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P channel mosfet as light dimmer

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merk

New Member
I am bulding some PCB design and need your help round mosfet. Since light bulb (H7) is grounded at the headlight I can only use P channel mosfet. I wish that my mosfet would be by default closed and if I am correct pmos is on when gate is low so I aded additional NPN that will bring gate of pmos low. I found this diagram:


so I made my diagram like this:
I am using dual PMOS capable of 7A and with Vth of -2V.
Now my question is will this work, do I need some additional protection?
Some say that I am missing GS resistors on mosfet and current limiting resitor for collector.
I really don't understand why they mean since 100ohm resitor limit current through collector-emittor and gate-drain.
diagram.JPG
 

ronsimpson

Well-Known Member
Most Helpful Member
Looks good.
I think the 100 ohm resistor on the 2N4401-C is too low. 120mA 1.5 watts. Did I do the math right.
How fast are you turning on/off the MOSFETs?
Think about 1000 ohms.

Also 12 Volts will be 14.5 Volts when the car is running.
 

crutschow

Well-Known Member
Most Helpful Member
If you are indeed running from 24V than D1's 5.1V drop is too low a voltage to protect the MOSFET gate.
For that D1 should be about 12V.

If you are running from 12V then you don't need D1.

To protect for either 12V or 24V operation can put a 12V D1 in parallel with R3 and add another resistor in place of D1 between M1's gate and the collector of Q1
 

audioguru

Well-Known Member
Most Helpful Member
In addition to the zener diode's voltage being too low, the one you selected has its voltage rated only when its current is very high at 49mA. At the low current in your circuit its voltage might be only 4V. Pick a 12V zener diode rated at 5mA instead.
 
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