# How do I tell which end of the LED is negative or positive?



## ruriimasu (Dec 23, 2008)

As per the title, how do I tell them apart? Kindly guide me, thanks


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## Resqueline (Dec 23, 2008)

Assuming you are talking about 3 & 5 mm LED's, the kathode usually has the shorter pin, and also a flat at the flange. The kathode is also the bigger part inside that the chip is bonded to. Kathode is negative.


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## ruriimasu (Dec 23, 2008)

thanks a lot! that is very informative  one more question, what will happen if i connect the wrong side to the battery? will it burn or..?


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## Black Rose (Dec 23, 2008)

It depends on the quality of the LED.

Nichia 5mm LEDs can take 5V of reverse voltage before they


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## zipplet (Dec 23, 2008)

I recommend a cheap LED tester device additionally. I have one that has testing positions for 1ma, 2.5ma, 5ma, 10ma, 20ma, 25ma and 50ma. If you will use LEDs often it's incredibly useful to check they work before you solder them in (that's bitten me before) and to check polarity if uncertain. Only downside is the current isn't particularly accurate depending on the vf of the LED.

I can provide links via PM (no shop linking allowed iirc?) if anyone wants


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## Mr Happy (Dec 23, 2008)

I think it should be possible to make an LED tester yourself from 3 AA cells (approximately 4.5 V) and a 1000 ohm (1k) resistor. Connect the resistor in series with the 4.5 V battery to limit the current to about 5 mA or so in the event of a short or a failed LED.

Touch the circuit momentarily to the LED connections. Most LEDs will light up if connected the right way round and will fail to light if connected the wrong way round. Mark the positive lead with a red marker pen or a dab of TippEx correction fluid or something.

Once you have the polarity identified, you can feed more current through the LED in the forward direction to make sure it lights up properly. Most miniature LEDs are limited to about 20 mA in normal operation.


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## ruriimasu (Dec 24, 2008)

Mr Happy said:


> I think it should be possible to make an LED tester yourself from 3 AA cells (approximately 4.5 V) and a 1000 ohm (1k) resistor. Connect the resistor in series with the 4.5 V battery to limit the current to about 5 mA or so in the event of a short or a failed LED.
> 
> Touch the circuit momentarily to the LED connections. Most LEDs will light up if connected the right way round and will fail to light if connected the wrong way round. Mark the positive lead with a red marker pen or a dab of TippEx correction fluid or something.
> 
> Once you have the polarity identified, you can feed more current through the LED in the forward direction to make sure it lights up properly. Most miniature LEDs are limited to about 20 mA in normal operation.



Wow. I got more than I asked for and now I can make my own LED tester  can i just use one 4.2V li-ion 18650 instead of 3 x AAs? what color is an 1000ohm resistor, i was thinking i maybe able to pull one out of an unused device  thanks for your info Mr Happy


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## Mr Happy (Dec 24, 2008)

ruriimasu said:


> Wow. I got more than I asked for and now I can make my own LED tester  can i just use one 4.2V li-ion 18650 instead of 3 x AAs? what color is an 1000ohm resistor, i was thinking i maybe able to pull one out of an unused device  thanks for your info Mr Happy


You can use any battery with a voltage between 4 and 5 volts. A 1000 ohm resistor is brown-black-red. The value is not critical, something between 500 ohms and 1000 ohms should be fine.

The circuit works because LEDs are diodes. They allow current to flow in the forward direction and block it in the reverse direction. However, unlike normal diodes, LEDs have a very low reverse breakdown voltage; the reverse breakdown voltage is the maximum reverse voltage you can apply before the diode is damaged. But pretty much every LED has a reverse breakdown voltage over 5 V, which is why the battery used should have a voltage of less than 5 V, so that reverse biasing the LED will not damage it.

If you are going to do any experiments with bare LEDs, you will really need to get a selection of different value resistors too. There is an advantage to buying these new rather than obtaining them from old equipment: the new ones have nice long leads which are much easier to connect things to or to plug into bread boards.


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## zipplet (Dec 24, 2008)

You could go one further on this idea and solder up yourself something on a bit of stripboard with several resistors connected (in parallel) to one pole of the battery, linked to one side of several 2-pin pin sockets. Link the other side of the pin sockets to the other pole of the battery.

Voila, LED tester with several positions for different currents (label them!)

Again I can't stress enough how useful something like this is


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## Lynx_Arc (Dec 24, 2008)

my led tester is 2AA batteries. The voltage is low enough it doesn't harm either 5mm or luxeons.


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## FLT MEDIC (Dec 24, 2008)

An LED tester is the best but if you have none, one can use a 3v lithium coin/watch battery (like CR2016, CR2032, etc.) to test 5mm to 10mm LEDs. 

Just insert the coin battery between the LED's wire terminals to see if it lights up. If it doesn't, reverse the LED terminals or flip the coin battery over to change polarity and see the LED light up if it still works.

Make sure you use a coin battery and not a larger 3v battery so the reverse polarity voltage when testing from the coin battery cannot burn out your LEDs. Good luck.


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## VanIsleDSM (Dec 26, 2008)

If you have a $10 DMM (digital multi meter) it'll most likely have a diode test setting, works great for testing LEDs, and if you connect it the wrong way, no damage done, it just won't light up.

Also very useful for lots of other stuff.


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