3W 24V LED Array

First off, hello I'm new to this forum, new to Arduino (Should hopefully arrive by the end of the week) and new to electronics in general although I do have programming experience (C++). What I am planning on doing with the Arduino, is running an experimental high power LED array over a fish tank. The specs for the LEDs are; DC Forward Voltage (VF): 3.6 ~ 4.0 Vdc DC Forward Current (IF): Typ 650mA I would like to run 25 leds (5 groups of 5 leds in series). Each group must be dimmable which, if I understand the Arduino specs correctly I should be able to accomplish using the PWM pins. My problem is that I plan on running each group of LEDs on 24V, which is higher than the Arduino's input voltage. Any suggestions on how I could do this?

You simply need a N-channel MOSFET for each group. The MOSFET would go in series with your string of 5 LED's and be controlled by the digital-out on the Arduino. Basically the MOSFET would be connected as follows:

Source: Ground Drain: Cathode side of your LED string Gate: Connected to the PWM Arduino digital-out

You could do the exact same thing with a NPN transistor, but the MOSFET is preferable because it has a lower on resistance (less voltage drop) and generally switch faster. Plus you don't have to worry about current limiting resistors for the digital control.

The only slightly complicated thing will be to find a MOSFET with ratings that will meet your needs. The key items will be:

source-drain current: > 650ma drain-source breakdown voltage: > 24V power dissaipation: > about 15W

However, based on your specs your drain-source voltage should never really exceed about 4V since you have about 20V of drop across your series LED's. Most MOSFET's have a breakdown voltage of more than 20V so you shouldn't have any problems finding one to fit your needs.

power dissaipation: > about 15W

I am not sure where this comes from. The power will be the 650mA times the voltage generated by this current going through the Ron resistance of the FET. As it is a FET this can be extremely low in the order of 0.1 to 0.01 ohm. This means the power burnt in the FET can be very small compared to the power burnt in the load.

I will bring up another restriction in your choice of MOSFET drivers. Most MOSFET devices will want a 10 volt gate drive in order to conduct fully. If you try to drive these with 5 volts, they will either not work, or burn up because they are partially on and have high resistance.

So you need to look for a device that is rated for 5V operation. These are typically marketed as Logic MOSFETS. They will not conduct their maximum possible listed current at 5V, but they will be close to minimum resistance and able to conduct significant current with a heatsink. In your application, 650mA, you may not need any heat sinking. This would be determined by inspecting the datasheet for the chosen MOSFET and using the figures quoted for Vgs = 5.0V to calculate the watts that will be dissipated at your desired current.

To throw some part numbers out there, the RFP12N10L and the IRLZ14 are nice general purpose devices for this kind of application, and not too expensive. They may be somewhat overspecced for this particular project, but you can determine that from the datasheets and perhaps find a smaller and less expensive MOSFET that will work. I would recommend buying a number of the RFP12N10L devices to get a quantity discount and using them in many different projects, unless space and per-part cost is an important consideration.

Thank for the input, now I just need to wait for the arduino to arrive.