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 Vgs is grounded and the n-channel MOSFETs turn on when Vgs is equal to Vcc. 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 don’t turn on at the same time.

When the output of the 555 IC is high, the right side of the H-bridge is HIGH and the left side of the H-bridge is LOW. MOSFETs Q3 and Q2 in the image turns on and the other MOSFETs are off. The current flows from left to right.

When the output of the 555 IC is LOW, the right side of the H-bridge is LOW and the left side of the H-bridge is HIGH. MOSFETs Q4 and Q1 in the image turns on and the other MOSFETs are off. The current flows from right to left. Hence, we get an AC output.

Overcurrent detection: The overcurrent detection used here is low side current sensing. There are two op-amps used- U2A as a non inverting amplifier and U2B as comparator. The op-amp U2A takes the voltage drop across shunt resistor and amplifies it as a non-inverting amplifier. The output of U2A is fed to U2B which acts as a comparator. The potentiometer is used to set a reference voltage level. When the voltage of the non-inverting side(output of U2A) becomes higher than the reference voltage the output of U2B becomes high and we know about the overcurrent in the circuit.

Refer to the attached images for better clarity.

NOTE: 1. This project is for demonstration purposes only and it lacks a lot of features to be implementable in real world scenarios. 

2.Resistor R5 in the circuit diagram should be of appropriate wattage as huge amounts of current can flow through it. Since this project is for demonstration purposes, a 0.5w resistor was used. 

3. Heat sinks can be used on the MOSFETs as they can get quite hot.

4. Project Design was derived from the project mentioned in the reference.