Heating the wire is a separate electrical system from moving the machine. It is low voltage but high current, and getting it right is key to both safety and cut quality.
A dedicated supply
Use a dedicated power supply for the wire — typically a current-limited bench or LED-strip supply delivering a few volts at several amps. Keep it electrically separate from the controller's logic supply. Size it for your wire length and diameter: longer/thicker wire needs more current for the same temperature.
Switching the wire
The controller switches the wire on/off (the M8/M9 commands) through a MOSFET driven from a PWM-capable output (often the "spindle" output). The MOSFET handles the high current; the controller just provides a low-current control signal. This keeps the heavy current away from the delicate logic board.
Setting the temperature (on the machine)
Beyond on/off, the heat is set on the machine side — either by hand (a bench-PSU voltage knob or a PWM dimmer module) or by PWM: rapidly switching the power so the average power, and therefore the temperature, is whatever you set. A higher duty cycle = hotter wire. cncfoam.com does not write a temperature value into the cut — it never knows your wire/supply — so any PWM target lives in the machine, not the G-code.
Closed-loop with current sensing
For consistency, a current sensor such as an INA226 (or measuring the wire's resistance, which rises with temperature) lets the machine hold a target rather than drifting as the wire ages or the supply sags. A closed temperature loop on the machine reacts in milliseconds — far faster than anything the G-code stream could do, and it keeps the heat constant whether the wire moves fast or slow.
Where control belongs
The cleanest division of labour: cncfoam.com emits only M8/M9 (wire on/off) — pure intent, no magnitude — and the machine owns the actual temperature, either manually or via a closed loop. That way the same cut file runs on any rig and survives a wire or supply change. See Wire temperature control (where it belongs).