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I have a 12V DC 0.4A brushless PC fan. For my application, I'm going to drive it at a low speed with 5V DC (I tested and it did spin). I want to control it with a 5V logic signal (simply on and off, no PWM). I do have a couple of logic level mechanical relays. But I also have some 2N7000s laying around. I thought a relay was a little clunky and cumbersome, and wondered if a mosfet would work.

  • I understand that 2N7000 can be driven at 5V. But its resistance is somewhere around 5 Ohm. Is that too big for a DC motor like this?

  • More importantly, is the starting current of the motor going to kill the mosfet?

Thanks!

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  • \$\begingroup\$ How much your fan draws at 5V and how much is the starting inrush current? And how much is current draw if you have 5 ohms in series? \$\endgroup\$
    – Justme
    Commented Jul 2 at 21:14
  • \$\begingroup\$ Yeah, that's the question. What is a good way to measure the inrush current with a multimeter? Or is there a good way to estimate based on the running current? \$\endgroup\$
    – Deling Ren
    Commented Jul 2 at 21:20
  • \$\begingroup\$ 3x running current is a good rule of thumb for starting current. \$\endgroup\$
    – vir
    Commented Jul 2 at 21:21
  • \$\begingroup\$ Thanks! Good to know! Lemme do some measurements. \$\endgroup\$
    – Deling Ren
    Commented Jul 2 at 21:24
  • \$\begingroup\$ Could parallel two FETs if you have them lying around to reduce the odds that you get the worst case 5 ohm resistance. \$\endgroup\$
    – user1850479
    Commented Jul 2 at 21:34

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Update: I measured the running current and it was a little over 60 mA. Going with the assumption that the start current is 3x the running current, it's pushing the limit of 200 mA. So I did end up with putting 2 MOSFETs in parallel and it worked great. Thanks for the suggestions and information.

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