MOSFET Rds

Discussion in 'General Electronics Chat' started by Flug540, Oct 17, 2013.

  1. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    I'm picking a MOSFET for a PWM motor control circuit and happened to stumble across several high-amp kits that are available for sale on the net that are using IRF7862PBF and IRF8734PBF. They seem to be very similar in characteristics and capable of handling up to 17A continuous current so I was going to order them for my project but then I noticed that they seem to have a very high Rds. Am I reading this wrong or is it true that they have around 3Mohm ON resistance? Why would you want Rds this high? I thought you want Rds as low as possible for switching circuits.
    Thank you!
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    3 milliohm ;)
     
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    That's 3 milli-ohms, not 3 Meg ohms. That's like 22 inches of 12 gauge wire. What do you want? micro ohms?
     
  4. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    Oh... oh... that makes sense :)
    Thanks for pointing out, still learning :D
     
  5. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    Another question I have is what should be the rating of the flyback diode connected in parallel with the motor? Should it be similar in rating to the MOSFET or the flyback current isn't that high?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    I believe I've learned here that 2X the motor's rated current is a good rule of thumb. I'm sure someone else will chime in to set us straight. Oh, and I believe you'd say it's anti-parallel.
     
    #12 likes this.
  7. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    Point in fact, whatever current the inductor (motor) is using at the moment of switch-off is what will demand to continue flowing until you can dissipate the energy. Theoretically, that's 1 X the run current, but we all know what happens when you try to work without any "safety margin", "excess capacity", such as that. Two X should be good, but if we're talking 18 cents difference, go for 3X.:cool:

    Another point: If there is any chance the motor might get switched off during a start surge, "locked rotor amps", use that for your standard and then add a safety margin.
     
    Last edited: Oct 17, 2013
  8. Flug540

    Thread Starter Member

    Mar 24, 2012
    60
    1
    Got it, thanks guys!
     
Loading...