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Picture of LEDs for Beginners
This instructable shows how to wire up one or more LEDs in a in a basic and clear way. Never done any work before with LEDs and don't know how to use them? Its ok, neither have I.

***If you have wired up LEDs before, this explanation might seem overly simplistic. Consider yourself warned.***
 
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Step 1: Get some LEDs

Picture of Get some LEDs
So I wasn't completely honest - I have used LEDs once or twice before for simple applications, but I never really knew what I was doing, and since so many projects on instructables use LEDs, I thought I might as well teach myself and post about it too.

I know that there are many projects already posted that contain information about how to wire LEDs for simple projects - LED Throwies, LED Beginner Project: Part 2 and 9v LED flashlight - teh best evarrr!, but I think that there could still be some use for a detailed step by step explanation about the basics of LEDs for anyone who could use it.

The first step was to buy some supplies and figure out what I would need to experiment with. For this project I ended up going to Radioshack because its close and a lot of people have access to it - but be warned their prices are really high for this kind of stuff and there are all kinds of low cost places to buy LEDs online.

To light up an LED you need at the very minimum the LED itself and a power supply. From what I have read from other LED instructables wiring in a resistor is almost always a good idea.

If you want to learn about what these materials are check out these wikipedia entries:
LEDs
Power supply
Resistors

Materials:

LEDs - I basically just reached into the drawer at Radioshack and pulled out anything that wasn't more than $1 or $2 per LED. I got:

2760307 5mm Red LED 1.7 V
2760351 5MM Yellow LED 2.1 V
2760036 Flasher Red LED 5 V
2760041 2 Pack Red LED 2.6 V
2760086 Jumbo Red LED 2.4V

Power Supply - I really didn't know what I would need to power them so I bought some 9V batteries and some 1.5V AA's. I figured that would allow me to mix and match and make enough different voltage combinations to make something light up - or at least burn those little suckers out in a puff of smelly plastic smoke.

Resistors - Again, I wasn't too sure what I would need in terms of resistors here either. Since I got a whole bunch of different LEDs with various voltages I knew that I would need a couple different types of resistors, so I just bought a variety pack of 1/2 Watt Carbon Film Resistors (2710306).

I gathered up a soldering gun, solder, needle nose pliers, electrical pliers, some primary wire and electrical tape too since I thought they might be useful.

Step 2: The LED

Picture of The LED
PICT0034.jpg
PICT0031.jpg
LEDs come in different sizes, brightnesses, voltages, colors and beam patterns, but the selection at Radioshack is pretty small and so I just picked up a couple different LEDs from what they had in a few different brightnesses and voltages. I kept close track of what LED was what voltage because I didn't want to accidentally send too much current through one of the low voltage LEDs.

The first thing I did with the LEDs was figure out which wire (its called an electrode) was positive and which was negative. Generally speaking the longer wire is the positive electrode and the shorter wire is the negative electrode.

You can also take a look inside the LED itself and see whats going on. The smaller of the metal pieces inside the LED connects to the positive electrode and the bigger one is the negative electrode (see picture below). But be warned - in the LEDs I picked up I didn't always find this to be true and some of the LEDs had the longer electrode on the negative when it should be on the positive. Go figure - its OK though, if it didn't light up I just flipped it around.

Once I knew what was positive and what was negative I just had to remember what the voltage of each LED was.

All my LEDs recommended 20mA of current. 20mA is standard for most LEDs.





Step 3: Power supply

Picture of Power supply
To make the power supplies I just soldered some wire onto the ends of the batteries I had bought so that I could easily attach the LEDs to them. The 9V battery served as my 9V power supply, one AA battery made a 1.5V power supply and three AA batteries bundled together made a 4.5V (1.5V + 1.5V + 1.5V = 4.5V) power supply. I didn't use alligator clips on the ends of the wire, but they would have been helpful here.

Step 4: Resistors

Picture of Resistors
I opened up the assortment pack to find that resistors aren't labeled with what value they are. The pack said it contained a whole bunch of different resistors from 100 ohms to 1 Meg ohm so I set out to see what was what. When I poked around online I found that all resistors have a coding system on them that tells you what value they are.

Here are two pages which explain in depth about how to calculate resistor values.

Do it yourself
or
Have it done for you

I'll go through the examples of how I calculated the values myself in the next few steps when I start wiring up my LEDs.

