I want to switch a 3W LED on and off from an Arduino pin. The LED will be powered by an external 12V source with GND common to the Arduino and will be on continually for periods of about 8 minutes with several minutes of off time between.
Can anyone recommend a suitable FET for this purpose ?
Before anyone asks. There is a resistor in series with the LED and I do realise that using a resistor wastes power and generates heat compared with a proper LED driver but space is limited, weight is important and cooling is not a problem.
How much current? All we know is the power and supply voltage - it could be
3 1W LEDs in series, 1 3W LED by itself...
For MOSFET you need to decide the max power dissipation you want (best to
go for < 0.3W then heatsink avoided), and select the relevant on-resistance
based on the load current and I-squared-R.
You will need a heatsink for the resistor perhaps?
The LEDs are on the front and back of a quadcopter. 2 white on the front and 2 red on the back to help orientation in flight. Each LED is connected to the 12V supply (in practice it starts at 12V or so and drops to 10.4V over a few minutes of flight) via a 100 ohm 1 Watt resistor so there are 4 LEDs and 4 resistors. Presently the LEDs are on whilst the Lipo is plugged in powering the quad. The resistors get hot but they are in fresh air and there is plenty of airflow around the quad when the motors are running.
The current through each resistor and LED is about 80 mA on an 11.4V supply (the Lipo was at its storage voltage at the time)
What I want to do is to have the LEDs come on when the quad flight controller is armed thus saving idle current, acting as an indicator that the quad is armed and to prevent me being blinded when working on the quad with it powered up (props removed of course)
I have the program sorted out to detect the flight controller being armed which will take an Arduino pin HIGH or LOW as required. Now I need to sort out turning the LEDs on and off.
Are you looking for throughhole parts, or SMD's? I would think for that application, to control weight, you'd want a surface mount part. I default to using SI2302/SI2312's, but there's nothing special about them - it's no problem finding a SOT-23 mosfet that will handle that.
Don't the high power LEDs thrash the battery life? 4x3W LEDs is a lot of power going towards light instead of flight.
I think it would still be a better idea (and not expensive) to use a LED driver. eBay has plenty of drivers designed for MR16 bulbs in about every LED wattage combination thinkable and they're all under $2. Most are based on the PT4115 chip and can be modified for dimming using either PWM or just a potentiometer.
DrAzzy:
Are you looking for throughhole parts, or SMD's? I would think for that application, to control weight, you'd want a surface mount part. I default to using SI2302/SI2312's, but there's nothing special about them - it's no problem finding a SOT-23 mosfet that will handle that.
Don't the high power LEDs thrash the battery life? 4x3W LEDs is a lot of power going towards light instead of flight.
Through hole will be fine. The weight is not that critical considering that the Lipo weighs 100g and the total weight of the quad with battery is just over 400g.
As to the current consumption of the LEDs, as I said in a previous reply each LED draws about 80mA which compared with the 4 motors drawing up to 10 amps each is not a lot. I have not noticed any dramatic change in flight times with or without the LEDs but there are a lot of variables such as how aggressively the quad is flown, how much I indulge in aerobatics, the temperature and the C rating of the Lipo pack, its age and how carefully is has been treated during its life.
Have you got any recommendations for suitable SOT-23 devices ?
Chagrin:
I think it would still be a better idea (and not expensive) to use a LED driver. eBay has plenty of drivers designed for MR16 bulbs in about every LED wattage combination thinkable and they're all under $2. Most are based on the PT4115 chip and can be modified for dimming using either PWM or just a potentiometer.
UKHeliBob:
Have you got any recommendations for suitable SOT-23 devices ?
Like I said, I default SI2302 or SI2312 (or higher spec ones in that series) for SOT-23 mosfets - but I don't think there's anything magic about Vishay's FETs, they just happen to be pretty common, and very cheap on ebay (at least the SI2302 is)
The LEDs are on the front and back of a quadcopter. 2 white on the front and 2 red on the back to help orientation in flight. Each LED is connected to the 12V supply (in practice it starts at 12V or so and drops to 10.4V over a few minutes of flight) via a 100 ohm 1 Watt resistor so there are 4 LEDs and 4 resistors.
Not totally clear, but I think you mean one 3W LED per MOSFET? But in that
case why current limited to 100mA level? Almost any switching MOSFET or transistor
will handle 100mA.
Did you think of stringing them in series for better efficiency?
Yes, I had thought of putting the LEDs in series but the first thing I am going to try is having just one LED front and back and seeing how I get on with orientation like that.
I think it would still be a better idea (and not expensive) to use a LED driver.
You could reduce your series resistor to 50 ohms and PWM the FET at 50% duty cycle. All the LED driver does is PWM. Be sure to select a "logic level" FET and don't forget to put about 200 ohms between the Arduino and the FET gate.
