A DC to AC converter, commonly called an inverter, works by rapidly switching direct current back and forth through a transformer to simulate alternating current. At its core, the circuit needs just three stages: an oscillator to set the switching frequency, power transistors to do the actual switching, and a transformer to step the voltage up to usable levels. You can build a basic working inverter with fewer than a dozen components, though getting clean, stable AC output takes more refinement.
How an Inverter Actually Works
Direct current flows in one direction. Alternating current reverses direction many times per second, 50 or 60 times depending on your country. An inverter fakes this reversal by using transistors as electronic switches that flip the current’s path through a transformer, first one way, then the other. The transformer then steps the voltage up from your DC source (typically 12V from a battery) to mains voltage (120V or 220V).
The switching pattern matters. The simplest inverters produce a crude square wave, which works for resistive loads like incandescent bulbs but can damage sensitive electronics. Better designs use a technique called sinusoidal pulse-width modulation (SPWM), where the switches flip on and off at high frequency in a pattern that, after filtering, produces a smooth sine wave closely resembling utility power.
The Three Main Circuit Topologies
Most DIY inverters use one of three switching arrangements, each with different power-handling capabilities.
- Push-pull: Two transistors alternate pushing current through opposite ends of a center-tapped transformer. This is the simplest topology and works well for low to moderate power levels, roughly up to a few hundred watts. It’s the most common choice for a first build.
- Half-bridge: Uses two transistors with a split capacitor bank instead of a center-tapped transformer. It handles more power than push-pull but requires careful capacitor balancing.
- Full-bridge (H-bridge): Four transistors arranged in an “H” pattern, with the load (transformer primary) across the middle. This is the standard for higher-power inverters because it can handle twice the power of single-quadrant designs. Commercial inverters almost universally use an H-bridge for the DC to AC conversion stage.
For a first project, push-pull is the easiest to build and debug. If you need more than 500 watts or so, plan for a full-bridge design from the start.
Choosing Your Components
The Oscillator
Something needs to generate the timing signal that tells the transistors when to switch. The CD4047 IC is a popular choice for beginners because it produces a clean 50% duty cycle square wave from just a resistor and capacitor. To hit 50Hz (standard in most of the world) or 60Hz (North America), you select component values using the formula: frequency equals 1 divided by 8.8 times resistance times capacitance.
Using a 0.1µF capacitor puts you in the 10Hz to 1kHz range, and a 250K potentiometer lets you dial in the exact frequency you need. A potentiometer is preferable to a fixed resistor here because it lets you fine-tune the output with a multimeter.
For more advanced builds, the SG3524 or TL494 PWM controller ICs pack an oscillator, error amplifier, and current-limiting circuitry onto a single chip. These are designed specifically for power conversion and can generate the SPWM signals needed for a cleaner sine wave output. The SG3524 directly supports push-pull topology with built-in pulse steering.
Power Transistors
MOSFETs are the clear choice over bipolar transistors for the switching stage. They require almost no input current to control the load current, run cooler, and have very low on-resistance, meaning less wasted energy. The IRFZ44N is a widely available, inexpensive MOSFET commonly used in 12V inverter projects. For a push-pull circuit, you need two. For a full H-bridge, you need four.
When selecting MOSFETs, pay attention to three specs: voltage rating (should be at least double your DC supply voltage to handle spikes), current rating (must exceed your maximum expected load current), and on-resistance (lower is better for efficiency).
The Transformer
The transformer steps your 12V DC up to your target AC voltage. The voltage ratio is directly proportional to the turns ratio. If you need 12V in and 120V out, you need a turns ratio of roughly 1:10. For 220V output, you need about 1:18.3. In a push-pull design, the primary winding is center-tapped, so each half of the primary carries the full input voltage alternately.
You can reuse a mains transformer from old electronics by running it “backwards,” applying your switched DC to the secondary (low-voltage) winding and taking AC from the primary (high-voltage) winding. This is the most practical approach for a DIY build since winding your own transformer requires careful calculation of core size, wire gauge, and turn counts to avoid saturation.
