Never having used high power LEDs before, I have one question: Both these options set a specific output current (via resistors). What happens to the the voltage output? Does ohms law work its magic to give me a perfect forward voltage for the powering the LEDs?
That is changed until the current matches what it is set to be.
This is important with a high power LED as the voltage will have to change as the LED warms up and also ages.
The DIY design you linked to is a linear current regulator, so quite a lot of power will be dissipated in the mosfet (i.e. (supply_voltage - 0.65 - LED_voltage) * LED_current). On the other hand, if you use a logic level mosfet in that design, you can drive it direct from a PWM pin (instead of connecting the top end of the gate drive resistor to the LED +ve supply, connect it to an Arduino pin). As the current you want is only 350mA, you could reduce the cost by using a small NPN power transistor instead of a logic level mosfet, along with a lower value drive resistor.
The ready-made one is a switched mode regulator, so it will waste very little power. However, it doesn't have a control input. Switching the input to it is probably a bad idea because it looks like it has a capacitor connected across the supply. Switching the output may be possible, but you'd need to establish whether the current regulator is on the high or the low side in order to determine how to switch it.
[EDIT: I just spotted the following comment on the page for the ready-made regulator:
"Can be dimmed. The integrated circuit for this LED driver is the PT4115 LED controller, which contains a "DIM" input. Applying a pulse-width modified digital signal, or a variable DC voltage to this DIM input provides continuously-variable dimming of the LED. For more information on how to do this, see the PT4115 datasheet (it is very easy to read, so even a beginner DIYer should be able to do it)."
The DIM input is compatible with the signal from an Arduino output pin. So using this module is a good solution if you are able to make the necessary modification to it.
Thank you for the info. I have quite a few NPN power transistors laying around. Is there any downside to using these along with a resistor to power the LED compared to that linear current regulator design? The power dissipated would just be through the resistor (1 watt? 1/2 watt?) instead of the MOSFET correct?
I realize the switch mode regulator would be the best option but if I can, I'd like to do this with only components I already have.
The downside is that the LED current will vary with the power supply voltage and how hot the LEDs are. Not too much of a problem if the power supply is a regulated one.
If your NPN power transistors can dissipate a couple of watts and have an hfe of at least 50 at 350mA collector current, why not use them in place of the mosfets in the constant current circuit? You will need to reduce the value of R1 (between 180 and 330 ohms should do). As I said before, you can get PWM control by connecting the top end of R1 to an Arduino output pin instead of the +ve supply.
Thanks.
After playing with the resistor values a bit, my power transistors do seem to work well and I get a consistent current output under various loads. Waiting for the LEDs in the mail now.
