Soft start fan controller

Discussion in 'The Projects Forum' started by Jack_K, Jul 22, 2013.

  1. Jack_K

    Thread Starter Senior Member

    May 13, 2009
    OK guys. I see the problem here. Because I said I'm using a fan from an automobile you won't let me post. Yes, it's a fan from a Ford. I got it at a wrecking yard. I'm using it to cool a pump.

    The pump motor is rated 12-16 VDC. That's why I got an automobile fan. It was CHEAP! It can run off the same batteries as the pump. The pump only runs after a big rain, but when it does run, it often gets hot.

    The fan has three wires and I determined they are high speed, low speed, and ground. I used a shunt resistor to measure the current draw of both fan speeds. Low speed draws about 15 amps at start up and 8-10 while running. The high speed draws over 50 amps at startup but only around 30 amps while running.

    I used a circuit with two thermistors to switch two relays, one relay for each speed. I wired the relays so when the high speed thermistor circuit turns on it removes the voltage from the low speed winding and only the high speed winding gets voltage. I suspect the fan would not like both windings on at the same time.

    It often blows 30 amp fuses when the high speed cuts in and that means I've lost both speeds. The pump could overheat and die. Is it OK to parallel two 30 amp fuses?

    Better yet, is there a way to reduce the startup current surge (soft-start)?

  2. donpetru

    Senior Member

    Nov 14, 2008
    Use a PWM soft start circuit or analog soft start circuit. With both circuits you can reduce startup current surge by automobile fan.
    For example, you can use a simple circuit like this:

    In the above circuit, replaces Q2 with a transistor like this:
    and replace a relay coil with your fan.
    Rx = (Vcc - 12)/I, Ry = (Vcc - 12)/I_fan
    Last edited: Jul 22, 2013
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    You could do that. But you would only want the 2nd one connected during fan startup. You need some kind of speed sensor connected to a relay to swich out one of the fuses once the fan is running to speed.

    I can't guarentee you still won't blow fuses. Better to have one hi amp fuse for starting and a lower amp one for running.
  4. Jack_K

    Thread Starter Senior Member

    May 13, 2009
    I guess I could use the IRF3703 that I already have?
  5. Jack_K

    Thread Starter Senior Member

    May 13, 2009
    Ry would be an awfully small value (0.06 ohm). That's difficult to do.