As you can see by the attached image my first attempt was quite noobish...although it does work. Now I know I need to use something like a "buckpuck" etc for full brightness. But what you can see from the image is what I wanted to achieve, basically I just want to have 9 digital pins control 9 luxeon LED's (maybe having those 9 sub daisy chained with a further 3 luexons or even use 3W/5W ones instead?).
The problem with this set up was that not enough power was getting to the luxeon through my poor attempt at a simple switching circuit. Although when powering it straight from the 9V battery (white & red) it was nice and bright
So if anyone could suggest a simple way to drive 9 luxeons, with simple circuitry and control via setting the digital pins to HIGH that would be great! Also please bear in mind that my electronic knowledge is fairly limited and this is all being controlled through flash as3.
First off I would consider if you could get away with other [regular] LEDs... the super high-brightness ones in 5 or 10 mm size may be sufficient. But if you still want to use the Luxeons, you'll either need power source capable of supplying a fairly significant regulated current (I'm reading ~100ma per LED from a Luxeon datasheet), or use a switching power supply, like the one you provided in the ebay link.
It's fairly important though to have an idea of what your goal is, however... are you looking to drive 1W, 3W, or 5W LEDs? Would you like to drive them in sequence (only one on at a time) or in some other pattern? If you intend to sequence them, one power supply for all 9 LEDs, controlled individually by suitable (>0.5A) transistors or MOSFETS should work.
I don't see any obvious problem with them not mentioning red LEDs, though someone more experienced than myself may see a problem.
I've educated myself a little better on the whole thing now and order 9 constant current regulators (ebay) link. However id does seem as you pointed out my problem was not using a transistor with a high enough amperage, therefore the led was underpowered.
Could you or anyone else suggest an appropriate transistor for switching high power luxeons? I've never used MOSFETS before...
International Rectifier IRLB8748PbF or Fairchild FDP8878 are
both 30V logic level FETs. Used as a switch you should be
able to get over 30A from either device.
I notice from the photo that you're using a bare luxeon emitter without heatsink. You'll need to attach it properly to a heat sink before cranking up to full power or it will fry. For 3W and 5W this matters even more!
I much prefer the luxeon's with the aluminium base (about 2cm across) that you can bolt onto a bigger heatsink.
You'll need to check the datasheets to see what current they need (as the voltage depends on the colour) and make sure the switching transistor/FET can take that.
Also the power supply will have to be more than a PP3 as you'll be taking several amps in total for 9 LEDs.
And each LED needs a series (power) resistor to limit the max current to its rated value. To avoid wasting power you probably want to either drive them from about 5V or drive them in series pairs at 9V.
But again this is substantial amounts of power and heat sinking will be needed.
I hope you don't mind the spam, but I have created an Arduino shield for exactly the kind of LEDs you are working with. It will make your life easier and help keep you from damaging your Arduino.
However, you MUST heatsink the LED or it will burn up in a matter of seconds. Usually DIYers buy the LEDs pre-mounted to the little aluminum PCBs called "stars". Get some Arctic Silver epoxy and mount it to a CPU heatsink before trying to light it up.
I bought the International Rectifier IRLB8748PbF you mentioned and replaced it for the transistor however I'm still unfortunately getting the same dimmed output from my luxeon...
Any suggestions anybody? It's all being driven from a wall wart so power shouldn't be an issue. I've also tried a range of resistors. I'm starting to think I should use relays instead? Thanks!
I bought the International Rectifier IRLB8748PbF you mentioned and replaced it for the transistor however I'm still unfortunately getting the same dimmed output from my luxeon
First off what is the schematic you are using, not a photo that is almost impossible to follow.
This is because the voltage drop across the LED is not stable with temperature and so a simple resistor will not work as it warms up.
However it sounds like you have either not wired it up correctly or have got too high a resistor because you are not facing the problems you should be facing with this poor arrangement.
The power to the LED is coming from your Arduino Vin pin. Those pins aren't designed to handle the kind of current you're pulling for the LED and is probably dropping the voltage. (It may be hard on the circuit board traces too) Try running the LED power directly to the breadboard and see if that helps. Make sure you keep a common wire between your Arduino and breadboard ground connections (But NOT the + connections). The Arduino will be powered from the USB and the LED will be powered from the wall wart.
However, there is important information missing from this thread:
How many volts and mA does the LED need?
How many volts and mA is your power supply?
How many Ohms is the FET when it is in the 'on' state?
That will factor into us providing a suitable solution.
For example (values pulled from thin air):
LED: 3.4V @ 350 mA
Power supply: 9 VDC @ 500 mA (switching and/or regulated)
FET 'on resistance': 1.54 Ohms
You would need a resistor that is 14.46 Ohms (use 15 Ohms) and 2 Watts for this example.
9V - 3.4V = 5.6V /.350A = 16Ohms - 1.54Ohms = 14.46Ohms
.35A * .35A * 14.46Ohms = 1.77Watts (for the resistor)
.35A * .35A * 1.54Ohms = 0.18Watts (on the FET, might need a small heatsink)
But I can't stress enough two things: You need a constant current driver for power LEDs (not a resistor) and they must be heatsinked or you run a very high risk of burning up the LED.
As a rule of thumb you should avoid using a resistor to limit the current if the resistor works out to be less than 30R. In these cases there is simply not enough safety margin to make using a resistor viable. In these cases you should use a constant current supply.
The questions posed:-
However, there is important information missing from this thread:
How many volts and mA does the LED need?
Is wrong an LED doesn't need any current, it is your design decision as to how much you are going to let it have.
How many volts and mA is your power supply?
This is only important for powering lots of LEDs, for a single LED it is hard to think of a power supply that will not cope.
How many Ohms is the FET when it is in the 'on' state?
The on resistance of FETs varies from about 2R to 0.001R in the context of the circuit this is totally negligible.
However:-
But I can't stress enough two things: You need a constant current driver for power LEDs (not a resistor) and they must be heatsinked or you run a very high risk of burning up the LED.
Provide the correct math and info for driving a power LED with a FET and resistor, since it didn't look like the constant current driver approach was being considered. The numbers I used were merely for the example. It was meant as sort of an "if you're going to do it that way, then you need to do this to do it right". Admittedly, I didn't make it clear that it was not a very good way to go.
Reinforce the proper way of running power LEDs by using constant current drivers and appropriate heat sinking.
After reviewing the thread again, I see that I missed the OP stating they had ordered drivers off of eBay. However, after that, the OP again discussed using the FET to drive the LED. It doesn't sound like the constant current drivers are being used.