How can I measure the forward voltage of LEDs?

[Guest]

I have some "XC209-20" LEDs along with some other package-less ones that are impossible to identify. I tried googling the identifiable LEDs to find the forward voltage so that I know what size of a resistor I need, but couldn't find anything. I saw that you can use a multimeter to do it but I tried that too, I might have messed it up but it didn't seem to work. How can I find the forward voltage? I also heard some LEDs have built-in resistors. Can I tell if mine do?

[givingtnt]

3 minutes ago, whitephoenix said:

I have some "XC209-20" LEDs along with some other package-less ones that are impossible to identify. I tried googling the identifiable LEDs to find the forward voltage so that I know what size of a resistor I need, but couldn't find anything. I saw that you can use a multimeter to do it but I tried that too, I might have messed it up but it didn't seem to work. How can I find the forward voltage? I also heard some LEDs have built-in resistors. Can I tell if mine do?

https://learn.adafruit.com/all-about-leds/forward-voltage-and-kvl

[W-L]

19 minutes ago, whitephoenix said:

I have some "XC209-20" LEDs along with some other package-less ones that are impossible to identify. I tried googling the identifiable LEDs to find the forward voltage so that I know what size of a resistor I need, but couldn't find anything. I saw that you can use a multimeter to do it but I tried that too, I might have messed it up but it didn't seem to work. How can I find the forward voltage? I also heard some LEDs have built-in resistors. Can I tell if mine do?

What kind of LED's are they a COB style ones with an aluminum backed PCB? If you can provide a photo it would give more information.  

[Guest]

9 minutes ago, W-L said:

What kind of LED's are they a COB style ones with an aluminum backed PCB? If you can provide a photo it would give more information.  

 

They look exactly like this

I was told that the packaging should have voltage but this packaging just says "XC209R-20 T1 red diffused LED (1.5mcd @ 10mA) " I looked up XC209R-20 and found nothing, but while looking up XC209R I found that they probably have a voltage of 2.8V. But I still have a few with almost no information. They have the same style but vary in size and color.

[W-L]

Just now, whitephoenix said:

They look exactly like this

-PIC-

I was told that the packaging should have voltage but this packaging just says "XC209R-20 T1 red diffused LED (1.5mcd @ 10mA) " I looked up XC209R-20 and found nothing, but while looking up XC209R I found that they probably have a voltage of 2.8V. But I still have a few with almost no information. They have the same style but vary in size and color.

For regular 3-10mm LED Diodes they operate between 2.0-3.3V where red orange and yellows fall into the lower voltage category:

http://www.oksolar.com/led/led_color_chart.htm