What sort of resistor will i need for 8 IR LEDs wired in series and powered by a 9v battery?

I plan on wiring 8 IR LEDs in series with a 9v battery to power them. i pulled the LEDs out of some remote controlled helicopters so I'm not sure of the values. if there's a safe way to go, what resistor/s should i use?

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You won't have enough voltage to get 8 series LEDs to run properly. You may struggle even with 4/4

Steve
tokymaru (author)  steveastrouk6 years ago
i have 10 leds, if i ran all ten and used 2 9v batteries, would that work any better?
IR LEDS will flatten a 9V cell very fast. You'd be better with a 6V (thats 4 AA) pack, then chains of 3.
janw6 years ago
Most IR-leds need 1,35V to 1,50V to run propperly. So if you have 9V you can run 5 of them + a a resistor of 75Ohm. If we look what the closest value is in the E12 series, that would be 82Ohm.
And what current ? What is "properly" ?

Steve
Properly means working. Most IR leds have U = about 1,50V I = 20mA so

R = (9 - 1,5*5)/0.02 = 75Ohm

But to make 8 of them work you can make 2 parallel series of 4

R = (9 - 1.5*4)/0.02 = 150Ohm
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The datasheets for IR emitters usually assume a forward current of ~50mA.

You are completely right Steve! When I answered, I looked quickly at the figures provided by my favorite LED shop and based my calculations on those.

I did a quick search and found only one type a ~25mA but most are as you correctly stated around 50mA or even 100mA.

Learned a new lesson today: 'Never trust you favorite LED shop!'
And in remote control type circuits AMPS of forward current are pulsed, to make the LEDs VERY bright, very briefly.

I hope you understand now why I said "properly".

Steve
Indeed. With properly I did mean continous use. I love it when I make mistakes as it is the best way to learn something!
seandogue janw6 years ago
+1.

Although to be specific...

Let Vf be the forward voltage for the led (find in datasheet for led)
Let If be the forward current for the led (find in datasheet for led)
the resistor needed for each group will be

(9V - (4 x Vf))/If