Capacity for P MOSFET as diode in 12v circuit

Discussion in 'General Electronics Chat' started by quietplace, Jan 23, 2016.

  1. quietplace

    Thread Starter New Member

    Jan 23, 2016
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    I am modernizing my car harness, and I am using a P Mosfet as a diode, powering the low beams. what will be the capacity for the P Mosfet in this application? the low beams draw about 8 amps, will an IRF9540N hold up?

    Cheers
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    What does the datasheet say? My bet is that it won't. Why not use an actual diode if that is what you want?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I think it will hold up but make sure you have a good heat sink. Note that the on-resistance is about 0.2 ohms. That means, at 8 amps, there will be a voltage drop of about 1.6 V across this device. Leaving you with only 10.4 volts.

    That will take your headlights from (12V x 8A down to 96 W) down to (10.4V x 7.5A = 80 W). So, I would not recommend this device. There are P-channel MOSFETS with much lower (under 0.05 ohms, even 0.02 ohms) that you could use with almost no voltage drop.
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    Isd (source to drain current) is listed as -19 Amps. But GopherT's comments tell a better story. By the graph I see about 0.9 Volts but there probably are better choices out there.
     
    Last edited: Jan 23, 2016
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    At 8A and Rds(on)= 0.2 Ohms that poor old FET will have to dissipate around 12W! I'd be looking for one with a much lower Rds(on).
     
  6. Dodgydave

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    Jun 22, 2012
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    Why do you need a diode, what are you trying to achieve?
     
  7. quietplace

    Thread Starter New Member

    Jan 23, 2016
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    I am using a series of relays and diodes to get, among other things, the headlights to automatically turn on when the ignition is switched on, while at the same time getting the light switch to function as normal when the ignition switch is turned off (while having as little current as possible go through the 45 year old light switch). I got the system itself to work as it should, but I do not have the headlights available yet. What would you recommend? Is a schottky diode the way to go? Or a better P MOSFET? Model? Maybe another relay would be better? I just started work on the harness two weeks ago, my specialty is metalwork, so this is all very new to me. I enjoy relays because they remind me of computer programming IF/ELSE etc.

    My car project: http://www.alfabb.com/bb/forums/gt-...68-1300-gt-junior-sicily-norway-w-photos.html
     
  8. crutschow

    Expert

    Mar 14, 2008
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    A MOSFET with sufficiently low on-resistance has the advantage over a Schottky diode of not requiring a heat sink.
    Just look for one with a maximum ON resistance of no more than about 10mΩ (.01Ω). That way the ON dissipation will be less than 0.7W.
    Note that the forward current direction of a P-MOSFET used as a diode (gate to ground) is drain-to-source with the blocking direction being the source positive with respect to the drain (drain acts as anode). This is opposite the normal current flow, but MOSFETs conduct equally well in both directions when ON.
     
  9. quietplace

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    Jan 23, 2016
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  10. hp1729

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    Browsing at Schottky rectifiers and MOSFETs rated at 20 Amps or so you are still going to have about 0.7 or 0.8 Volts lost across the rectifier. One of the other suggestions is a 100 Amp rated device. That gets your resistance down to milli-Ohms.
    Or were you just trying to work with stuff you already had on hand?
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Those certainly have a low ON resistance but I think they are all N-MOSFETs and you need P-MOSFETs.
    Go to a supplier webpage, such as Digikey and search through their P-MOSFET listings for a device with the desired ON resistance (≤10mΩ∨), voltage rating, package type, and cost.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Your headlights will come on wth a relay fed from the headlight switch, i would locate the relay and feed its coil with a diode from the ignition switch, ideally get the wiring diagram for the car.
     
  13. quietplace

    Thread Starter New Member

    Jan 23, 2016
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    How about something like a MBR6045 60A 45V TO-247 Schottky Diode?
     
  14. Dodgydave

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    Jun 22, 2012
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    Overkill, why use a diode when a relay is better???
     
  15. quietplace

    Thread Starter New Member

    Jan 23, 2016
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    I am going to see if I can draw up a wiring diagram to add to the thread
     
  16. crutschow

    Expert

    Mar 14, 2008
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    Dodgydave in post #12 seems to have the best idea, just a low current (1A) diode from the ignition switch to the headlight relay (assuming it already has one).
     
  17. quietplace

    Thread Starter New Member

    Jan 23, 2016
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    The wiring is a bit more complicated than that at this point, the 87a-connection is getting used on several circuits to make ignition on/off, and hi/lo-beams behave correctly, I am trying to rewire everything as we speak, lights already have 4 relays, trying to avoid another
     
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