For the time being I just admired their little colored stripes and moved on to trying to get just one LED to light up.

Step 5: One LED, no resistor

Picture of One LED, no resistor
I thought that I would start as simply as I possibly could - just one LED with no resistor. First I had to decide what power source to use and which LED to light up. This may seem obvious, but this was my first time through so I might as well be as clear as possible...

LEDs require sufficient voltage to light them. Sometimes if you give them too little voltage they wont light at all, other times they will just shine dimly with low voltage. Too much voltage is bad and can burn out the LED instantaneously.

So ideally you would like the voltage of the LED to match the voltage of your power supply, or even be slightly less. To do this you can do a couple of things: change your power supply voltage, change the LED your using, or you can use a resistor that allows you use a higher voltage power supply with a lower voltage LED.

For now I just wanted to get one lit up so I chose my the power supply that had the lowest voltage - the single AA battery which outputs 1.5V.

I chose to light the red 1.7V LED since the battery outputs 1.5V and I knew I wouldn't kill the LED with too much power.

I wrapped my positive wire from the battery to the positive electrode of the LED and wrapped the negative wire from the battery to my negative electrode and presto - let there be LED light!

This first experiment was pretty easy to do - just some wire twisting and enough knowledge to know that the 1.5V power supply would light the 1.7V LED without need a resistor.

Step 6: One LED with a resistor

Picture of One LED with a resistor
It was just a coincidence that I bought an LED that was 1.7V and that it ended up working being able to be powered by my 1.5V power supply without the use of a resistor. For this second setup I decided to use the same LED, but up my power supply to the three AA batteries wired together which output 4.5V - enough power to burn out my 1.7V LED, so I would have to use a resistor.

To figure out which resistor to use I used the formula:
R = (V1 - V2) / I

where:
V1 = power supply voltage
V2 = LED voltage
I = LED current (usually 20mA which is .02A)

Now there are lots of calculators online that will do this for you - and many other instructables reference this as a good one, however, the math really isn't too hard and so I wanted to go through the calculation myself and understand whats going on.

Again, my LED is 1.7V, it takes 20mA (which is .02 A) of current and my supply is 4.5V. So the math is...

R = (4.5V - 1.7V) / .02 A
R = 140 ohms

Once I knew that I needed a resistor of 140 ohms to get the correct amount of voltage to the LED I looked into my assortment package of resistors to see if I could find the right one.

Knowing the value of a resistor requires reading the code from the color bands on the resistor itself. The package didn't come with a 140 ohm resistor but it did come with a 150 ohm one. Its always better to use the next closest value resistor greater than what you calculated. Using a lower value could burn out your LED.

To figure out the color code you basically break down the first two digits of the resistor value, use the third digit to multiply the first two by and then assign the fourth digit as an indicator of tolerance. That sounds a lot more difficult than it really is.

Using the color to number secret decoder website found here, a 150ohm resistor should have the following color code...

Brown because the first digit in the value resistor I needed is 1
Green because the fifth digit is 5
Brown because in order to get to 150 you have to add one 0 to 15 to get to 150.
Gold - the resistors I got all have 5% tolerance and 5% is represented by gold

Check out the decoder page link above if this isn't making sense.

I looked through all the resistors, found the one that was brown, green, brown, gold, and wired it in line on the positive electrode of the LED. (Whenever using a resistor on an LED it should get placed before the LED on the positive electrode).

Low and behold, the LED lit up once again. The 150 ohm resistor stopped enough of the 4.5V power supply from reaching the 1.7V LED that it lit up safely and kept it from burning out.

This is just the process that I went through to figure out what resistor to use with my particular LED with my particular power supply. You can easily use the formula above to figure out what value resistor to use with whatever LED and power source you happen to be using.

Step 7: Wiring up multiple LEDs in series

Picture of Wiring up multiple LEDs in series
Now that I knew how to wire one LED with various combinations of LED voltages and power supplies, it was time to explore how to light up multiple LEDs. When it comes to wiring more than one LED to a power supply there are two options. The first option is to wire them in series and the second is to wire them in parallel.

To see an in depth explanation about the difference between series and parallel check out this page. I'm going to cover wiring LEDs in series first.

