variable power supply

3V to 24V Variable Power Supply

This 3V to 24 volt variable-regulated power supply can be adjusted from 3 to 25 volts and is current limited to 2 amps as shown, but may be increased to 3 amps or more by selecting a smaller current sense resistor (0.3 ohm). The 2N3055 and 2N3053 transistors should be mounted on suitable heat sinks and the current sense resistor should be rated at 3 watts or more.

Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. The 1458 may be substituted in the circuit below, but it is recommended the supply voltage to pin 8 be limited to 30 VDC, which can be accomplished by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively. The power transformer should be capable of the desired current while maintaining an input voltage at least 4 volts higher than the desired output, but not exceeding the maximum supply voltage of the op-amp under minimal load conditions.

The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will provide regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is obtained using the center tap of the transformer with the switch in the 18 volt position. All components should be available at Radio Shack with the exception of the 1558 op-amp.

Variable Power Supply Circuit Diagram

variable power supply


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  • Jim Keith

    I used the Motorola MC1558 back in the early 1970’s–one of the first dual op amps–good device

  • Krokkenoster

    Hi guys the I,C. in this circuit is just a lower noise double 741 as only one unit is used a single 741 will be cheaper There are a number of different equivalent numbers in the market for the ancient workhorse 741 virtually any bi polar op amp will work

  • JogiSant

    HI guys, The circuit seems to be quiet calculated one. I wanted to confirm one thing , if we can design the same rating (3-25 Volt, 4 Amps ) supply without transformer. please suggest me on the same.

    • Jim Keith

      Three issues:
      1. A 4A capacitor limiter is unreasonably large, expensive and unavailable.

      2. The inrush when the power is switched on would potentially trip branch circuit circuit breakers.

      3. A variable supply that is used for experimentation requires transformer isolation for safety.

    • P. Marian

      By email from Rahul Mittal:
      “Dear Jim Keith,
      Thanks for highlighting issues. Please suggest me an appropriate design and its factors for designing for 4 Ap and then I need to step down the power from 220 V Ac, which could also surge to 600 volts. Look forward for your help.”

  • cezar


    I was just wondering, where did the 18 and 36 volts unregulated output came from, when the transformer secondary was a 12.6 volt center tap.



      Cesar If you multiply 12,6 with 1.4142 then you get 17.81892 and if you use the full secondary then you get 35.63784 according to my Chinese “abacus” If you do this practically and take the losses over your semiconductors and the ohmic or D.C. resistance of the secondary into account the values will be maximum use-able be 14 and 32 volts respectively The transformer should actually be 15-0-15 volt then it will work nicely and give some headroom for regulation at 35 volt output NO magic if you look properly!

    • cezar

      hi, Krokkenoster

      I built the circuit, as described with a 12.6 center tap transformer, using a typical EI core, rated at 2 amps, as the load approaches 2 amps, the circuit starts to lose regulation, especially on power surge demands like keying a transmitter amplifier. but the transistor at quite modest in term of heat generated, next time I will run it using a 26 volt transformer and see the regulation, and heat generated.

      I will keep you updated.


    • cezar

      I ran it using the 26 volt transformer, 13-0-13 secondary; the circuit holds regulation at 24 volts with a 2 amp load, the regulation is stable even on current surge demands, using a VHF amplifier, however as the load current increases, the voltage drop on the secondary is about 1.4 volt for every 600 mA of load added thus at 2.5 amps of load the circuits starts to lose regulation, the secondary voltage drop seems to be sharp as the load current increases.
      The heat generated by the Transistors are quite modest if the output is close to the transformer voltage, but gets quite hot between 14.5 and 20 volts.
      In my opinion this circuit will works best at 26 volt secondary (13-0-13) for a regulated 24 volt output and 13 volt secondary for a 11.8 volt output with a load of about 2 amps.
      The low dropout characteristic of this circuit is a quality by its self.

    • cezar

      Yes, you are right on the button, the EI core does not perform very well like a ferro resonnat ones that exhibit excelent regulation,the capcitor that I used was a 2×4000 micro farrad 50 volts, LDO qualities are a very good plus for this circuit, I like it.

      best regards and more power.


      Hi Cesar
      The regulation problem that you encountered is the transformer regulation and the voltage drop that is too small for the regulator to operate correctly as you noticed What is the size of your filter capacitor that can have an effect but nowadays, the cost of caps is not like when I started off in 1960 . Those days, a 1000 microfarad cap was the price of a complete amp of one watt with those coupling transformers common then. Complimentary symmetry transistors for amps of more than two watts were VERY dear Tubes (valves) were then relatively cheap and transistors just starting to come in I found one of those old caps today and 500 microfarad was 75 millimetres long and 27 millimetres diameter Spec 500 microfarad @ 12 volts D.C. You know an amp with 17 watts output only had a pi filter with two 50 microfarad and a resistor of 10 kiloohm of the series component The voltage was 300Volts

    • Jim Duly


    • cezar


      Okey, i can live with that…

      cheers ;))


    Super but for simplicity in the construction I used a 50 amp Darlington as I had one on hand but a TIP142 giving ten amps KEEP AN EYE ON W=IE that is the WATTS THAT YOUR TINY POWER TRANSISTOR must handle when the voltage drop is high i.e. 3 volts with 25 volt input and the current is 3 amps or more. a lot of crocodile tears were wept by not keeping this in mind. The TIP 3055 Is safe at ONLY 50 WATTS (plastic pack) 115 Watts in the metal version but then you ONLY use R.C.A. Motorola manufacture

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