Capacity for P MOSFET as diode in 12v circuit

Thread Starter

quietplace

Joined Jan 23, 2016
10
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
 

GopherT

Joined Nov 23, 2012
8,012
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
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.
 

hp1729

Joined Nov 23, 2015
2,304
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
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:

Alec_t

Joined Sep 17, 2013
11,388
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).
 

Thread Starter

quietplace

Joined Jan 23, 2016
10
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-1963-1977/198287-project-1968-1300-gt-junior-sicily-norway-w-photos.html
 

crutschow

Joined Mar 14, 2008
25,103
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.
 

hp1729

Joined Nov 23, 2015
2,304
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
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?
 

Dodgydave

Joined Jun 22, 2012
9,197
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.
 

crutschow

Joined Mar 14, 2008
25,103
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).
 

Thread Starter

quietplace

Joined Jan 23, 2016
10
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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