Hey guys, wanted to PWM a 3W (3,6V forward voltage) LED with the Photon…
what circuit should I be looking at?
Looked for a really simple one that used a N-MOSFET, but I’m not sure if Photon’s Vin can get over 5.5V…
This is the one:
What kind of LED is it ?
Dependant on the current you could maybe just use a transistor if you need a on / off switch or a mosfet using 5V supply and the 3.3V from the photon as the switch…
If you need to be able to make it dimmable you could maybe use a driver ic?
As I said, it is a high power LED.
Forward current of 750mA approx… can Photon drive this with a appropriate power supply?
I’m not sure of what IC Driver to use, can someone suggest a simple circuit for me to use?
Thanks
The N-channel MOSFET you linked to at Sparkfun or this one at Adafruit would be fine. I would not use the 10k gate resistor with a 3.3V drive device like Photon, so you might want to think about bringing that down to 4.7k or even 2.2k to be sure you turn on the transistor.
The thing you need to realize is that you are going to need a separate power supply of some sort for the LED since the regulated power on a Photon cannot supply enough current (nor could a USB connection). You may or may not want to power the Photon off of this same supply via the Vin pin but you need to be prepared to deal with any switching noise if you use the same supply. Or you might want a multi output power supply. It really depends on the physical case you want to stick this all in.
Another point to consider is a heat-sink for the LED. It would probably like one on the back side since it will likely get quite warm.
1 Like
I have a ready heat sink for all those components
I’m going to try suppying Vin = 5V with only one power supply. What kind of noise reduction should I be looking at?
Vin = 5V and R1 = 2.2k, is that it?
Thanks for helping!
So the on resistance Ron for the MOSFETs is very low and you don’t have any active current limiting, so if you are going to use 5V power your LED with Vf = 3.6 V, you will need some kind of software current limiting. In other words, if you turn on the digital output without any PWM control, you are very likely to burn out your LED. Software control could still be OK, but you will need to experiment a bit and be open to the possibility of burning an LED out by accident.
You may be limited in the number of steps of PWM control you have since you cannot take it up too far. To average 3.6V out of a 5V supply you need a PWM setting of around 183/255 for instance. You may need faster PWM as well to avoid overheating–lots of LED drivers run at over a 1 MHz.
A series power resistor would mitigate the burnout problem but might limit the full brightness. At 5-3.6 = 1.4 V and a full current of 0.75A, you need a 1.86 ohm resistor rated for say 5W. The nearest standard values are 1.8ohm and 2.2ohm in 5W resistors. It will waste a lot of power too.
Another approach is active current limiting with more circuitry. Or a real LED driver (try Maxim or Linear Tech).
It is really hard to answer a question like this since I feel that maybe you don’t have a lot of experience designing electronics and there are literally thousands of ways to do this. I have been just going along with your original direction using the MOSFET, but there are many other ways you could go depending on your skill, budget, space, etc.
3 Likes
Indeed I have absolutely no experience in designing circuitry! But thanks for the help even though I’m a newb =D
Since I’ve done some research before, of course I stumbled upon those drivers. But they didn’t look begginer friendly so I tried to stay away from them. Also, here (Brazil) it’s very hard to find those components, and international mail will always get stuck in the customs.
So I was looking for something simple and easy to find. I have access to a osciloscope so I’m pretty much sure I’m going for the software current limiting solution (sadly it is inefficient)
1 Like