I want to install a PWM controller to a small 12 volt furnace fan in a camper. I want the fan to run slowly after furnace shutdown, to quietly stir the air for more even heating.

My thought is to install the PWM controller parallel to the furnace's own fan circuitry, so the fan will run full speed while heating and will continue to run slowly after shutdown. This arrangement would backfeed the PWM controller with full fan voltage while the furnace is heating.

Is this a problem? I could isolate the PWM controller with a diode if necessary. Will this work? Is it dangerous? Can these systems live together side by side? Any help would be great appreciated.

  • \$\begingroup\$ Possible yes. But you need to determine the existing switching topology of the furnace controller. \$\endgroup\$
    – sstobbe
    Apr 10, 2019 at 15:27

1 Answer 1


It would be safer to isolate the PWM using a SPDT relay with a 12V coil:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ excellent answer \$\endgroup\$
    – scorpdaddy
    Apr 10, 2019 at 16:00
  • \$\begingroup\$ Thanks l like this solution.simple and effective. I'm on it. \$\endgroup\$
    – Hillsider
    Apr 11, 2019 at 19:05

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