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### Topic: Strange resistor value (Read 1 time)previous topic - next topic

#### SouthernAtHeart

##### Feb 23, 2011, 05:18 am
Could someone please tell me what resistor to use on the emitter on this page: (the one connected to pin 3 on the diagram half way down)
http://letsmakerobots.com/node/19030

It says 39R, is that 39 ohms?  That doesn't seem like a normal resistor value to use on an LED to me, but I'm not familiar with IR LEDs.
thanks

#### RuggedCircuits

#1
##### Feb 23, 2011, 05:27 am
It is indeed 39 ohms. It seems a bit low in my opinion, but it depends on the supply voltage. If you are supplying +5V and you want 50mA, and the emitter does indeed have a forward voltage of 1.7V then you would calculate:

R = (5V - 1.7V) / 0.05A = 66 ohms

--
The Aussie Shield: breakout all 28 pins to quick-connect terminals

#### SouthernAtHeart

#2
##### Feb 23, 2011, 05:29 am
Thanks! And thanks for explaining how to figure out the value!

#### cachehiker

#3
##### Feb 23, 2011, 05:31 am
39 ohms is right for a 3.3-3.6 Volt power supply like single Lithium Ion cell or 3xNiMH cells.

I'd use 68 or 82 ohms with a 5 Volt power supply.

#### retrolefty

#4
##### Feb 23, 2011, 05:40 am

Could someone please tell me what resistor to use on the emitter on this page: (the one connected to pin 3 on the diagram half way down)
http://letsmakerobots.com/node/19030

It says 39R, is that 39 ohms?  That doesn't seem like a normal resistor value to use on an LED to me, but I'm not familiar with IR LEDs.
thanks

Well 39R does mean 39 ohms. However the info is incomplete, to say that 39 is the correct value or not. They don't say what the applied voltage is. Back calculating it by adding voltage drop across the resistor (Vr=.05 X 39) + (Vir = 1.7) = 3.65 volts doesn't make much sense either. Also if you read the datasheet for the device, you will see that 50ma of emitter current is the absolute maximum current rating, not the recommend current of 20ma.

So assuming you want to power this emitter with an Arduino output pin, you would calculate the resistor value as R = (5.0v - 1.7v) / .020 = 165 ohms.

Lefty

#### SouthernAtHeart

#5
##### Feb 23, 2011, 05:42 am

Could someone please tell me what resistor to use on the emitter on this page: (the one connected to pin 3 on the diagram half way down)
http://letsmakerobots.com/node/19030

It says 39R, is that 39 ohms?  That doesn't seem like a normal resistor value to use on an LED to me, but I'm not familiar with IR LEDs.
thanks

Well 39R does mean 39 ohms. However the info is incomplete, to say that 39 is the correct value or not. They don't say what the applied voltage is. Back calculating it by adding voltage drop across the resistor (Vr=.05 X 39) + (Vir = 1.7) = 3.65 volts doesn't make much sense either. Also if you read the datasheet for the device, you will see that 50ma of emitter current is the absolute maximum current rating, not the recommend current of 20ma.

So assuming you want to power this emitter with an Arduino output pin, you would calculate the resistor value as R = (5.0v - 1.7v) / .020 = 165 ohms.

Lefty

Ah, I think this is it!

#### retrolefty

#6
##### Feb 23, 2011, 05:45 am
Quote
Ah, I think this is it!

No think grasshopper, just do.

Lefty

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