Trying to power a 1W Luxeon star LED. It says: Forward Voltage: 3.5V, Forward Current 650ma. So I tried a 3.6V battery with a minimal resistor. Very low current. Then I tried without the resistor but still only about 100ma current. In order to get 650ma to flow through the LED I have to raise the voltage to about 11V and of course I already blew the LED. So... how can I get 650ma while only using 3.5V. Seems like my battery has too much internal resistance. Thanks much for your help.
If you want to get any decent length of life out of it, you don't want to be thinking in terms of voltage. It needs a constant current source. Try Googling "LM317 constant current source". It's a cheap and simple way of doing it with just a couple of resistors to set the current. The voltage that's used is then arbitrary, as long as it's enough. The voltage across the LED will settle to whatever is its natural value for the colour in question - maybe 3.5v, as you're expecting. Alternatively, use one of the electronic drive modules made by Luxeon, especially for the job.
What he said. I've used the LM317 to drive a LED headlight in a model railroad locomotive. Simple and works great; nice constant brightness regardless of track voltage.
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Found--the gene that causes belief in genetic determinism
"PeterD" wrote in : : Post an actual specifications sheet (or link to it), not an abstract. : There's more informaton that is missing from the above list of specs. : : The above specs are wrong in that they are missing critical : information about conditions for forward current for example.
IFM is Max Forward Current and is usually for a very short pulse condition, like 10mS on with a duty cycle of x%, and not steady state. Art
I've used LM317's as a constant current source for driving all sorts of LEDs for a long time, and never suffered a problem with a switch-on failure, so I guess the answer to "how fast is an LM317"?" is "fast enough ..." It's not a bad idea to have a decoupling cap across the output anyway, and the initial charging current that this will 'steal', should be plenty enough to ensure that the '317's output current has settled to the desired value for the LED, by the time that the cap's effective resistance has come up towards that of the LED.
Driving with any kind of constant current source, is superior to current limiting with a resistor from a constant voltage source for any kind of demanding use, although just using a resistor is fine for simple indicator type uses. The way to get the best performance and life from any high power LED, is to pulse drive it. As someone else commented, the max forward current quoted in specs, is for a short duration pulse. The level of these that some high power LEDs can withstand, is staggering, compared to the maximum continuous forward current. It is not impossible to put together a little circuit to pulse drive a LED satisfactorily, but it is easier to just use one of the ready made modules.
Your use of 11 Volts suggests you may have connected the LED backwards!
There is something wrong with your interpretation of the specifications. It is a one Watt device, but you seem to be trying to make it burn 2.275 Watts (3.5 Volts times 0.65 Ampere). I think the 3.5 Volts and 0.65 Amps are peak or maximum values, not for continuous use.
An LED is a diode. Voltage drop is just that, the Voltage measured across the device when it is conducting in its forward direction. You apply a current and measure the Voltage drop, not the other way around. For a 1 Watt LED, I would expect around 3 Volts drop for 1/3 Ampere of current (3 Volts times 1/3 Ampere equals 1 Watt).
To use a 3.6 Volt battery in theory, the series resistance would be 1.8 Ohms (0.6 Volts divided by 1/3 Ampere). With this approach, the light output will dim fairly rapidly since the internal resistance of the battery increases as it drains.
Use of a higher voltage with the LM317 constant current arrangement suggested by Arfa, is probably the best approach. Just make sure you don't set it for too high a current!
An LM317 with 1.92R (1.8R + 0.12R) between the common & output pins will give you a constant 650mA.
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In general, it's not so much about applying a current and measuring the voltage with LEDs, although your calculations are of course all correct. The forward voltage drop on a LED is what it is - i.e. the voltage will be pretty much constant irrespective of the current which that voltage causes to flow. A bit like a zener diode. The actual voltage varies a great deal between colours of LED, and even within colours, depending on LED power and other factors. The key is that as long as you apply enough voltage to exceed the forward voltage requirement for the LED in question, with a sensible margin available, then that *actual* applied voltage is arbitrary, the key requirement then being that the current is restricted in some way, to give the desired light output / life expectancy, without exceeding the manufacturers maximum figure for continuous (DC) operation, or pulsed drive.
It also has to be remembered that high power LEDs generate quite a bit of heat, and this needs to be removed fairly efficiently. If the LED is allowed to heat up exessively, its life will be considerably shortened. Its forward voltage drop will also vary with temperature, so if you are feeding it with a simple resistive current limiter, and running it close to max spec, you might run into additional problems with the current increasing further than you intend. Which is why it's an all-round better solution to drive with a simple constant current source, as Fred agrees.
Remember also, that if you are intending to drive multiple examples of the same LED in some kind of array, the normal way to do this is to put them into series 'strings'. So, as an example, the LEDs you are using here have a quoted forward voltage drop of around 3.5v. Lets say that you wanted to run five of them at perhaps 150mA. So, multiply 3.5 by 5 to get 17.5v. Add a bit of overhead to allow the LM317 to work, and call it 21v or so. A 15v transformer with a bridge and decent sized filter cap on the end would be just about right. Set the resistor values to give the 150mA, hook the five LEDs in series, and away you go. You might need a small heatsink on the LM317, but provided you don't go wild with the voltage that you're starting out with, or the drive current that you're asking it for, the device should not dissipate a lot of power.
Each LED in the string, will develop its own forward voltage drop, and this may well be slightly different for each example, and may vary slightly differently for each example as the temperature rises. No matter. The LM317 will adjust its output as required, to maintain 150mA through the string. You don't have to worry about voltages, or matching currents.
If you wanted more than five LEDs, you just put more in series, and raise the driving voltage appropriately, taking care of course, not to exceed the maximum ratings of the LM317, and not to go to a level that could be dangerous if touched. It is possible to parallel up strings to drive bigger numbers of LEDs at lower voltages, but this requires some slightly more complicated balancing arrangements, and protection against the current through the remaining strings increasing, if one string fails by an LED going open.
Well, those 'specs' are not a specification sheet, and not full specifications. That is what happens when you buy cheap Chinese crap...
What you should have asked for, before bidding, was the specicification sheet for the part. I bet the seller doesn't have it, I bet it doesn't even exist, in that the part was probably cobbled together without any real engineering or design, just a copy of some other maker's part.
I'd suggest you neg the seller for failing to provide specifications, get my money back, and move one.
I have a IR LED rated at 10A pulse current. I would like to do some experiments with it. I have hesitated as I do not want to damage it with overcurrent. The LM317 is rated at most at 1.5A depending upon the package type. I would probably start out with a current of 1A and increase it. How could I beef up the LM317 current carrying capability? There must be a circuit available that uses the LM317 and a power boosting transistor. Yes, I know, the radiant power is deadly to the eyes and all experiments would be in a light tight box.
I asked about the speed of the LM317 response as I would like to pulse the diode with pulse widths ranging from, say 1 uS to several mS. I would use a fast FET on the anode side for the switch.
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