variable power supply

Variable Power Supply with UA78G/UA79G

A stable variable power supply with an adjustable output voltage from 5 volts to 30 volts can be easily constructed with the regulator ICs UA78G or UA79G. These ICs differ from the common three-terminal regulator since their output voltages are adjustable by a voltage level at their control inputs. The maximum current delivered by these ICs is 1 ampere.

The unregulated voltage must be at least 5 volts higher than the desired output level to maintain stability. The input voltage however must not exceed 40 volts. The maximum dissipation of the IC is 15 watts.

The variable power supply circuit presented here is designed to give maximum voltage level of 28 volts. If P1 is replaced with 25K potentiometer, the regulator can deliver up to a maximum of 30 volts. Capacitors C1 and C2 stabilize the IC and they must be connected as close as possible to the IC terminals.

The IC UA79G delivers negative voltage level. Take note that the two ICs have different terminal connections.

UA78G/UA79G power supply circuit diagram

variable power supply

Variable power supply PCB layout

variable power supply pcb


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  • stonetrails

    I need a schematic for a variable (adjustable) power supply 5 VDC to 24 VDC, capable of delivering 25 amps at full voltage to a .58 hp gearmotor. I will typically operate the motor with a dc voltage of between 7-15 VDC. Any thoughts?

  • h

    can this circuit 12.6volt and 1ampere ???
    i want use 12volt for input

  • Paul Yak

    Hi All,
    thanx for the great site. I am starting electronics after 20+yrs toying with PCs. Now I want to really get to the board level of devices and understand how they work.

    I am recovering from double pneumonia and my immune system is totally shot, the simplest sore throat can attack my lungs and hospitalise me…the ONLY thing that stops the infections are my salt pipe inhaler and now I’ve found “Colloidal Silver” its MY Life Saver, but I can’t afford to buy the devices or buy the actual product at £10 for 3 teaspns when I take that much everyday….lol.

    The power supply I require is variable from Max 27+ DC to 5– 10 V DC Min. I’ve been informed that the Milliamp current is more important for the smallest size of silver colloids which enter my bloodstream easier. I need a Max Current of; 600 milliamp, and would ideally prefer tiny currents of 25 to 50 milliamp if possible.

    Can anyone please assist, as this is very confusing as I can’t seem to find an easy way to reduce the mA current of something such as a cellphone charger to modify the power/current to my needs…. sorry if this sounds confusing, I am a novice with years of medical research to get to this point and now I’m making the silver medicine, but just want to make it even better…by the way guys IF ANY of you have infections and diseases you find difficult to clear with anti-biotics talk to me and hear my personal story, I want NO money I only tell people what I’ve researched then give them info to do themselves too.


    My comments are as above


    Dear sir,

    I have gone through your above circuits and they are simple and good for many small applications.
    We would like to build a circuit for controlling the PMDC A motor works on 12 volts dc at 3.5 amps. current. We would like to control the speed by varying the input voltage from 1 volts to 12 volts. with a good regulation. Kindly let us know whether you can provide the circuit details with discription so that our technican can build it. You can also prvide with and without some good feedback system so that regulation will be good.
    Looking forward for your favorable response.
    With best regards,

  • Themisive

    In view of the high output current; the PCB used for this circuit should be of fibreglass construction.

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