There are basically two ways:- 1, analogue this is where you use a power transistor to wind up/down the voltage to the bulb. The disadvantage with this is that the transistor will get very, very hot as its dissipating up to 30 Watts or so, so you have to bolt the transistor to a heat sink, like a mini radiator to get rid of the heat. The second way is digital, you still have a power transistor feeding the bulb, but you switch the transistor on hard for different periods of time to get a different power in the bulb. The power transistor will either have .2 V X 5 A = 1watt dissipation or 12 V X .0001 A = .0012W dissipation. The system works like , if you switch the bulb on for 50 % of the time, where the time is a very short period, say .01 seconds, before the filament "lights up", the power is off again, so the average power across the bulb is 30 W(50 % X 60W). Its advantage is NO heat sink and very good efficiency, but its far more complex in terms of electronic devices.
Frank
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Another thought has come to me, because filament bulbs run so hot (>1200 degs C), their resistance changes a great deal (low resistance when cold, high when hot), so if you truely want a linear voltage to light intensity law then you will have to build some sort of law compensator, because I think you will find that at 70 % of the rated voltage there will be virtually no light output, yet the current will be 80 % of the rated current.
Frank
Thanks again everyone for replies.
I wanted to run by a final check of my circuit before i buy the MOSFET.
https://imgur.com/Ca8kJce
And the specific MOSFET I was looking at is the following:
https://www.digikey.ca/product-detail/en/FQP30N06L/FQP30N06L-ND/1055122
It is a logic level gate with maximum power of 75W.
My understanding is that by using PWM output of arduino that the MOSFET will act as a switch and this will solve the concerns with respect to power dissipation and therefore heatsinking the device. I'm obviously looking for the simplest solution here. Is there anything else I should be concerned with before going ahead?
Thanks!
First, isn't a car light bulb 55/60Watt? Why not use a mosfet to drive both!? at 75Watt you would be on the limits for one lamp only.
Second, really bad practice to drive the mosfet straight from the arduino pins. For one its dodgy - the mosfet will certainly overheat since there is not enough power to switch on/off quickly enough and the mosfet will work some time in the linear region as it goes from fully off to fully on, for other if the mosfet ever fails it will put 12/14V on the Arduino and will ruin your circuit.
To overcomplicated you haven't stated your PWM frequency. This has a major impact in what kind of driver or power dissipation you will have on the mosfet. For a light bulb 1KHz is more than enought. Higher freuencies will increase linear losses on the mosfet as per the explanation given earlier.
You are missing a gate resistor in series between the mosfet and your uP. This helps avoid ringing.
Simple is not always the best long term solution
You are correct. I looked at only the first page of the datasheet where it says 10V for the gate.the MOSFET is a logic level gate, doesn't that mean that the arduino output should provide enough power to switch on and off the mosfet to prevent overheating?
Thanks for the reply.
I only need to drive a single headlight for this application; that's why I chose 75W. I added a 300ohm resistor between the arduino pin and the gate of the MOSFET. My understanding is this will prevent the arduino from being ruined if the MOSFET fails.
PWM frequency for arduino is 490Hz.
Second, since the MOSFET is a logic level gate, doesn't that mean that the arduino output should provide enough power to switch on and off the mosfet to prevent overheating?
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