LEDs wired in series are connected end to end (the negative electrode of the first LED connects to the positive electrode of the second LED and the negative electrode of the second LED connects to the positive electrode of the third LED and so on and so on...). The main advantage of wiring things in series is that it distributes the total voltage of the power source between all of the LEDs. What that means is that if I had a 12V car battery, I could power 4, 3V LEDs (attaching a resistor to each of them). Hypothetically this could also work to power 12, 1V LEDs; 6, 2V LEDs; or even 1 12V LED if such a thing existed.

Ok, let's try wiring 2, 2.6V LEDs in series to the 9V power supply and run through the math.

R = (9V - 5.2V) / .02A
R = 190 Ohms
Next higher resistance value - 200 Ohms

Now the variety package of resistors didn't come with a 190 or 200 Ohm resistor, but it did come with other resistors which I could use to make a 200 Ohm resistor. Just like LEDs, resistors can be wired together in either series or parallel (see next step for an explanation on wiring things together in parallel).

When same value resistors are wired together in series you add their resistance. When same value resistors are wired together in parallel you divide the value of the resistor by the number of resistors wired together.

So, in the most simplified sense, two 100 Ohm resistors wired together in series will equal 1 200 Ohm resistor (100 + 100 = 200). Two 100 Ohm resistors wired together in parallel will equal one 50 Ohm resistor (100 / 2 = 50).

Unfortunately, I learned this key point after I wired my resistors together for the experiment. I had originally wanted to wire two 100 Ohm resistors together to equal the 200 Ohms of resistance I needed to protect my LEDs. Instead of wiring them in series, as it should have been, I wired my resistors in parallel (did I mention I am beginner with resistors?) So my resistors were only providing 50 Ohms of resistance - which apparently worked out OK on my LEDs in the short duration of the experiment. Having too much power getting to the LEDs would probably burn them out in the long term. (Thanks beanwaur and shark500 for pointing this out.)

I took my resistors and placed them in front of the positive lead of the first LED that was wired in series and hooked them up to the battery and once again, there was LED light!

With three different combinations of LEDs and battery power supplies and no puffs of plastic smoke yet things were looking good - aside from my little confusion between wiring resistors in series and in parallel.

Step 8: Wiring up multiple LEDs in parallel

Picture of Wiring up multiple LEDs in parallel
Unlike LEDs that are wired in series, LEDs wired in parallel use one wire to connect all the positive electrodes of the LEDs your using to the positive wire of the power supply and use another wire to connect all the negative electrodes of the LEDs your using to the negative wire of the power supply. Wiring things in parallel has some distinct advantages over wiring things in series.

If you wire a whole bunch of LEDs in parallel rather than dividing the power supplied to them between them, they all share it. So, a 12V battery wired to four 3V LEDs in series would distribute 3V to each of the LEDs. But that same 12V battery wired to four 3V LEDs in parallel would deliver the full 12V to each LED - enough to burn out the LEDs for sure!

Wiring LEDs in parallel allows many LEDs to share just one low voltage power supply. We could take those same four 3V LEDs and wire them in parallel to a smaller power supply, say two AA batteries putting out a total of 3V and each of the LEDs would get the 3V they need.

In short, wiring in series divides the total power supply between the LEDs. Wiring them in parallel means that each LED will receive the total voltage that the power supply is outputting.

And finally, just some warnings...wiring in parallel drains your power supply faster than wiring things in series because they end up drawing more current from the power supply. It also only works if all the LEDs you are using have exactly the same power specifications. Do NOT mix and match different types/colors of LEDs when wiring in parallel.

OK, now onto to actually doing the thing.

I decided to do two different parallel setups.

The first one I tried was as simple as it could be - just two 1.7V LEDs wired in parallel to a single 1.5V AA battery. I connected the two positive electrodes on the LEDs to the positive wire coming from the battery and connected the two negative electrodes on the LEDs to the negative wire coming from the battery. The 1.7V LEDs didn't require a resistor because the 1.5V coming from the battery was enough to light the LED, but not more than the LEDs voltage - so there was no risk of burning it out. (This set up is not pictured)

Both of the 1.7V LEDs were lit by the 1.5V power supply, but remember, the were drawing more current from the battery and would thus make the battery drain faster. If there were more LEDs connected to the battery, they would draw even more current from the battery and drain it even faster.

For the second setup, I decided to put everything I had learned together and wire the two LEDs in parallel to my 9V power supply - certainly too much juice for the LEDs alone so I would have to use a resistor for sure.

To figure out what value I should use I went back to the trusty formula - but since they were wired in parallel there is a slight change to the formula when it comes to the current - I.

