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

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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steveastrouk
steveastrouk

10 years ago

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

Steve

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tokymaru
tokymaru

Answer 10 years ago

i have 10 leds, if i ran all ten and used 2 9v batteries, would that work any better?

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steveastrouk
steveastrouk

Answer 10 years ago

IR LEDS will flatten a 9V cell very fast. You'd be better with a 6V (thats 4 AA) pack, then chains of 3.

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janw
janw

10 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.

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steveastrouk
steveastrouk

Answer 10 years ago

And what current ? What is "properly" ?

Steve

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janw
janw

Answer 10 years ago

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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steveastrouk
steveastrouk

Answer 10 years ago

The datasheets for IR emitters usually assume a forward current of ~50mA.

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janw
janw

Answer 10 years ago

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!'

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steveastrouk
steveastrouk

Answer 10 years ago

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

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janw
janw

Answer 10 years ago

Indeed. With properly I did mean continous use. I love it when I make mistakes as it is the best way to learn something!

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seandogue
seandogue

Answer 10 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