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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    63 Discussions

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    studleyleeDharmendraM2

    Reply 6 weeks ago

    Not sure what you are asking. If unsure use the 1K ohm and a 9v battery and find the polarity.

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    Nikusstudleylee

    Reply 2 months ago

    Thanks for pointing out, I didn't know about that, never faced such LEDs, I will update step 3

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    studleyleeNikus

    Reply 2 months ago

    Good instructable, it made me think over some things that I was fuzzy on. Thank you!: funny that I never paid attention to the longer lead being positive.

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    studleyleestudleylee

    Reply 2 months ago

    The anvil( larger base) can be bonded as the cathode. It's up to the manufacturer or led technology used. It does seem the 90% of google search say the anvil is the cathode, but I have many in hand led that are positive anvil. Maybe the industry settled on this convention at some point. Heat transfer goes with the electron flow from negative toward positive( not intuitive see welding polarity and https://en.wikipedia.org/wiki/Thermoelectric_effec... ) so it would make sense that the anvil should have been chosen to be positive to dissipate heat from the die, hmm maybe the opposite to draw heat away from the die... The anvil is also often the reflector forming a conical 'pit' around the die.

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    erwin

    2 months ago

    Good one! Easy to remember. Thanks

    1 reply
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    DharmendraM2

    Question 7 weeks ago

    In the above projects cost isnot mentions & block diagram

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    MuhammadH345

    Tip 2 months ago on Step 3

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

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    cb77305

    Tip 2 months ago on Step 6

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

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    Alimess

    2 months 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

    2 months ago

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

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    DaveS162

    2 months 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 2 months ago

    Good comments on IR transmission devices.

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    ImAtechDaveS162

    Reply 2 months 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

    2 months 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.

    3 replies
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    NikusJayGeeBSE

    Reply 2 months 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 2 months 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 2 months 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."