Help! The polarity of LEDs

[trenzafeeds]

Really hyped for the new Pok3r firmware update, and I'm looking to solder some of the leds from my old QF TK (and one desoldered from the Pok3r capslock) onto the PCB. All the people posting about the update are saying you have to reverse the polarity of the LEDs, and, being an electricals noob myself, I can't tell how to do this. How do I know which end is which on the LED and the PCB?

[sethk_]

long leg on LEDs = positive, and on the Poker 3 they have the LED silk screen backwards. All you need to do is stick the long leg in the - side.

[Findecanor]

If the LEDs are desoldered, then the legs are cut... so there is no leg that is longer than the other.

"Reversing the polarity" may sound like something out of Star Trek, but it is as simple as to swap + and - .

If the LEDs are somewhat transparent then you will be able to see the innards if you have good eye sight. Those innards are often a bit asymmetric and once you have figured out which legs are + and - on one LED in the pile then you should be able to find the polarity on the others as well.

Or you could build a LED tester. Connect a 3V battery pack, a resistor and two empty terminals in series. Then test the LED against those terminals to figure out which direction works. The resistor is necessary to not burn the LED.
I dunno about the value of the resistor though ... You could check a web site that sells LEDs what the LEDs are rated at, then google for a "LED Resistor calculator".

[MJ45]

When in doubt about the polarity I just insert the LED in the pcb if it lights up ok, if not reverse it. Easy way to test no equipment needed also verify polarity find defective (doa) LEDs then do my soldering. 

[quochung1989]

https://www.youtube.com/watch?v=iUAWsaMEEhE

[ghostjuggernaut]

When in doubt about the polarity I just insert the LED in the pcb if it lights up ok, if not reverse it. Easy way to test no equipment needed also verify polarity find defective (doa) LEDs then do my soldering.

This is the easiest way, if you are unsure. The amount of voltage needed to damage a diode in the reverse biased direction, is much larger than 5V. So you can feel safe using this method.

[trenzafeeds]

When in doubt about the polarity I just insert the LED in the pcb if it lights up ok, if not reverse it. Easy way to test no equipment needed also verify polarity find defective (doa) LEDs then do my soldering.

This is the easiest way, if you are unsure. The amount of voltage needed to damage a diode in the reverse biased direction, is much larger than 5V. So you can feel safe using this method.

Awesome, this is most likely what I'll do. Thanks everyone!

[yobinad]

You can quickly know led polarity just by visual inspection.
The small cup in the center is the minus

[bpiphany]

Perhaps not highly recommended for all types of LEDs, but this is a very quick way to test one