# Looking for the safest resistance for Parallel-series LEDs

I have 8 identical infrared LEDs (datasheet as shown below) connected like so:

I am trying to determine the correct R1 and R2 values to achieve 80 mA current in each of the LEDs. Help

• You will NOT be able to pick a value to guarantee just under 100mA. 100mA is also the MAX... life will be short at that, 50mA is likely better. But if you need it that tight to the limit you would be better driving them with constant current drivers. Commented Aug 9, 2017 at 19:53
• @Trevor I thought it would light up brighter if its closer to max, but if it is not worth it, I would prefer it to be at 80% of the max current (80 mA). Edited my post Commented Aug 9, 2017 at 19:57

You will NOT be able to pick a value to guarantee just under 100mA. 100mA is also the MAX... life will be short at that, 50mA is likely better.

In order to calculate R you need to subtract the total forward voltage of the diodes from the supply voltage. Normally you would use the typical value of 1.4V shown here, that's 5.6V across the diodes so 3.4V remaining. That would be an R value of 34R for 100mA.

HOWEVER, since there is no minimum value for Vf you can assume some will be less than 1.4V, so for 34R, your current will exceed the max of 100mA.

As such you need to guess and derate the current to perhaps 50%.. e.g. 50mA. Your R value will then be @75R.

But if you need it that tight to the limit you would be better driving them with constant current drivers or using a select on test resistance value.

EDIT Since you changed it to 80mA, R value would be 43R half watts.

• Thanks for confirming, I computed it to be 42.5 ohms @ 0.448W per branch Commented Aug 9, 2017 at 20:10

According to the information given, the LEDs work with 1.4V typically on IF=100mA. With that information, you can easily find R1 and R2, knowing that their current will also be 100mA and solving for one of the branches (we assume them equal). $$V_{R1} = 9V - 4 * 1.4V\\ V_{R1} = 3.4V$$ Solving Ohm's Law for R1: $$R1 = V_{R1} / I\\ R1 = 3.4V / 100mA\\ R1 = 34 \Omega$$

R1 and R2 are equal. Remember to use the appropiate power rating in these resistors.

• Ya but 100mA is max and Vf is only typical... it can be less.. and the LEDS go boom. Commented Aug 9, 2017 at 20:02
• @Trevor I agree, but these calculations were made using the original question. In that case, I would try to use the LED I vs V graph to check more in detail the right point. As you said in your answer, 80% of the current does the trick.
– jgmh
Commented Aug 9, 2017 at 20:11