During a multi-day power outage a couple of years back, I sat there with a perfectly good car battery outside and a house full of dead outlets, wishing I had some way to bridge the two. That frustration is exactly what got me building my first power inverter — a simple circuit using a CD4047 IC that converts 12V DC from a battery into a rough AC waveform capable of running small AC appliances. It's not going to run your refrigerator, but it kept a lamp, a fan, and a phone charger running for two days, which felt like a genuine win at the time.
This guide walks through building a basic square-wave inverter using the CD4047 — a solid, well-understood starting point before you consider anything more advanced like a pure sine wave design.
An Honest Note Before We Start
This project involves stepping voltage up to mains levels (220V or 110V depending on your region) and handling that voltage on the output side. Please take that seriously — insulate exposed connections properly, don't touch the output side while it's powered, and if you're not confident working with mains-level voltage, treat this as a "build and understand the theory" project rather than something you plug real appliances into without proper enclosure and insulation first.
Also worth knowing upfront: this design produces a square wave output, not the smooth sine wave that mains power normally is. This works fine for resistive loads like incandescent bulbs and simple heating elements, and reasonably well for many basic AC motors and chargers, but it's not suitable for sensitive electronics or devices that specifically require a clean sine wave (some sensitive audio equipment and certain medical devices, for example).
Why the CD4047 Specifically
The CD4047 is an astable/monostable multivibrator IC, which is a fancy way of saying it's very good at generating a steady, predictable oscillating signal — exactly what you need as the foundation of an inverter. It's simple to wire, doesn't need much beyond a resistor and capacitor to set its oscillation frequency, and it's been a reliable choice for basic inverter designs for a long time because of exactly that simplicity.
What You'll Need
- 1x CD4047 IC
- 2x Power MOSFETs (IRF3205 or similar, rated for the current your load will draw)
- 1x Center-tapped step-up transformer (12V-0-12V primary to 220V secondary, rated for your intended power level — a 5A transformer gives you a reasonable amount of headroom for small loads)
- 1x 10kΩ resistor and 1x 0.01µF capacitor (these set the CD4047's oscillation frequency — tuned for roughly 50Hz or 60Hz depending on your region's mains frequency)
- 2x heatsinks for the MOSFETs
- A 12V battery (car battery or a 12V deep-cycle battery both work)
- Appropriate gauge wiring for the high-current primary side — this matters more than people expect
How This Circuit Actually Works
The CD4047 generates two alternating square-wave outputs that switch out of phase with each other. Each of those outputs drives one of the two MOSFETs, which alternately connect each half of the transformer's center-tapped primary winding to the battery's positive terminal.
Here's the part that makes this work: as current alternately flows through each half of the primary winding, it creates a rapidly alternating magnetic field in the transformer core. That alternating field induces a much higher voltage on the secondary winding, proportional to the transformer's turns ratio — this is the same basic transformer principle used throughout electrical engineering, just running in the "step-up" direction instead of the more familiar step-down direction you'd see in a wall adapter.
The Wiring, Step by Step
- CD4047 Pin 4 (Oscillator output) and Pin 5 (complementary oscillator output) → these drive the gate of each MOSFET respectively, through a current-limiting resistor
- The 10kΩ resistor and 0.01µF capacitor connect to the CD4047's timing pins (Pin 1 and Pin 2), setting the oscillation frequency
- Pin 14 (VCC) → 12V supply, Pin 7 (GND) → ground
- Each MOSFET's drain connects to one end of the transformer's center-tapped primary winding
- Each MOSFET's source connects to the battery's negative terminal
- The transformer's center tap connects to the battery's positive terminal
- The transformer's secondary winding becomes your AC output
Circuit Diagram
(See the schematic below — a CD4047 IC configured as an astable oscillator with its timing resistor and capacitor, its two complementary outputs each driving the gate of a power MOSFET, with each MOSFET's drain connected to one end of a center-tapped transformer primary, the center tap tied to the 12V battery's positive terminal, and the transformer's secondary winding providing the stepped-up AC output.)
Building and Testing Safely
Build and test this in stages, not all at once. First, confirm the CD4047's oscillator is actually running at the right frequency using an oscilloscope if you have access to one, or at minimum, confirm the MOSFET gates are switching by checking for a changing voltage with a multimeter set to AC mode. Only once you've confirmed clean oscillation should you connect the transformer and battery for a live test — and even then, start with a very light load, like a single incandescent bulb, rather than jumping straight to whatever appliance you actually want to power.
Common Problems (And How I Actually Fixed Them)
No output voltage at all on the secondary side. Check your oscillator is actually running first — a dead CD4047 or incorrect timing component values means nothing downstream will work. Verify with a multimeter or oscilloscope before troubleshooting anything else.
The MOSFETs get extremely hot, very quickly. This usually means they're not switching cleanly, and are instead sitting partially on for extended periods, which dissipates a lot of heat. Double-check your gate drive signal timing and make sure your heatsinks are properly attached — this is a common failure point on a first build, and it's worth catching early before a MOSFET actually fails.
Output voltage seems low relative to what the transformer should provide. This can point to insufficient primary-side current handling — thin wiring or an undersized battery connection will cause voltage sag under load. The primary side of a step-up transformer draws surprisingly high current for a given output power, so this wiring needs to be genuinely heavy-gauge, not just "whatever wire was in the drawer."
The inverter powers small loads fine, but struggles or shuts down with slightly bigger ones. This is expected behavior for a basic design like this — you're working within the current limits of your specific MOSFETs, transformer, and battery. Scaling up meaningfully means upgrading all three components together, not just swapping one part.
Why Not Just Buy One?
Fair question, and for a genuinely important backup power need, buying a proper commercial inverter with real safety certifications and a pure sine wave output is honestly the more sensible choice for most people. This project is worth building for the same reason most projects on this blog are worth building: understanding exactly how power conversion works at the component level teaches you something that buying a sealed box never will, and it's a solid stepping stone toward more advanced designs — like pure sine wave inverters — once you're comfortable with the fundamentals here.
Final Thoughts
This build taught me more about transformers and MOSFET switching than almost anything else I've built, mostly because getting it working reliably forced me to actually understand why each part of the circuit exists, rather than just copying a schematic. It's not a project to rush, and it's not one to skip the safety considerations on, but if you take your time with it, it's a genuinely rewarding one to finally see a lamp light up, powered entirely by a car battery and a handful of components you wired yourself.
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