# How to use a Led calculator to find the correct size resistor for your led projects



## hpropman (Jul 27, 2008)

Hello All,

In this tutorial I am going to teach you how to find the correct resistor value for you led projects such as led spots or led eyes. I have listed several led calculator links below. They are all very similar to use. One thing I want to note is that we will always use leds is series and not use leds in a parallel arrangement. All that means is the way in which we wire the leds (leds connected is series means one led connected to the next- see the second picture below). The reason we do not use leds in parallel is that they use a lot more current and is inefficient. Here are the links to some led calculators:

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

http://www.quickar.com/noqbestledcalc.htm

http://led.linear1.org/led.wiz

Now before we can select a resistor to use with our leds we will need to know a few things. we will need to know the voltage drop of the led, the maximum current of the led, the source voltage, and how many leds we are going to use in our circuit. This information is given when we purchase the leds. Lets look at a typical led from an on line retailer.










As you can see from the led description that the voltage drop per led is 2.1 volts and the max forward current is 20ma (ma = milliamps - this is normal for most leds). Now lets use the led spot light circuit as an example. We know the voltage that we will be using is 12 volts and that we will have 3 leds in the circuit. lets populate a calculator and see what it tells us.










We see from the calculator that with 3 leds is series (meaning one led connected to the next like in the picture above) we will need a 330 ohm resistor. There is also two other things I want to make note of. One is the power rating of the resistor. As we can see from the picture above that the power rating of the resistor must be at least .19 watts which is below .25 (or quarter watt) resistor. So a 1/4 watt 330 ohm resistor would be suitable in this case for every three of these leds that we would use at 12 volts. most resistors come in either 1/4 watt or 1/2 watt (.50 watts). I would not recommend using an 1/8 watt resistor it may not be able to handle the current. The other common question that comes up a lot is why not use 5 volts with this circuit. The answer is that it would not have enough voltage to power the 3 leds. Each led requires 2.1 volts (in this case) which is 6.3 volts just for the three leds so 5 volts would not be enough we would need a least 6.5 to 7 volts minimum. I hope this answers any questions that you might have on this but if not please ask me and I will do my best to answer it.


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## GhoulishCop (Sep 25, 2009)

Joe,

Perfect! That is _exactly_ the information I need so I know what to order. I've always been so confused by those calculators I had no idea what I was doing and just let others handle it for me.

Allowing others to make complicated orders is sometimes a good thing, but knowing what to do yourself is so much better. Thanks!

Rich


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## Dead Things (Apr 4, 2009)

Joe, refering back to your LED tutorial, you wired 4 LED's in series with a resistor (1 group), then wired those groups together in parallel (one lamp). And from what I could tell from Rich's rca tutorial, the rca's are wired in parallel, correct? Could you explain the reason for this?


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## niblique71 (Dec 2, 2009)

Dead Things said:


> Joe, refering back to your LED tutorial, you wired 4 LED's in series with a resistor (1 group), then wired those groups together in parallel (one lamp). And from what I could tell from Rich's rca tutorial, the rca's are wired in parallel, correct? Could you explain the reason for this?


Hi DT, Love your video's, especially the PicAxe exploration.

I'm fairly certain that Joe wired 3-LED's with 1 resistor, and then did that 4 times in Parallel to make 12 LED's in one fixture. I've not seen a calculator that will let you do more than 3 LED's in series.

Now as for why that is a rule??? I would love a simple explanation as well.

EDIT: Actually I get the series parallel thingy, but What inhibits connecting 4 LED's in series?


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## hedg12 (Jul 6, 2008)

niblique71 said:


> Hi DT, Love your video's, especially the PicAxe exploration.
> 
> I'm fairly certain that Joe wired 3-LED's with 1 resistor, and then did that 4 times in Parallel to make 12 LED's in one fixture. I've not seen a calculator that will let you do more than 3 LED's in series.
> 
> ...


When you wire LEDs in series, the voltage requirement is cumulative. You add the forward voltage of each LED in the series, & you can't exceed your supply voltage. If you're running LEDs with a 3.4v forward voltage off a 12v power supply, 3 * 3.4 = 10.2v, but 4 * 3.4 = 13.6v. If you use a higher voltage supply, you could wire more in the series.

Load in series increases voltage draw - load in parallel increases current draw. That's why you use series/parallel arrays - you can maximize the number of LEDs on each supply.


