2N3055 Variable Power Supply

This simple variable power supply circuit has a low production cost and delivers an output voltage between 1,5 V and 15 V with a maximum current of 500 mA. Its stabilization is better than 2% if the current consumption does not exceed 350 mA. The variation of the power supply voltage can be made with a potentiometer and when overloading occurs a buzzer sounds a alarm.

T4 compairs the P1 slider voltage with the output voltage. Then the P1 slider voltage is 0,65 V higher than the adjusted voltage, T2 opens, which stops the T3-T5 Darlington base current. The 18V, 1A transformer voltage must be filtered with B1 and C1. When the output current is more than 500 mA, the Bz1 starts the alarm (overloading). Bz1 must be a 24 V type, auto-oscillating.

2N3055 variable power supply circuit schematic

2N3055 variable power supply


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  • V.Sambath kumar

    Hi Mihai,

    Thanks for correcting me i made a mistake shorting R1 . suppose R1 is
    there as per the original ckt and no changes for ci also ,but if i
    connect the base resistor of T1 to the +ive of the rect supply and connect the emitter of the t1 to the collector of t2 will it work.

    The load is sensed by the 1.5ohm 5watts resistor and there by activating t1 to trigger the buzzer when the current reaches 0.65A,as per this TI will be forward biased and the buzzer will be continuously on as per the orginal ckt.

    want to know whether the suggestion i mentioned above here will work or not.

    Regards V.Sambath kumar.

    • Mihai

      Quote: «If I connect the base resistor of T1 to the +ive of the rect supply and connect the emitter of the t1 to the collector of t2 will it work.»

      NO! It will not work. Base-emitter of T1 will be forward biased(and in parallel with R4) irrespective of overload condition.

      BTW, you didn’t mention about collector(T1). Does it stay as it is?

      Anyway, what you are trying to do doesn’t make sense.


  • V.Sambath kumar

    V.Sambathkumar on 15, Dec 2015

    I think Q1 connection is wrong , you can disconnect R2 from the ground and connect it to the +ive of cap C1, short R1 and disconnect the emitter of Q1 and connect it to the collector of t2.In this condition when the current exceeds 0.65A the buzzer sounds.


    • Mihai

      If R1=ZERO(short, as you said above) how do you get current limit at 0.65A?

      You only have to connect C1 across +/-(output) of the rectifier bridge, not after R1.


    Guys I agree wholeheartedly with you but the little block get the value written inside the block and then the values cannot be confused

  • Brian Gardner

    To be honest a little complicated I have seen more simplified circuits with the same outcome

  • Brian Gardner

    Wise up guys a symbol is a symbol learn to read them any way they come just like the metric/imperial measuring tape it all means the same.

  • mehdi

    500ma is so low!

  • Jim Keith

    It appears to be a beeper or buzzer that goes off when the current is above about 0.65A.

  • ren

    what component is Bz1?

    • jOSE A. Ortiz

      BZ1 I think is a crystal,but crystal oscillators are use in transmitter circuits and other circuits that at this point I don’t know.

  • Jose Ortiz

    I will never agree with those new electronic symbols;they looks crazy,I will keep using my own electronics symbols the way I learned when I studied electronic; I don’t care if that is the new standard of the organization that controls the electronics business but I will keep my own old symbols;these new symbols looks stupid;look at the variable resistor symbol,look at the resistor symbol they are a box that is what they looks like;there is nothing like my old symbols.Nothing like my old electronic symbols

    • Sonny

      actually that is still the standard the difference is the one is block diagram and the other is the schematic diagram

    • Mihai

      As a matter of fact, there are two standards: IEEE (zigzag symbol used mainly in the USA) and IEC (European symbol like in the diagram above). Both of them are of the same age. Nothing has been changed lately regarding those symbols.

    • jOSE A. Ortiz

      Funny symbols,looks at the variable resitor looks like a box with a axe. Let me keep my old beautiful symbols forever until the end of the world

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