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Transistor based 3 Phase Sine Wave Generator Circuit

Transistor based 3 Phase Sine Wave Generator Circuit

The post explains a very simple bipolar transistor based 3-phase sine wave generator circuit, only three to be precise and a few passive components for initiating the desired three phase output.

How it Works

Referring to the 3 phase sine wave generator circuit we can see three identical transistor stages configured in a cross coupled manner, having equivalent RC timing constants across their bases.

The 10k resistor and the 1u capacitor essentially become responsible of providing the required delay effect for generating the intended 3 phase signals with 120 degree phase shift.

When power is switched ON, the stages may seem to undergo a locked sequence, however since all the capacitors cannot have a precisely same value, the one which has a shade lower value than the other charges up first, triggering a sequential conduction across the transistor.

Let's assume that due to inconsistency in values, the middle transistor base capacitor gets charged first, this enables the middle transistor to conduct first which in turn grounds the base of the extreme right transistor preventing it from conducting for that instantaneous moment, but in the meantime the base capacitor of the left or the right transistor also gets charged in tandem which forces the middle transistor to switch OFF and release the right transistor conduction.

Push Pull Cycle

The above mutual push and pull procedure induces and settles into a continuous sequential train of conduction across the transistors causing the intended three phase signal pattern to appear across the collectors of the transistors. Owing to the gradual charge and discharge pattern of the caapcitrs, the resultant signal shape is a pure sine wave.

The 2K2 resistor shown in yellow strangely becomes crucial in initiating the 3 phase signal generation sequence, without which the circuit seems to stall abruptly.

As mentioned before the degree of phase may be altered by changing the RC values across the bases of the transistors, here it's configured to produce a 120 degree phase shift.

Circuit Diagram

Oscilloscope trace, 3 phase waveform



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

37 thoughts on “Transistor based 3 Phase Sine Wave Generator Circuit”

  1. Dear Swagatam,
    I would like to have your opinion on my choice as to the power of section in ultrasonic area.
    I attach a video (YT) that shows some patterns of sections.
    Please let me know the most powerful for ultrasounds (I want to use it for dog repeller in distance).
    In the video I have 5 section.
    What would you suggest to me for each one.
    I’ll use piezoelectric transducer.

    best regards

    • kels, yes you can do it, I have posted quite a few dedicated articles related to this topic which you can search in the website and refer them…

  2. Hello Sir,
    I want to take a strong signal for some experiments.(To reinforce the signal)
    What can I do? Do you have a schematic in order to I can multiply the multi watt phases (as more are better).
    best Regards
    P.I e-mail sunrisezero@yahoo.gr

    • Hello sunrisezero you can try connecting TIP122 BJTs with the output leads of the circuit, and get strong outputs from the collector or the emitter of the BJTs

    • It is very easy actually, just connect the bases of the respective TIP122 BJTs with the phase1,2,3 outputs, connect their collectors with the positive supply, and finally strong signals can be obtained from their respective emitters.

    • The plan suggested by me is right, and you must try exactly the same plan.

      the collector/emitter voltage of TIP122C is 100V, in the emitter follower mode as suggested by me, the base pattern and voltage will be replicated at its emitter, so if the above BC547 3 phase circuit is operated with 12V, the emitters of the TIP122 would also replicate the same sequence at a little less than 12V

    • Hello Bhanu, can you please specify why do you think that this circuit could be relevant to a differential 3 phase shifter?

  3. Sir can it be applied to three phase inverter circuit to produce three phase sine wave?
    Does the moasfet drivers give sine wave output if we use this as input?

    • Athri, no that won't happen, because the mosfet driver will convert it into square waves, and the output will be also a square wave only.

  4. Great work as always. I refer a lot on this site. Great source for hobbyist like me. good work. BTW, is this 3 phase output shown is with or without load? will it be disturbed on some minimal load?

    • I'm glad you liked my site….yes the shown output is without load and will get disturbed with any kind of load…a buffer stage will be required for accessing the 3 phase output correctly

    • connect NOT gates with each of the transistor collectors…the o/p from the gates could then be used for the half bridge

    • BLDC circuts work by sensing back emf or signals from the hall-effect sensors….you won't need a 3 phase generator circuit for it.

  5. Hello! I was wondering if I can simply use a 12v supply instead of the 9v in schematic, at the present components ratings? If not and components need to be swapped out for different rated ones, what would they be or where do I find the information to know? Thank you, Tim

  6. hai mr. swagatam,
    can i use this circuit to make it as input for the 3 phase H-bridge Mosfet circuit to drive the BLDC rotor from honda motorcycle safely? or i need another complicated circuit again? thanks..

    • Hi Dhyaksa, I think it'll work, at least there's no harm in trying it out….since it's designed to produce a 3 phase output with a 120 degree phase shift, it should work as expected.

    • okay i will try it.. because of my old built in ECU for triggering the BLDC was died, and to buy a new one is quite expensive, i try to make it one, is there any potential problem to use this circuit for drive a High and Low mosfet together in each Phase of final Mosfet 3 Phase circuit then? what do you think? before i start to make it.

    • if you are using a driver IC for driving the mosfet bridge then the above circuit can be tried without much fear, because the driver IC will be smart enough to detect any issue and prevent the mosfets from blowing of.

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