I am having a hard time figuring out what the forward current would be so I can't figure out what size resistor to use.
Also...do you think it would be okay to control these through an arduino and a TIP120? so i'd have the LED cathode grounding controlled through the TIP 120, the anode would have a resistor on it, going into 12v as well as the tip120 would have a 1k resistor going to arduino pin
anyone know much about these? the info on instructables i've found didn't seem to be exactly what I wanted...
and also...anyone know where to get good heatsinks for them?
thanks for any help!
These types of high power leds really need to be driven by a constant current driver module, not switched on and off via a transistor and series current limiting resistor. And of course you will need to know what the recommended maximum continuous current rating is for the led before being able to select the proper CC driver. Also a heatsink of proper size and mounting method is also a requirement. The problem with a lot of those low cost Asian devices is that they often don't supply enough source data to actually be able to utilize the device properly.
I am having a hard time figuring out what the forward current would be so I can't figure out what size resistor to use.
10W at 12V is 830mA. They make 'standard' drivers for 850mA so I guess that would be the current (the 12V will be approximate, LEDs are driven by current, NOT voltage).
My experience is that eBay power ratings are optimistic. You might be able to run them at the stated rating in a laboratory with some sort of super heat sink but it's best not to try it at home. Those things get HOT. Burn-your-house-down type hot.
I'd try running it at about 60% (eg. 500mA) and see how it goes. Even so, make sure you get a big heatsink.
treebykooba:
Also...do you think it would be okay to control these through an arduino and a TIP120? so i'd have the LED cathode grounding controlled through the TIP 120, the anode would have a resistor on it, going into 12v as well as the tip120 would have a 1k resistor going to arduino pin
What's the reason for doing it? I assume you're going to try PWM control. If so, 1k for the gate resistor is much too high. 150 Ohms is the right size.
It might be OK to do it that way at 60% power but don't try going anywhere near 850mA with just a resistor to control the current. Even at 850mA you'll need a really high wattage resistor, eg. 10W, because the resistor is going to generate a lot of heat of its own.
To figure out the right value for the resistor you can put a potentiometer on the gate of your MOSFET and use it to adjust the MOSFET's resistance. Start at zero gate voltage then turn it up gradually until you get 500mA flowing through the circuit. Measure the voltage drop across the MOSFET. Use that voltage drop+current reading to calculate the value of the resistor you need. Be quick because your MOSFET is getting hotter all the time, even with the heatsink. Two multimeters is very useful here (one for the current, one for the voltage).
treebykooba:
and also...anyone know where to get good heatsinks for them?
That sort of LED die can be cooled with a chipset cooler or similar - 5cm square heat sink with 12V fan cooling -
fan can run from the raw 12V nicely. You will need good solid mounting with thermal-paste to transfer that
amount of heat well. LEDs must be kept cool - cooler than silicon devices I believe, so don't skimp on cooling.
The device is probably three 3W chips in series, it will probably pull about 10V at 0.85A, so a constant-current drive
running from 12V would work. Offhand the CAT4101 driver might be a good match - check the datasheet though.
I've seen 4-chip 12W LEDs in a similar format wired as 2-in-series-in-parallel.
To expand a bit on the voltage & current issue... LEDs (all diodes actually) are non-linear. A voltage change of 1/10th of a volt might change the current flow by a factor of 2. On top of that, the forward voltage drop (at a constant current) varies with temperature, and from part-to-part. So the proper solution is to use "constant current" power supply. A constant-current power supply tries to supply the same current no matter what load is attached. (At some point if the load resistance is high-enough, the power supply cannot supply enough voltage, and the current will be lower. But, within it's normal operating conditions, the current is (approximately) constant.)
With "normal" low-power LEDs, a constant-voltage power supply and a current limiting resistor can approximate a constant-current source, and thats how it's done with "regular" LEDs. The higher the supply voltage, and the higher the voltage-drop across the resistor, the better it approximates a constant-current source. But, the resistor usually wastes more power than the LED is consuming, so a current limiting resistor is not efficient and not practical for higher-power LEDs. A constant-cuirrent switching supply (which uses an inductor) can be nearly 100% efficient. Just about anythhng you buy that has 1W or higher LEDs is going to have a constant-current switching supply. The downside to a constant-current (switching) supply is complexity and cost.
A "normal" power supply is constant voltage... A good well-regulated 12 power supply will supply a (approximately) constant 12V as long as the input voltage and current-load are within spec. Actually, most "things" are constant voltage... For example the Arduino's output pins are (about) 5V (when "on" ), as long as you don't exceed the 40mA output-current rating.