PWM · duty cycle · fans and pumps
PWM control wiring: how to control a fan or pump
A relay gives a fan two speeds: off and full. Pulse-width modulation switches the power on and off many times a second, and the share of “on” time — the duty cycle — sets the speed. Here is how the two are wired and when PWM is worth it.
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Relay vs PWM
| Relay | PWM | |
|---|---|---|
| Speeds | Off / full | Any, from the duty cycle |
| Switching element | Mechanical contact | Transistor (MOSFET) in a controller or the ECU |
| Typical use | Fans, pumps, lights | Fans and fuel pumps with a PWM controller, boost and idle valves |
| Noise and inrush | Full inrush at every start | Soft start, quieter at low speed |
How duty cycle works
- Duty cycle = on-time ÷ period. At 50 % on 12 V a brushed motor sees about 6 V on average and runs at roughly half speed.
- Brushed motors (most fans and fuel pumps) can be driven directly by a PWM power stage — never by a relay, whose contacts would burn.
- Brushless fans with a separate PWM input take constant 12 V power and only a low-current speed signal from the ECU.
- The motor’s coil needs a flyback (freewheeling) diode across it; good PWM controllers have it built in.
Wire and fuse: size for 100 %
PWM lowers the average current, not the current while the switch is on — and at some point the controller runs at 100 %. Size the fuse and the wire for full speed: a 300 W fan on 12 V draws 25 A, takes a 35 A fuse and 4 mm² over 1.5 m.
Size the circuit in WireForge
- The simulator calculates steady-state DC, so it models the fan at full speed (100 % duty) — which is exactly the case you size the wiring for.
- Draw Battery → Fuse → Relay or a Custom block “PWM controller” → Cooling Fan → Ground.
- Set the fan power and the wire lengths, simulate and click the fan wire to read the current.
- Pick the fuse and the cross-section so the simulator shows no overload or voltage-drop warning.
Questions
Can I PWM a fan through its relay?
No. A relay is too slow and its contacts arc at every switching. Use a PWM controller or an ECU output with a suitable power stage.
What frequency is used?
It depends on the load and the controller — from tens of hertz for valves to a few hundred hertz or more for motors. Use what the fan or pump maker specifies.
Can every fuel pump be PWM-controlled?
Brushed pumps usually can; brushless pumps with their own electronics need their own speed input. Check the pump’s documentation first.