How do you test LEDs?

[BhujangiJedi]

So if you want to run an LED with a resistor, how do you test the LED to determine the optimum resistor? What are you testing - the forward voltage? Can you do it with a simple DMM? (I have very little knowledge of electronics and haven't used a DMM in a decade.)

[Atomic]

I've been using this LED Calculator (pointed to me by Novastar, thanks again!)

http://led.linear1.org/1led.wiz

Resistors are not an exact science from what I can tell (since they have a 5% or 10% variance) but I think it gives you a good idea.

I have some resistors on order. I plan to do some testing with some white Cree P4s I have to see what they can take. I used the calculator and plan to keep downgrading the resistors.

[xwingband]

To do it you'd need a power supply to vary the current. It's an expensive tool though.

So... I'd say if you have one with the bin numbers you should track down the one that is the voltage range. It's not exact but it can help if your LED is out of the range that is "standard". Then you'd have to calculate the resistor (like on the link above) and get it close as you can.

Atomic, those LEDs can take a lot... you'll see it go blue if your heatsink can take it.

[BhujangiJedi]

Xwing - are you saying that the forward voltage of LEDs is tighter within bins? So if you don't know the bin, you know the vf range is say 2.95 - 3.25, but if you know the bin, you can narrow it down to something like 3.05 - 3.15?


With a variable power supply, you dial in the desired current and the PS adjusts the voltage as necessary (and displays it) in response to the load, correct? So now we know the correct vf for that specific LED and knowing our power source and desired current, we can calculate the required resistor. (Please correct me anywhere I'm off.)

So what if you have a puck, so now you know the current. Can you connect it to the LED and use a DMM to measure the voltage somewhere... at the battery pack, at the LED... ? And then figure out the forward voltage and calculate the optimal resistor for that LED?

[xwingband]

Yup, on all counts.

To clarify though... bins are a lot of things not necessarily all tied together. It's color, lumens, forward voltage range, etc... that's why the code is so long!