The project was originally completed on March 2025

The project here describes a 555 timer-based square wave inverter designed for DC to AC conversion. The circuit utilizes a 555 timer IC operating in astable mode to generate a PWM signal. This signal is then used to drive a MOSFET H-bridge circuit consisting of IRF9540 and IRF540N MOSFETs, which effectively switch the DC input to produce an AC output. To improve system reliability, the inverter uses an overcurrent sensing circuit using an LM358 operational amplifier, which detects excess current flow. The system is powered by a 12V DC supply. 

Circuit Overview: 555 IC used in astable multivibrator mode generates a 50% duty cycle pulse. A MOSFET H-bridge is present.  One half of the H-bridge receives the input from the astable multivibrator while the other half of the H-Bridge receives the inverted input from the BJT. The BJT acts as a NOT gate which inverts the output of 555 IC. This ensures that all the MOSFETs are not on at the same time. The H-bridge generates the ac waveform. There is an overcurrent detection circuit which uses low side current sensing.

555IC in astable multivibrator mode: The trigger and threshold pins are connected together so that there is no need of external trigger pulse. Initially the voltage source will start charging the capacitor through the resistors R₁(as we used a diode to bypass R₂). During this time, the trigger comparator will output 1 because the input voltage at the trigger pin is lower than the 1/3 of the supply voltage. So, the Q’ of the flipflop will give 0 and the discharge transistor is closed. The output of the IC is high. Once the voltage across the capacitor reaches 1/3 of the supply voltage, the trigger comparator will give zero but the flipflop output is unchanged as, both the inputs are zero (memory state). So, the voltage across the capacitor will rise to 2/3 of the supply voltage and the threshold comparator will output 1. Now, flipflop inputs are R=1 and S=0, so, the output of Q’ is 1 and the capacitor will start discharging through the discharge transistor. So, the IC output is low. Now, the capacitor voltage drops to 2/3 but there will be no change in the flipflop output. When the capacitor voltage drops to 1/3 of the supply voltage, the trigger comparator will output 1 and the discharge transistor will turn off. The capacitor starts charging again and the process repeats.

We can calculate the on and off time using the following formula.

T₁=0.693(R₁+R₂) *C

But since the diodes are used to charge the capacitor through R1 only,

T₁=0.693(R₁) *C

T₂=0.693(R₂) *C

For 50% duty cycle, T₁=T₂ which means R₁=R₂

We have taken R₁=R₂ =1.2k, C=10uF. This combination generates approx. 50Hz 50% duty cycle signal.

BJT as a NOT gate: The input is connected through resistor R(refer to image) to the transistor’s base. When no voltage is present on the input, the transistor turns off. When the transistor is off, no current flows through the collector emitter path. Thus, current from the supply voltage flows through resistor R2 to the output. In this way, the circuit’s output is HIGH when its input is LOW. When voltage is present at the input, the transistor turns on, allowing current to flow through the collector-emitter circuit directly to ground. This ground path creates a shortcut that bypasses the output, which causes the output to go LOW.

MOSFET H-Bridge: The H-bridge in the inverter circuit consists of four switches(MOSFETs) arranged in a manner that allows current to flow in alternating directions through the load.

The H-bridge consists of 2 p-channel MOSFETs and 2 n-channel MOSFETs. The p-channel MOSFETs turn on when V_gs is grounded and the n-channel MOSFETs turn on when V_gs is equal to V_cc. The left side gets the inverted signal from the astable multivibrator due to the BJT while the right side gets the signal directly from the astable multivibrator. This ensures that all the MOSFETs...