Putting the Circuit Together
A basic push-pull inverter wires up like this: the CD4047’s two complementary outputs (pins 10 and 11) each drive the gate of one IRFZ44N MOSFET. The drain of each MOSFET connects to one end of the transformer’s center-tapped primary winding. The center tap connects to the positive terminal of your 12V battery. Both MOSFET sources connect to the battery’s negative terminal. The transformer’s secondary winding is your AC output.
Between the oscillator IC and the MOSFET gates, add small resistors (10 to 47 ohms) to limit current surges during switching. Place a capacitor across the battery input terminals, as close to the MOSFETs as possible, to absorb voltage spikes. A 1000µF electrolytic in parallel with a 0.1µF ceramic capacitor handles both low-frequency and high-frequency noise.
On the output side, an LC filter (an inductor followed by a capacitor across the output terminals) smooths the square wave toward something closer to a sine wave. Without this filter, you get a raw square wave that is fine for simple motors or heaters but harsh on electronics.
Efficiency and Where Power Gets Lost
Well-designed commercial inverters achieve efficiencies above 90%. A DIY build will typically land lower, in the 75% to 85% range, depending on component quality and layout. The biggest sources of loss are switching losses in the MOSFETs (energy wasted each time a transistor transitions between on and off states), conduction losses from MOSFET on-resistance, and core and winding losses in the transformer.
Switching losses increase with frequency. Higher switching frequencies make filtering easier and produce cleaner output, but they also increase gate-drive losses because the MOSFET’s gate capacitance must be charged and discharged more often. For a basic 50/60Hz inverter, switching losses are relatively modest. For high-frequency SPWM designs switching at tens of kilohertz, MOSFET selection and gate driver design become critical.
Inductor and transformer losses break down into winding losses (resistance in the copper wire generating heat) and core losses (energy absorbed by the magnetic core material during each switching cycle). Using a transformer rated for your power level, rather than an undersized one, helps significantly.
Keeping MOSFETs Cool
Every watt of power lost in a MOSFET becomes heat. Without adequate cooling, junction temperatures climb until the transistor fails. Heat sinks work by providing a larger surface area for heat to dissipate into the surrounding air. The basic relationship is: thermal resistance equals the temperature rise above ambient divided by the power being dissipated.
For a 12V, 200-watt inverter, each MOSFET in a push-pull pair handles roughly 100 watts of throughput. Even at 95% MOSFET efficiency, that’s 5 watts of heat per transistor. At room temperature, without a heat sink, a small MOSFET package might rise 60 to 80 degrees Celsius above ambient, well past safe limits. Bolt each MOSFET to an aluminum heat sink with thermal paste between the surfaces. For loads above a couple hundred watts, consider adding a small fan for forced-air cooling, which dramatically reduces thermal resistance compared to passive convection alone.
Safety and Circuit Protection
The output of your inverter is potentially lethal. At 120V or 220V AC, the same voltages that come out of a wall outlet, a shock can kill. Treat the output side with the same respect you would give to mains wiring.
Install a fuse on the DC input, rated slightly above your expected maximum current draw. For a 12V, 300-watt inverter, peak current from the battery is around 25 amps, so a 30-amp fuse is appropriate. This protects against short circuits in the switching stage and prevents battery cable fires.
On the AC output side, use a fuse or circuit breaker rated for your intended load. The chassis of the inverter should be grounded. Connecting the chassis to earth ground allows fault current to flow through the ground path and blow the fuse rather than flowing through anyone who touches the case. The AC ground wire needs to be thick enough to carry the full fault current until the fuse blows, so size it at least equal to the fuse rating’s current capacity.
If you plan to connect any AC device that has a three-prong plug, you need a proper ground reference. Without reliable grounding, residual current devices (GFCIs, RCDs) cannot function, and metal-cased appliances become shock hazards during a fault. Enclose all high-voltage wiring and connections inside an insulated housing, and never test the circuit with the output exposed on a breadboard.