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

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