Theory & Background

A line-frequency (LF) inverter switches at 50Hz (low switching frequency). The H-bridge driven by a low-frequency square/SPWM pattern drives a step-up transformer directly. Advantages: simple, highly efficient (>90%), robust. Better alternative: high-frequency (HF) inverter uses a small HF transformer at 20–100kHz then rectifies to DC bus, then SPWM inverter stage. More complex but smaller/lighter. For educational purposes, the LF SPWM design with EI transformer is clearer to understand.

Components & Requirements

10 components required for this project.

#	Component	Purpose	Qty
1	IRF3205 Power MOSFET (55V/110A)	Full H-bridge for LF inverter stage	x4
2	IR2110 Gate Driver IC	High-side and low-side MOSFET driving	x2
3	EI-66 Transformer (12V:220V, 500VA)	Voltage step-up and isolation	x1
4	Arduino Uno (SPWM generation)	Generating SPWM control signals	x1
5	10000µF/25V Capacitor	Input DC bus filter	x2
6	100µF/450V Electrolytic Cap	AC output filter	x1
7	ZMPT101B Voltage Sensor	Output voltage monitoring and feedback	x1
8	12V 100Ah Lead-Acid Battery	DC input source	x1
9	60A ANL Fuse + Holder	Battery over-current protection	x1
10	Cooling Fan (120mm)	MOSFET thermal management	x1

Step-by-Step Implementation

Follow these 6 steps carefully.

1

Inverter Topology Selection

A line-frequency (LF) inverter switches at 50Hz (low switching frequency). The H-bridge driven by a low-frequency square/SPWM pattern drives a step-up transformer directly. Advantages: simple, highly efficient (>90%), robust. Better alternative: high-frequency (HF) inverter uses a small HF transformer at 20–100kHz then rectifies to DC bus, then SPWM inverter stage. More complex but smaller/lighter. For educational purposes, the LF SPWM design with EI transformer is clearer to understand.

2

SPWM Signal Generation

Generate complementary SPWM signals for the H-bridge. Modulate a 50Hz sine reference against a 4kHz carrier triangle wave. When sine > triangle: S1(top-left) and S4(bottom-right) ON. When sine < triangle: S3(top-right) and S2(bottom-left) ON. Never turn on both top and bottom switches on the same leg simultaneously (shoot-through). Add 2µs deadtime using IR2110. Output feeds the low-frequency transformer.

3

IR2110 Gate Driver Wiring

The IR2110 drives both high-side and low-side MOSFETs on one inverter leg. HIN (high-side input) and LIN (low-side input) connect to Arduino PWM pins. Floating supply for high-side gate driver is generated by bootstrap circuit: bootstrap capacitor (100nF) charges through bootstrap diode when low-side is ON. Verify: VCC=15V, VB=15V above VS pin (the high-side source). Output swing: HO switches between VS and VB, LO between 0 and VCC.

4

Transformer Connection and Testing

Connect the 12V winding to the H-bridge output (center-tapped or full-bridge depending on transformer). The 220V secondary is the AC output. Before connecting load, test with oscilloscope on secondary: should show ~300V peak sine wave (220V RMS). Verify frequency is 50Hz. Check THD with oscilloscope FFT: target < 5% THD for sine wave. If waveform looks square with rounded edges, the LC filtering is insufficient.

5

Output Voltage Regulation

Under load, output voltage drops due to transformer winding resistance and leakage inductance. Sense output voltage with ZMPT101B sensor. Calculate actual RMS: compare with setpoint (220V). Implement feedback: if Vout < 220V, increase SPWM modulation index; if Vout > 220V, decrease it. This closed-loop control maintains ±2% output voltage regulation from no-load to full-load — critical for sensitive electronics.

6

Protection and Monitoring

Implement: DC input undervoltage lockout (stop inverter if battery < 10.5V — prevents deep discharge), AC output overvoltage (>250V — hardware trip), overcurrent (sense DC input current with shunt resistor — trip at >50A), overtemperature (MOSFET heatsink > 85°C), and low battery warning alert at 11.2V. Display battery voltage, output voltage, output current, and estimated runtime on LCD.

Code & Implementation

Core code for inverter_spwm.ino:

inverter_spwm.ino
  C/C++

// 50Hz SPWM using Timer1 on Arduino   #include <avr/io.h> #include <avr/interrupt.h>  const int SINE_SAMPLES = 200; uint8_t sine_table[SINE_SAMPLES]; volatile int sample_idx = 0; float mod_index = 0.85;  void init_sine_table() {   for(int i = 0; i < SINE_SAMPLES; i++)     sine_table[i] = (uint8_t)(127.5 * (1 + mod_index * sin(2*PI*i/SINE_SAMPLES))); }  ISR(TIMER1_COMPA_vect) {      OCR1A = sine_table[sample_idx];                           OCR1B = SINE_SAMPLES - sine_table[sample_idx];            sample_idx = (sample_idx + 1) % SINE_SAMPLES; }  void setup() {   init_sine_table();      TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM11);   TCCR1B = _BV(WGM13) | _BV(CS10);   ICR1 = 255;        TIMSK1 |= _BV(OCIE1A);   sei(); }

Testing & Troubleshooting

Test DC to AC Inverter (12V → 220V) by verifying each subsystem individually before full integration.

!

Troubleshooting Tips

Verify power voltages, check ground connections, use serial monitor for debug.

Real-World Applications

*Battery-powered backup systems

*Solar off-grid power conversion

*Vehicle auxiliary power (12V battery)

*Rural remote area power supply

*Emergency power during outages

*Field equipment power supply

*Educational power electronics lab

*DIY UPS construction

Extensions & Next Steps

Add grid-tie capability with synchronization to mains
Implement battery charger mode (bidirectional inverter/rectifier)
Build a 3-phase inverter by adding a third H-bridge leg
Add Bluetooth monitoring via HC-05 module
Replace transformer with HF inverter + transformer for smaller form factor

Interactive Playground

Coming Soon

An interactive simulator will be available here — simulate circuits and run code in-browser without hardware.

Frequently Asked Questions

What is the efficiency of a DIY inverter compared to commercial ones?

Commercial inverters achieve 92–96% efficiency through optimized MOSFET selection, minimal transformer losses, and sophisticated control. A well-designed DIY LF inverter can reach 88–92% efficiency. Main losses: transformer core losses (hysteresis + eddy current, 10–20W), MOSFET conduction losses (I²×Rds_on), and gate drive losses. MOSFET selection is critical: IRF3205 has low Rds_on (8mΩ) but high gate charge — suitable for LF inverters.

Why does my inverter hum loudly?

Transformer hum at 50Hz is caused by magnetostriction of the iron core. It amplifies significantly when operating near magnetic saturation (caused by DC bias current in primary, incorrect modulation index, or operating at lower than rated voltage). Solutions: ensure no DC component in the switching waveform (symmetric deadtime on both H-bridge legs), use a center-tapped transformer with push-pull topology which inherently cancels DC bias, reduce load, and mount transformer on vibration-absorbing pads.

What loads can my 500W inverter NOT power?

Inverters struggle with: induction motors (require 6–8× rated power during startup — a 500W inverter may trip on a 200W pump motor start), UPS systems with active power factor correction (they reject non-sinusoidal waveforms from modified sine inverters), some medical devices requiring very clean power, and large capacitive loads that draw high peak current. Pure sine inverters handle these better than modified sine.

Introduction