Good luck

---------- Post added at 22:27 ---------- Previous post was at 22:17 ----------

---------- Post added at 22:37 ---------- Previous post was at 22:27 ----------

Last edited:

By varying Vref the output vary between 33v and 45v, and at 4v Vref I get variable output between 35v t0 44v like a ripple

What are you doing ? I can't understand that why you can not earn success ? if you change the vref from zero up to six volt , you will have the out put voltage from zero up to 50 volt ! .How did you try to change the vref ?????!!!!!!!!!!!!!!!!!!!!!!!!!

See below , please:
Best Regards

Dear dselec
How are you ?
As you can see at the original question he want make a variable power supply for experiment the circuits . ( up to 50 volt and up to 5 amp ) but your attachments are constant voltage .

here are the respective outputs at various reference voltages


  • output 1.pdf
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  • output 2.pdf
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  • output 3-unstable varying bw 37v-45v.pdf
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  • output 4.pdf
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Ooops . sorry my friend . the problem is that you should change the collector and emitter of PNP transistor .
Good luck

by swaping this PNP transistor it shows constant 48v output

did you change the vref ?

---------- Post added at 20:43 ---------- Previous post was at 20:41 ----------

my mean by vref is v2 in my latest attachment

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