5 Simple Ways to Determine LED Polarity

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About: Hi, my name is Nikodem Bartnik. I live in Poland, I'm 18 years old. I like to program and create robots, devices and things. In the future, I want to start a company that creates new technology. Right now I ...

LEDs are probably the most-liked elements by all beginners maybe even by everyone involved in electronics projects. One of the most important factor to use them properly is to connect them the way they should. Of course, usually you need to use a resistor in order to limit the current and avoid burning your LED, but that's not what I am going to write about in this instructable. I want to talk about even more basic thing: checking the LED polarity. In electronics you can find various components, we can divide them into polarized ones (LEDs, electrolytic capacitors, transistors, microcontrollers and many others) and ones that don't have polarity (resistors, coils and other). Polarized component means that it has to be connected in a specific way in order to work. Connecting it the other way may damage it, it may even explode (electrolytic capacitors) or your circuit will just not work properly. So polarity is very important as you can see. In this instructable, I will show you 5 ways to determine LED polarity. Let's start!

Step 1: There Is a Reason Why Those Are Different

Leads length. That's the easiest way to check LED polarity, at least if you have brand new LEDs. If you already used them in a project or unsoldered them from an old device this wouldn't work for you but no worries I have other 4 methods that may work for you :)

As you can see on the image above leads of an LED have different length and that's not a manufacturing error, it's done on purpose. It's the easiest way to determine where is a plus and minus of an LED. The longer lead is a positive one and shorter lead is a negative one. In case you cut out part of leads it wouldn't work for you, if you are not sure if you did, follow the next steps to be sure that you determined polarity right.

Step 2: #2 Nothing Is Exactly the Same on Both Sides

And so the sides of LEDs aren't. If you will take a closer look at the LED you will notice that one side of it is flat and again that's not a manufacturing error, that's a mark that lets you easily determine LED's polarity.

Lead next to this mark is negative another one is positive.

This method is probably the best, it's super easy, always work and you don't need any gear to check that. It's almost impossible to destroy this mark, maybe if you would sand down the other side of LED, but why? I don't know :)

Step 3: #3 Small Things Matter, Take a Closer Look...

Sometimes it's worth to take a closer look at small details, those can tell you a lot, you just have to know where to look. Here is a tip: look at the inside of an LED. Do you see those two metal plates inside a plastic part that depends on what type of LED you have can be clear, red, blue, yellow or green? As you may notice they are not the same when it comes to the size. One is smaller and another one is bigger. The bigger plate is always connected to the negative lead and smaller one to positive lead. Still a very simple method, just have to look closely and maybe in some cases you will need a magnifier for that.

As throbscottle and studleylee pointed out in the comments, there are some LEDs that are built differently, and this method wouldn't work with them. Bigger plate inside them may be connected to the positive lead. Those are very rare so there is a small chance you will find some of them, just wanted to make it clear that this method may not always work.

Step 4: #4 Multimeter Will Help Because It's Multi

The multimeter is super useful in electronics and if you don't have one yet, you should definitely get one. It can measure a whole bunch of values and save you a lot of time on troubleshooting and trying to find a resistor value. Most of the multimeters (even cheaper ones) have a function of measuring diodes (this kind of diodes that don't shine) and we can use this function to check the polarity of an LED. Just touch leads with probes if it don't shine swap the probes and it should be fine. The positive lead is where you red probe touches an LED and negative lead is where you have a black probe. This method is fast and easy but you need a multimeter for that, you may not have a multimeter in your pocket (I always have one with me, except when I am at the swimming pool, because I don't have pockets in my swimming trunks) all the time and that's why the first 3 options are better.

Step 5: Sometimes Smaller Is Better - a Coin Cell Battery

Small coin cells batteries are mainly used in watches, but we may also it to determine the polarity of an LED. Why a coin cell battery? Because it is small enough to fit it in between leads of an LED. You can also use a bigger battery let's say an AA battery but you will need some cables to connect it to the LED. If LED lights up after placing a battery between LED leads the positive lead of LED is where it touches a plus of a battery if it doesn't shine, swap the battery polarity and it should shine. This method would be great but coin cell batteries are not that popular so it's not the most practical method.