R = (V1 - V2) / I

where:
V1 = supply voltage
V2 = LED voltage
I = LED current (we had been using 20 mA in our other calculations but since wiring LEDs in parallel draws more current I had to multiply the current that one LED draws by the total number of LEDs I was using. 20 mA x 2 = 40 mA, or .04A.

And my values for the formula this time were:

R = (9V - 1.7V) / .04A
R = 182.5 Ohms

Again, since the variety pack didn't come with that exact value resistor I attempted to use the two 100 Ohm resistors bundled together in series to make 200 Ohms of resistance. I ended up just repeating the mistake that I made in the last step again though, and wired them together in parallel by mistake and so the two 100 Ohm resistors only ended up providing 50 Ohms of resistance. Again, these LEDs were particularly forgiving of my mistake - and now I have learned a valuable lesson about wiring resistors in series and in parallel.

One last note about wiring LEDs in parallel - while I put my resistor in front of both LEDs it is recommended that you put a resistor in front of each LED. This is the safer better way to wire LEDs in parallel with resistors - and also ensures that you don't make the mistake that I did accidentally.

The 1.7V LEDs connected to the 9V battery lit up - and my small adventure into LED land was completed.

Step 9: Extrapolation

While I didn't actually end up making anything besides a couple of lit LEDs, this information can be used to make all kinds of cool things!

The take away concepts hopefully were:
- Power a whole bunch of different value LEDs using the same basic principals.

- Figure out what is the positive electrode and what is the negative electrode of an LED by looking at it and testing it.

- Use resistors, or combinations of resistors wired together in series or in parallel to supply the correct amount of power to the LED.

- Make calculations to determine what resistor is needed using the formula, or using web sites that do it for you.

- Wire LEDs in series or in parallel depending on the application.

- Make LEDs light up!

This was the most basic kind of walk through for LEDs possible - and I learned a whole lot along the way. LED arrays and wiring schemes can get significantly more complicated - but for the most part, LEDs are pretty simple to work with, and with relatively little knowledge I was able to light them up - all be it if I sent a little too much juice through them towards the end of the experiment. I don't fear the LED now. They are my friends.

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is the supply current doesnt matter? if i have a power supply with 12V and 2A should i use it like a normal 12v 20mA power supply?? pls clear my doubt

Please also bear in mind, everyone, that led's are still DIODES.. Polarity matters, and they do have a breakdown voltage, where they will become hot and self-destruct if reverse biased. They should remain dark and block voltages at least as high as their forward voltages, however, when reverse biased... If you can find a data sheet on the particular led you are working with (good luck) it will tell you this info...

In theory, you could connect 60 pairs (in reverse parallel) of 2V forward voltage led's in series in a long string (how about inside a burned-out 4ft fluorescent tube?) and power it with 120v AC usa mains power to create a rather efficient yet bright replacement for it... No starter or balance resistor needed, just straight 120v AC... It would use about 1.2 Watts.

Why parallel pairs? AC -- One would light during the positive phase, the other during the negative phase... And being diodes, they would block the current in the wrong direction, giving it all to their partner, each time the current flipped direction...

The light would flash at 60Hz, just like the fluorescent tubes they would replace.

The number of led's would vary with their forward voltages, and if your mains voltage is less than the 120v standard.

GrantK529 days ago
Just to throw this out there in case any of you start playing with resistors in series and parallel. the method he stated determing equivalent resistance in series is correct. But the method he used for resistors in parallel is only accurate for the same sized resistors. The actual equations are as follows:

n=total number of resistors

Series
Rtotal = R1+R2+...+Rn

Parallel
1/Rtotal = 1/R1+1/R2+...+1/Rn
or simplified
Rtotal = 1/[1/R1+1/R2+...+1/Rn]

Hope this helps anyone messing with differet value resistors figure out any issues they have been having. Other than that great ans simple instructions on LEDs!

GK

Well done - thank you! I want to teach my 13 year old grandson a little about electronics using LEDs. It's been a long time since I did this kind of stuff & I didn't want to embarrass my self too badly (or flounder around and have him get bored).

Yeah - really basic, but that's what I was looking for. It helped.

Dave851 month ago

I thoroughly enjoyed this beginners guide to LED. I did a lot by trial and error but this eliminates my need to do it inefficiently. Great post!!!

