I want to switch a 3W LED on and off using a transistor that is controlled by Arduino.
Specs of the LED:
3.0-3.6v forward voltage
700mA constant current
I am currently using a tip122 that is controlled by Arduino but I want to make a PCB with an SMD transistor that can do the same as tip122 but more efficiently. (I know that the tip122 may be overkill, it also needs to work well in regard to heating, etc.)
Any help is appreciated
Try AO3400
May be a good choice, but is a MOSFET, not a transistor. So the schematic may need to be changed...
You're right, but I haven't used transistors in years.
Morsfets are better for digital applications.
Power LEDs are best driven with a constant current LED driver. That ensures constant light, independent of supply voltage and LED temperature variations, and they don't get hot (the LED still does). Example here (click).
Leo..
I am using constant current LED drivers, I forgot to mention it, and since I never used MOSFETs I wanted to be sure that they don't heat up more than they should.
What kind of resistor do I need between the Arduino digital pin and the gate of the MOSFET? (AO3400)
Do I need a resistor before the LED since I'm using constant current drivers?
It's not critical; anything between 0 and 1k or so will do. It depends a bit on how fast you want to switch the LED; I'm assuming something below 10kHz or so.
It's usually a good idea to tie the gate of the MOSFET to GND with 10k or 100k or so, so that the gate doesn't float if the connected microcontroller GPIO is set to a high impedance state. This can prevent behavior like a brief flicker of the LED as the system powers up or weird behavior (LED goes on/off all by itself after some time or if you touch something) if you set the GPIO to INPUT instead of OUTPUT for whatever reason.
Connect like this:
The 'power' signal is your GPIO. I used a 2n7002 here which is not suitable for your application; you'll use something like the AO3400 mentioned (good choice IMO). R37 can be made anything between let's say 10k and 220k. R36 can either be left out entirely or anything up to let's say 1k.
If you need to switch the LED really fast (>10kHz or so), the choice of R36 in my diagram becomes more critical, but again, I assume this is not the case here.
No. The LED driver handles the LED current.
What kind of LED driver do you have? Many of them have a dim/PWM pin. In this case you don't need an additional MOSFET at all and you can simply enable or disable the LED driver by applying a signal to its dim/PWM pin.
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