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I am spinning my wheels here. How do you calculate for the correct resistor?

For example I have an IR LED that runs a 1.2 forward voltage and 100mA forward current. My power source is 5v. From what I calculated I need a 41 Ohm resistor. My calculation is ((5v / 1.2v) / .1), problem is when I use lets say a 47 Ohm resistor and I check it with a multimeter I am getting about 2.2 volts, but when I use a 1k Ohm resistor I am getting below 2v on the measure.

Any ideas?
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(5v - 1.2v)/0.1A = R needed
Basically (V across the resistor)/current = resistor value
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Thank you, at least now I know I am not going crazy.

Now to figure out why I am still getting 2+ volts on my multimeter.
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How are you connecting the leads of your multimeter?
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Are you measuring the voltage across the LED or across the resistor.  If the resistor then it means the LED is drawing less than 100mA.  The specified value of 100mA is the maximum that you should permit to flow but the device may actually draw less depending upon its individual characteristic; even the forward voltage (1.2 volts) will vary.  LED control using passive resistors isn't an exact science.  If you want accurate control of current then you need a constant current source device which will adjust its output voltage to maintain a constant output current.

LEDs may be classed as "current driven devices" so when setting up LED systems I'd recommend measuring the current flow and adjusting the limiting resistor accordingly.   
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I am spinning my wheels here. How do you calculate for the correct resistor?

For example I have an IR LED that runs a 1.2 forward voltage and 100mA forward current. My power source is 5v. From what I calculated I need a 41 Ohm resistor. My calculation is ((5v / 1.2v) / .1), problem is when I use lets say a 47 Ohm resistor and I check it with a multimeter I am getting about 2.2 volts, but when I use a 1k Ohm resistor I am getting below 2v on the measure.

Any ideas?

Can you provide a link to the spec sheet of the IR LED? Maybe the forward voltage is 2.2 instead of 1.2?
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Quote
My power source is 5v.

Yes but from where?

If it is an arduino output pin then there is not enough capacity to drive a load at 100mA at 5V.Therefore the voltage will droop. Measure the voltage across both the LED and the resistor. Then measure the voltage from the source, those first two numbers should add up to the third measurement.
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