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## hpropman (Jul 27, 2008)

Yes Niblique and Hedg are correct about the leds. As for the RCA tutorial - those RCA boxes are used to distribute power to the led spot lights. The power distribution boxes as I like to call them plug into a main power line with female RCA pigtails. This 12 volt main power line is connected to a computer power supply. Now if you look at an RCA connector it has two conductors one being the center post (on the male plug) and the outer ring is the second conductor. The power distribution box is just 8 female RCA jacks on one plate. All we are doing is connecting all the center contacts together (one ones that connect to the male center post) these are the two outside strips of connectors. The center of the plate has the outer ring (ground) connections conveniently leaning towards the middle and almost touching so we solder all eight connection together and connect the ground wire there. the outer two strips on the rca plate get connected together and connect to the positive wire. the wire exits the box and has an rca plug attached that plugs either into the main 12 volt power line or into another power distribution box which is already powered. This makes it easier to route power wherever you need it. It is very a flexible system. One last benefit of the RCA plugs it that they will pull out it the wire gets caught or you trip on a wire or something. This saves the lights from damage because you just plug it back in. This has already happened to me so I know it works. I hope this answers your questions but if not let me know and I will see about getting some pictures up.


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## Dead Things (Apr 4, 2009)

niblique71 said:


> Hi DT, Love your video's, especially the PicAxe exploration.
> 
> I'm fairly certain that Joe wired 3-LED's with 1 resistor, and then did that 4 times in Parallel to make 12 LED's in one fixture. I've not seen a calculator that will let you do more than 3 LED's in series.
> 
> ...


Thanks Niblique, I appreciate that, especially coming from you, I'm a big fan of your work. And I stand corrected, 3 not 4!



hedg12 said:


> When you wire LEDs in series, the voltage requirement is cumulative. You add the forward voltage of each LED in the series, & you can't exceed your supply voltage. If you're running LEDs with a 3.4v forward voltage off a 12v power supply, 3 * 3.4 = 10.2v, but 4 * 3.4 = 13.6v. If you use a higher voltage supply, you could wire more in the series.
> 
> Load in series increases voltage draw - load in parallel increases current draw. That's why you use series/parallel arrays - you can maximize the number of LEDs on each supply.


Thanks hedg12. Ok, I think I get this. If I used only series (wiring individual lamps in series AND connecting all lamps to the main supply in series), I would quickly outstrip the 12 volts of a wallwart or CPS. If I wired everything in parallel, I would quickly outstrip a wallwarts amperage (assuming it has 1 amp). However, I could still connect more lamps to a CPS because it has higher amperage, correct?



hpropman said:


> Yes Niblique and Hedg are correct about the leds. As for the RCA tutorial - those RCA boxes are used to distribute power to the led spot lights. The power distribution boxes as I like to call them plug into a main power line with female RCA pigtails. This 12 volt main power line is connected to a computer power supply. Now if you look at an RCA connector it has two conductors one being the center post (on the male plug) and the outer ring is the second conductor. The power distribution box is just 8 female RCA jacks on one plate. All we are doing is connecting all the center contacts together (one ones that connect to the male center post) these are the two outside strips of connectors. The center of the plate has the outer ring (ground) connections conveniently leaning towards the middle and almost touching so we solder all eight connection together and connect the ground wire there. the outer two strips on the rca plate get connected together and connect to the positive wire. the wire exits the box and has an rca plug attached that plugs either into the main 12 volt power line or into another power distribution box which is already powered. This makes it easier to route power wherever you need it. It is very a flexible system. One last benefit of the RCA plugs it that they will pull out it the wire gets caught or you trip on a wire or something. this save the lights from damage because you just plug it back in. There has already happened to me so I know it works. I hope this answers your questions but if not let me know and I will see about getting some pictures up.


Thanks hpropman, very helpful. This is the year I build these (haven't been able to find the temporary end caps at HD, but I will come up with something)


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## hedg12 (Jul 6, 2008)

Dead Things said:


> Thanks hedg12. Ok, I think I get this. If I used only series (wiring individual lamps in series AND connecting all lamps to the main supply in series), I would quickly outstrip the 12 volts of a wallwart or CPS. If I wired everything in parallel, I would quickly outstrip a wallwarts amperage (assuming it has 1 amp). However, I could still connect more lamps to a CPS because it has higher amperage, correct?


Yep, that's right. You probably wouldn't want to load any power supply to its max rating, though. I'd say stay at about 80% of its rating is probably safe.


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