DerekP4 months ago

I am new to electronics, and I have a Nintendo 64 that I want to wire roughly 16 LEDs into. I have seen many people use a 5 volt power output on the board. Should I wire the LEDs in parallel or series?

T0BY DerekP1 month ago
Are you pimping your N64? How does it look?
FarisB DerekP1 month ago
Parallel :)
PrashantS51 month ago
sir I want to install 2 10 w led with 12 v power supply. so what's the steps to install it . Please describe either I join this in parallel or series. what is the power of resistence. would I have to use led driver.
dhermann1 month ago

This was an excellent beginners tutorial. I had forgotten a lot over 30 years of not using my basic electronics knowledge I learned in the military and this helped me remember it.

Good work with your explination and instructions.

mizami5 months ago

Say i have 12V power supply and 15 LEDs. i decide to make three loops of parallel LEDs (3.6V each) and arrange them in series (i.e one loop has five LEDs arranged in parallel and the three loops are connected by series). This means for every loop, there is approximately 3V to power adequately.

Question is do i still need to put resistor in front of LEDs since theoretically i have used voltage power effectively? If yes, do i put the resistor in series of each LEDs or in series of every loop?

FarisB mizami1 month ago
Hello, What I assume it is better to use a resistor for longer lasting of LED.
JinkyDrew1 mizami5 months ago

i dont thiink you would need one. but im not certain, send a pic :D

Hey man I love shrek, shrek is love shrek is life

james.forey.32 months ago

How do i turn a led on cause im dum and dontthr no what im doin so pls help me wire up a led thnks

For beginners I suggest to have these items to test and understand the concept of making an electronic circuit


2 AA BATTERY 1.5V
1 220 Ohm Resistor
1 Breadboard
Set wires (1xRed)(1xBlack)
2 Light Emitting Diode(LED)
Switch or Variable switch

Test in series first than parallel

In series your first LED should be brighter than the second LED

In parallel both of your LED should have the same brightness

Here's a tip
When you're running parallel the voltage will remain but the current don't
If you run in series the voltage will increase while the current remains.

Hope this helps :))

pwrmac76002 months ago

So If I understand this correctly. If I am wiring 5 LEDs in parallel that are 1.9v per, and I am powering them off of a 9.6v 300ma power supply, then I would need a 60ohm resistor in between each LED? does it matter how many watts the resistor is?

MickiG12 months ago
priandoyo2 months ago

Thank you for clear explanation, I spend days to realize this.

dave.callista3 months ago

I have a machine that has two indicator lights and neither one is working. I opened one up thinking it had a burned out bulb, but instead, it contained 3 LEDs. The machine's wiring diagram shows 6 volts AC applied to the indicator terminals. I tested and 6 volts AC is present. I must assume that there is some circuitry tied to the LEDs that rectifies the AC. I would like to get this fixed but have no idea what to do. I can use a multi meter, and soldering iron, if someone will tell me what I need, and how to connect the components.

Oh so whit u hav to do m8 is recharche ur battery so yeah shild fix it r8 me out of 8

cause I r8 8/8 m8 cya later mssg me im starting a utube channel Tips wif James

Dave, LEDs are dioides, thus they are rectifiers themselves. If one of the LED's polarity doesn't match with the others', no current will go through. I would suggest that you test each one independantly with a AA, then make sure they go back into your machine in series (+ to -).

I have tried linking up a resistor and my lead would not glow even with 2 batteries. I have tried connecting to leads in series and thy refused to glow up. PLEASE HELP.

I want to link you about 15-20 LED's in series and power up with 12 volts.

I have already done some projects with LED's in parallel and they are working perfectly.

sko565 years ago
The only reason the LED doesn't Fry from adding 1.5V is because the Alkaline Battery has an internal resistance. Ohms Law Holds true I (current) = V(volts)/R(resistance in Ω) . If there was no resistance the current would be near Infinity causing the Wattage to go through the roof as well (W=I^2*R). same reason a "throwie" works without a resistor.
How do you make an ohm's symbol on a keyboard?

ALT 234 Ω

The easiest way, in Windows, is by bringing up charmap, scrolling down the list until you find the "Ω" symbol, copy it, and then paste it into the window.
You can also hold down the ALT key and type the decimal value of the character but wasn't successful getting the character to print out. My guess is it has something to do with the character map I was using. In any case, I think using charmap is probably the best way to print non-keyboard generated character.
For Mac, just press Option+Z.
In Word or Excel make change the font to "Symbol" and then hold the 'alt' key while typeing 89 on the numeric pad. Or use the Insert” tab, click , and then choose the symbol you want.

