Pure Sine Wave Inverter, Using IC 555

The proposed circuit generates accurately spaced PWM pulses which imitates a sine wave very closely and thus can be considered as good as its sine wave counter part design. Here we use two stages for creating the required PWM pulses, the stage comprising the ICs 741 and the other comprising the IC 555. Let’s learn the whole concept in details.


How the Circuit Functions – The PWM Stage

The circuit diagram of the proposed pure sine wave inverter can be understood with the following points:


The two op amps are basically arranged to generate the required sample source voltages for the IC 555.


The couple of outputs from this stage is responsible for the generation of square waves and triangular waves.

The second stage which is actually the heart of the circuit consists of the IC 555. Here the IC is wired in a monostable mode with the square waves from the op amp stage applied to its trigger pin #2 and the triangular waves applied to its control voltage pin # 5.


The square wave input triggers the monostable to generate a chain of pulses at the output where as the triangular signal modulates the width of this output square wave pulses.


The output from the IC 555 now follows the “instructions” from the op amp stage and optimizes its output in response to the two input signals, producing the sine equivalent PWM pulses.


Now it’s just a matter of appropriately feeding the PWM pulses to the output stages of an inverter consisting of the output devices, the transformer and the battery.

The Output Stage

The above PWM output is applied to the output stage as shown in the figure.


Transistors T1 and T2 receive the PWM pulses at their bases and switch the battery voltage into the transformer winding according to the duty cycles of the PWM optimized waveform.

The other two transistors make sure that the conduction of T1 and T2 takes place in tandem, that is alternately so tat the output o from the transformer generates one complete AC cycle with the two halves of the PWM pulses.

Parts List

R1, R2, R3, R8, R9, R10 = 10K,
R7 = 8K2,
R11, R14, R15, R16 = 1K,
R12, R13 = 33 Ohms 5 Watt,
R4 = 1M preset,
R5 = 150 K preset,
R6 = 1K5
C1 = 0.1 uF,
C2 = 100 pF,
IC1 = TL 072,
IC2 = 555,
T1, T2 = BDY29,
T5, T6 = TIP 127,
T3, T4 = TIP122
Transformer = 12 – 0 – 12 V, 200 Watts,
Battery = 12 volts, 100 AH.