Building Your Own PWM Controller for HHO: Cheaper Than Buying

The Core Circuit

A PWM controller requires three functional blocks: an oscillator (generates the switching frequency), a duty cycle control (adjusts the on-time ratio), and a switching element (passes high current to the cell). Commercial units combine these in one package; a DIY unit uses an Arduino or 555 timer as oscillator and duty control, with a separate power MOSFET as the switching element.

555 Timer Approach (Simplest)

A 555 timer IC in astable mode generates a square wave with adjustable duty cycle via a potentiometer. Total component cost: $5. Limitation: the 555 approaches aren't easily modified for current limiting. Best for builders who want simplicity and are comfortable setting a fixed duty cycle manually.

Arduino Nano Approach (Most Flexible)

An Arduino Nano ($5) generates PWM on its digital output pins at programmable frequency and duty cycle. Adding a current sense resistor (0.01 ohm, 5W) and op-amp amplifier allows closed-loop current control — the Arduino reads actual cell current and adjusts duty cycle to maintain the target. Cost: $20 total including MOSFET and current sensor.

MOSFET Selection

The MOSFET must handle peak current and switching losses. For a 20A HHO system: IRF3205 (55V, 110A rated, 8 milliohm Rdson) is overkill in the best sense — minimal switching losses and thermal stress. Heat sink to an aluminum plate and optionally add a small fan for continuous high-duty-cycle operation.