Is it possible to prewired LEDs with resistors this way to a 12v battery

itsjustjayok4 months ago

Hi can you tell me if this is correct - I purchased these 50 LEDs connected in series powered by 3 x AA batteries (4.5V) i want to switch to a 4.5V multivoltage adapter instead of batteries but need a resistor to prevent dimming and dieing of the leds.

I know the Voltage is 4.5v and the Current is 10mA so using Ohms Law would i be right in saying that based on a "white" LED generally having a forward voltage (vF) of 3.3V i would need to take the 3.3v from 4.5v to give 1.2v for ohms law

Resisance = 1.2 (v) / 0.01 (10mA)

This gives me a 120 resistor.

adding this in the series would this help? or am i totally out lol

Thanks :)

i have 3x6v led lights that each take 4x1.5 AA battery's

im wanting to wire them to all up so they can be powered by a wall adapter

what power size adapter would i need to do this

thanks

Maybe someone on here can answer this for me. I have aftermarket headlights (spec d) on my car. Had em for 2 years and now I have two leds blown out that are part of the eyebrow on the lights. They are wedge style leds with one led on each light. There are 5 LEDs wired in series for each eye brow. You can't buy the LEDs from the company they don't sell them and they won't tell me where to get them. My problem is I tried a 12v led and it wouldn't light up. I checked the voltage and each led in the series only uses 3volts. Where can I find a 3volt wedge (looks like t10) led?!?? Email me at gualano_365@yahoo.com
Rushmoore4 months ago

This was so awesome! Very clear and easy to understand! I had been looking at other sited the whole morning and this was the only explanation that made sense. Thanks you!!

rania.riya.74 months ago

i take an led dim light an remove the 2 resisters and plugged on the switch i can see a lightinig!! and it stoped what does it mean???

You probably blew the LED because of too much power going into it. Try again with a new LED and a bit more resistance.

ahwang4 years ago
Hi.
I have a question but btw, you have an awesome website for ppl like me who's interested in learning to light up some LEDs!!!

Forward voltage: 3.0-3.2
Current: 20ma
Battery: 6V

1. Following your instruction, I first tested my 2 of my LED, in a series without any resistors to my 6V and baam! it lit up! thanks!
but after about 20 secs, the LEDs started to get hot. Is that normal? I don't think I'd need any resistors or I wouldn't even know how since I would get 0 ohm resistor value

2. My project is to light up 8 LEDs. 4 pararell LED each side, then 2 of 4 pararell LEDs in series. so like. (1+1+1+1) + (1+1+1+1).
so I'm thinking (3V, .08ohm) + (3V, .08ohm) so it would be 6V with .08ohm right?
then it would require no resistor since 6v-6v=0 right?
Or would that get too hot and require resistors?

any help is appreciated!!!
Thanks!!
JinkyDrew1 ahwang5 months ago

do the first one with resistor, u were giving the led toooooooooooooooooooooooooooooooooooooooooooooo mich current abd volts

ronym83 ahwang11 months ago

6 volt battery sometimes have more than 6 volt

( it's depend on battery state... it's full... or near empty )

so we must measure the exact voltage of battery using volt meter

if the voltage of battery is... say 6,5 volt

devide it by two we get 3,25 volt...

that's why your LED are too hot... because you over voltage it

.

just like 12 volt battery which can be 10,7 to 12,8 volt

( it's depend on charge state of battery )

it also can be 14 volt ( if the battery are connecting to the charger )

cellis6 ahwang3 years ago
LEDs power is based on Amperage not Voltage, that's why they are getting hot. Think of them like a resistor, when you put 2 resistors in series, you drop voltage each resistor but Amperage never changes.

So if your LED requires Forward Voltage of 3.0-3.2Vf, and 20mA, and you want to run it off 6V. If you were just trying to power one LED it would be (6V-3.2V)/.02mA. Meaning you'd need a 140 ohm resistor before attaching the LED to power.

As far as hooking them up in series/parallel, you should be able to do it with one resistor at the beginning of each series of 4 of 140 ohms. Which would make each series 20mA and 3.2 Volts after the resistor. Hope this helps.
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