How to Build a Simple Inverter Circuit Using MOSFETs: A Step-by-Step Guide

Inverters are essential devices that convert DC (Direct Current) power into AC (Alternating Current). Whether you're powering appliances from a solar battery or building a backup system, a MOSFET-based inverter is a great place to start for hobbyists, students, and engineers alike.

In this article, I’ll guide you step by step on how to build a simple square wave inverter using MOSFETs, explain its working principle, and answer some frequently asked questions.

🔧 What You'll Learn:

• Basics of how an inverter works

• Components required

• Circuit diagram and working principle

• Step-by-step construction process

• Example calculations for sizing

• Safety tips and testing

• Frequently asked questions

⚙️ 1. Understanding the Basics

❓ What is an Inverter?

An inverter converts DC power (e.g., from a battery or solar panel) into AC power that can be used by household appliances. The most basic inverters generate square waves, while advanced models produce sine or modified sine waves.

⚡ Why Use MOSFETs?

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are efficient power switches. They are ideal for inverter applications because of:

• Fast switching speeds

• Low power loss

• High current handling capability

• Availability and affordability

🧰 2. Components Required

🔌 3. Circuit Diagram (Square Wave Inverter)

     +12V DC
        |
    [Battery]
        |
    ----------
   |          |
   | MOSFET1  | MOSFET2
   |    D     |   D
    \        /
     \      /     <-- Transformer Primary
      \    /____|
        |        \
     CD4047      |
        |        |
       GND      GND

🔄 4. Working Principle

1. The CD4047 IC is configured in astable mode, producing square wave pulses (~50 Hz) on its Q and ~Q outputs.

2. These outputs alternatively turn the MOSFETs ON and OFF.

3. The MOSFETs switch current through the center-tapped transformer primary winding, causing alternating magnetic flux.

4. The transformer steps up the 12V DC pulses to 220V AC (square wave) on the secondary side.

🧪 5. Construction Steps

Step 1: Build Oscillator Circuit

• Use CD4047 IC in astable multivibrator mode

• Adjust frequency with resistor and capacitor values:

  $$f = \frac{1}{4.4 \times R \times C}$$

  For 50Hz, typical values might be R = 100kΩ, C = 0.1µF

Step 2: Connect the Outputs to Gate of MOSFETs

• CD4047 provides Q and ~Q outputs

• Connect each output to the gate of an N-channel MOSFET through a resistor (e.g., 220Ω)

Step 3: Connect MOSFET Drains to Transformer

• Center-tapped transformer primary is connected to +12V

• Each end of the primary goes to the drain of a MOSFET

• Sources of both MOSFETs go to ground

Step 4: Add a 12V Battery Supply

• Connect the positive terminal to the center tap of the transformer

• Ground connects to MOSFET source and IC GND

Step 5: Attach a Load to the Transformer Output

• Use a bulb or small AC fan (do not connect sensitive electronics)

🧮 6. Example: Component Sizing

Assume you want to power a 40W AC bulb at 220V.

• Power (P) = 40W, Voltage (V) = 220V

• Required Current (I) = P / V = 40 / 220 ≈ 0.18 A

Transformer must be:

• Rated for at least 50W

• Primary: 12V-0-12V

• Secondary: 220V, 0.23 A

Battery:

• 12V, 7Ah lead-acid battery gives ≈ 84 Wh

• Enough to power 40W for about 2 hours

MOSFET:

• Choose one with >10A drain current and low Rds(on) (e.g., IRF540N or IRFZ44N)

⚠️ 7. Safety Tips

• Always use a fused connection from the battery.

• Use heat sinks on the MOSFETs.

• Don’t run inductive loads (like refrigerators) on basic square wave inverters.

• Always test the output voltage using a multimeter before connecting any load.

• Never touch the transformer output while the circuit is powered.

❓ FAQs

Q1: Can I use this inverter to run my laptop or refrigerator?

No. This simple inverter produces a square wave, which is not suitable for sensitive or inductive loads. Use a pure sine wave inverter for those.

Q2: Can I use IC 555 instead of CD4047?

Yes, but CD4047 is more stable and already provides complementary outputs, which simplifies the design.

Q3: How do I get a sine wave output?

You need a PWM (Pulse Width Modulation) controller, a microcontroller, and filter circuits (LC filter) to generate a sine wave. That’s a more advanced design.

Q4: How efficient is this inverter?

Efficiency depends on component quality, load type, and transformer losses. A simple design like this may offer 70–80% efficiency.

Q5: How can I increase the power capacity?

• Use higher-rated MOSFETs

• Upgrade the battery and transformer

• Ensure proper cooling and wiring

✅ Conclusion

Building a simple inverter using MOSFETs is an excellent project for anyone interested in power electronics. It teaches essential concepts like switching, waveforms, and transformer behavior.

While it won’t replace a commercial-grade inverter, it’s a great learning tool and can power small loads efficiently.