Headlight modulator circuit design

I am building an Arduino-based headlight modulator for a motorcycle. What seems to stand out as different than all other posts I have read, is the need for A) Positive side switching; and B) Normally Closed design.

I have deduced that a Depletion Mode P-channel MOSFET is required.
I will be using PWM on the Arduino to switch a LED headlamp @ 12V, 10A between 100% and 20% @ 240Hz.

What I am wondering, can I use this circuit design, substitute a FQP27P06 for a depletion-mode P-channel, to achieve my goal?

PFetSwitch.png

Any help would be greatly appreciated.

PFetSwitch.png

I have deduced that a Depletion Mode P-channel MOSFET is required.

Why do you think that?

That circuit is standard, and using the FQP27P06 should work well, but I would lower the gate resistor to 1K or less.

The circuit will switch between full on and full off at the PWM frequency, with the average lamp current being roughly proportional to the duty cycle.

In the circuit posted, the MOSFET is normally OFF (open) until the npn transistor is switched ON. Is that what you want? Also, your resistor arrangement is no good.

For 'normally on' you need a third drive transistor, or a mosfet driver chip with a 'normally low' output.
That circuit is dangerous, because a power spike >25volt will kill the gate of that mosfet.
A zener (~18volt) across source/gate is needed for automotive applications.
1.2mA gate pull up (10k) is rather low (switching losses/heat). Make it 1k.
Hope you are not trying to dim halogen lights. They have a shorter lifespan dimmed.
Leo..

jremington:
Why do you think that?

That circuit is standard, and using the FQP27P06 should work well, but I would lower the gate resistor to 1K or less.

Switching to a depletion-mode P-channel MOSFET allows the circuit to achieve “Normally Closed” when the Arduino is off or if the controller fails. This is a safety feature of the circuit to ensure the headlight will continue to operate whether the controller is functioning or not.

JCA79B:
In the circuit posted, the MOSFET is normally OFF (open) until the npn transistor is switched ON. Is that what you want? Also, your resistor arrangement is no good.

No, it needs to be Normally ON until the NPN is switched on. This is why I was thinking a depletion-mode transistor. Unless there is another way to design this circuit so common stocked parts are used.

Wawa:
For ‘normally on’ you need a third drive transistor, or a mosfet driver chip with a ‘normally low’ output.
That circuit is dangerous, because a power spike >25volt will kill the gate of that mosfet.
A zener (~18volt) across source/gate is needed for automotive applications.
1.2mA gate pull up (10k) is rather low (switching losses/heat). Make it 1k.
Hope you are not trying to dim halogen lights. They have a shorter lifespan dimmed.
Leo…

Thank you everyone for the 1k Gate recommendation. Adding the zener in serial with the 1k gate pull up should protect against an power spikes while switching, correct?

the circuit to achieve "Normally Closed" when the Arduino is off or if the controller fails.

This requirement was not made clear in the original post. You will need a different circuit. Google "normally closed mosfet switch" for plenty of ideas.

As others have mentioned, motor vehicle electrical systems are electrically very noisy, so you need high voltage spike and reverse voltage protection for any circuitry you add.

What, exactly, does this mean?

switch a LED headlamp @ 12V, 10A between 100% and 20% @ 240Hz.

niuhuskie13:
Adding the zener in serial with the 1k gate pull up should protect against an power spikes while switching, correct?

No.
The zener goes between source and gate, across the 1k resistor.
Look at the fourth diagram on this page.

Remove the two resistors on the base of the existing transistor, and use a 100k resistor from it's base to 12volt (vcc).
Use a second NPN transistor, and connect it's collector to the base of the exixting transistor.
Connect it's emitter to ground.
And use the resistors you just removed on it's base.
That will give you a 'normally on' circuit.
PWM will be inverted. 0 is full on, 255 is off.
Sorry, no time today to draw a circuit.
Leo..

Wawa:
No.
The zener goes between source and gate, across the 1k resistor.
Look at the fourth diagram on this page.

Remove the two resistors on the base of the existing transistor, and use a 100k resistor from it's base to 12volt (vcc).
Use a second NPN transistor, and connect it's collector to the base of the exixting transistor.
Connect it's emitter to ground.
And use the resistors you just removed on it's base.
That will give you a 'normally on' circuit.
PWM will be inverted. 0 is full on, 255 is off.
Sorry, no time today to draw a circuit.
Leo..

Thank you for the clarity and links. I have recreated the drawing based on your feedback and guidance. I appreciate your time!