# Help with LED's in parallel resistor

Discussion in 'General Electronics Chat' started by CrazyAl, Aug 27, 2010.

1. ### CrazyAl Thread Starter New Member

Aug 27, 2010
20
0
Hi I am new here, I have made a simple infra-red LED array and put them in a circuit and installed them in a project box and it all works fine except I am a bit confused as to what value of resistor I need to put in as many on-line resistor calculators only calculate for LED's in series OR LED's all in parallel and not both at the same time.

Each LED's forward voltage is 1.6V and uses 140mA.
I have got two sets of 4 LED's in parallel with each other running of of a 9V battery and have got two 12ohm resistors, one at the start of each LED array. Now I do have dyscalculia which might be why I am getting comfused with what value of resistor I need.

I have attached a basic circuit diagram if that helps, Thanks.

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2. ### Ghar Active Member

Mar 8, 2010
655
72
You can add together the voltages of the LED's and treat it as a single one.
So, 4x1.6V = 6.4V

With a 9V battery and 140 mA, you get this:

9V - 6.4V = 140mA * R
R = 18.6 ohms

The current will drop as the battery discharges so you can play around with that a bit.

With a 12 ohm resistor you will have approximately:
I = (9V - 6.4V)/12 = 216 mA

3. ### CrazyAl Thread Starter New Member

Aug 27, 2010
20
0
Sorry, but I am having a hard time understanding what you are saying.

I have 8 1.6 volt 140mA LED's in total, I have 4 in series behind a 12ohm resistor and another 4 in series behind another 12ohm resistor in parallel with the other 4, What ohm resistor do I need to use as I don't think 12ohm's is enough.

Edit: I have just realised what you mean that with the on-line LED resistor calculators I can just put 2 LED's in the calculator but just 6.4v as the forward voltage of the LED's.

But I am still confused with the current, where it asks what is the current of each LED do I need to times that by 4 as-well? so instead of putting 140mA I put 560mA.

Thanks.

Last edited: Aug 27, 2010
4. ### Ghar Active Member

Mar 8, 2010
655
72
You can consider each string by itself since a battery has very low resistance compared to your components.
That means you can work with a single string of 4 LEDs and then just build it twice off the same battery.

Current in a series circuit is equal for all the components so you'd leave it as 140mA.

I showed the calculation, you would use two 18+ ohm resistors, one per string of 4 LEDs.

This calculator considers series/parallel and even suggests schematics for you:
http://led.linear1.org/led.wiz

That calculator gives 22 ohm resistors because you can't get an 18.6 ohm resistor.
http://www.elexp.com/t_eia.htm
With 10% resistors you go from either 18 ohms to 22 ohms. They pick the higher one because they're assuming you don't want to exceed the current you specified.

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If you are using a 9v "transistor" battery, you will drain it very quickly. Your light output will diminish noticeably in the first few minutes of installing a fresh battery. 9v "transistor" batteries are rated at around 150mAH when discharged at a 20 hour rate (7.5mA); when you discharge them with a higher load, they go dead much more quickly.

Consider using an array of six AA alkaline batteries in a battery holder. They will be less expensive, and will last far, far longer.

6. ### CrazyAl Thread Starter New Member

Aug 27, 2010
20
0
Thankyou, that really made things easy to understand for a beginner like me!

I guess I am quite lucky that my LED's have not burned out yet using only 12ohm resistors and I will have a look through my big box of resistors tomorrow and swap them for some 22ohm ones.

7. ### CrazyAl Thread Starter New Member

Aug 27, 2010
20
0
I am just using a normal 9V energizer battery.

Edit: Alkaline I belive so according to wikipedia I should get around 565mAh

Last edited: Aug 27, 2010
8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
OK, I was using rechargeable NiMH numbers instead of alkaline.

Still, you will have a very short battery life span even @ 565mAH. That's discharging at a 28.25mA constant rate for 20 hours, until the battery reaches around 7v. You want to discharge at a 140mA x 2 rate, or 260mA. That's almost 10x the 20 hour discharge rate.