Step 6: Conlusion

There you have it, 5 simple methods to find LED polarity. I hope you will find them useful for your next electronics project! If you have any other LEDs related tips, share them in the comments! Thank you for reading :)

Happy making!

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    59 Discussions

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    MuhammadH345

    Tip 7 days ago on Step 3

    How to identyfy the colour of led
    I mean cristle led colour.

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    cb77305

    Tip 10 days ago on Step 6

    wonderful article. I'm a beginner so this was REALLY helpful THANK YOU!

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    erwin

    13 days ago

    Good one! Easy to remember. Thanks

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    Alimess

    13 days ago

    Good subject, simple and useful. Thank You
    May be It is interesting to add that in case we have to cutoff the LED legs to fit into its dedicated place (example breadboard prototyping), it is useful to think cutting the cathode slightly shorter than the anode to preserve the some visible sign.
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    Tally O

    14 days ago

    Made that real simple for me. Things like the above make this site a pleasure to peruse.
    Nooice!

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    DaveS162

    16 days ago

    Someone mentioned that most of this wouldn't work with IR (infrared) diodes. A simple trick to see if they are actually lit up is to look at them through the camera on your cell phone. It will show infrared light as visible.
    BTW, this also works with IR remote controls like the one for your TV. Fast way to check the batteries.

    2 replies
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    kenfillupsDaveS162

    Reply 15 days ago

    Good comments on IR transmission devices.

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    ImAtechDaveS162

    Reply 15 days ago

    I did not know this! I just tried it and Wow this will come in handy showing my grandson IR circuits. Been a Etech for 40 years. I'm embarrassed.

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    JayGeeBSE

    17 days ago

    For some one who's first language is not English, this is very well written - until you got right to the end with "6. Conlusion". A typo obviously.

    I'm a little worried about the suggestion of powering an LED from AA cells with no series resistor - you've already pointed out the risk of frying LEDs that way. You will be OK with button cells as they have limited current output - though you might need 2 or 3 to get enough volts to work an LED. These later options won't work with IR LEDs of course.

    5 replies
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    NikusJayGeeBSE

    Reply 17 days ago

    Thank you! LED wouldn't burn at 1.5V, they usually operate at around 2V, so it's even not enough to run it on full brightness

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    JohnH848Nikus

    Reply 16 days ago

    Do not try it with a lithium cell that are 3V, and some of these small round cells can put out a lot of current up to 40A in a short circuit.

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    UdonJohnH848

    Reply 16 days ago

    40A?
    That would mean I could melt steel using a 3V lithium cell. I use this method to teach my students how not to run LEDs, but the results are more often than not, underwhelming. 40 amps would be spectacular.

    "I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhäuser Gate. Lithium cells, putting out 40 amps. All those moments will be lost in time, like tears in rain."

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    JohnH848Udon

    Reply 15 days ago

    At 40A any 100/200 mAh button or coin cell will be discharged in a blink of an eye, so there is not enough energy to melt steel, but it generally enough to damage a semiconductor, actually the cells that could pot out that much current were some now obsolete rechargeable NiCad button cells, modern cells are built to limit the current, as a short circuit could make them explode, and in the case of lithium cells catch fire, as happened to several celular phone models.

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    UdonJohnH848

    Reply 15 days ago

    Certainly, a coin cell is a safe-ish way to kill LEDs for demonstration purposes.

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    kukopia

    17 days ago

    The diode mode of multimeters is not to "shine" the LED. When connected correctly (red-pos., black-neg.) the meter will show the voltage drop. When connected incorrectly, it will show "OL" (Open Loop).

    5 replies
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    JohnH848kukopia

    Reply 15 days ago

    In diode mode most multimeters put out a constant 5 mA current to test semiconductors

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    LesBkukopia

    Reply 17 days ago

    Despite it's purpose, to my experience the diode mode will light the LED slightly

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    NikusLesB

    Reply 17 days ago

    it will, and that's just enough for your eye to see it

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    Nikuskukopia

    Reply 17 days ago

    But also can be used to check the polarity of LED, just another use of this function