I am not sure about what voltage to use in my Ohm's Law calculations when using IR333-A infrared LED.

The retailer I'm purchasing from states they are "Approx 1.6V forward voltage"


The DigiKey website lists the same product and describes it as "940 nm, 1.2 V, 100 mA, 7.8 mW/sr @ 20 mA". IR333-A.

And I'm not quite sure how to interpret the data sheet, as it has a table with multiple conditions.

For reference, I am planning to power these in series from a 5 V supply off of a Raspberry Pi Zero W.


  • 2
    \$\begingroup\$ Note that 100mA and 1A are pulsed-duty only (it will fail if driven continuously at these levels.) \$\endgroup\$
    – rdtsc
    Commented Oct 28, 2021 at 21:35

1 Answer 1


With a 5 V supply you have plenty of leeway. IR LEDs are usually about 1.4 V and your datasheet supports this in that the range is 1.2 to 1.5 V at 20 mA.

If you design for 1.2 V you'll have \$ R = \frac V I = \frac {3.8}{0.02} = 190 \ \Omega \$.

If it's 1.5 V and you use the 190 Ω resistor the current will fall to \$ I = \frac V R = \frac {3.5}{190} = 18.4 \ \mathrm{mA} \$.

If you're really worried you could use a constant current driver.

enter image description here

Figure 1. A constant current sink.

See my article for an explanation of operation.

  • \$\begingroup\$ Good answer. My project is actually more complicated, I want to wire up about 12 no. of these ( 2 in parallel x 6 in series) and the purpose of this post was only to see what the proper voltage was to use in the calculation. I also want to include an NPN transistor to switch it on/off from 3.3V GPIO pin. I will make a new thread with the full query and diagram. \$\endgroup\$
    – Zach Work
    Commented Oct 28, 2021 at 21:49
  • \$\begingroup\$ The Figure 1 circuit would be suitable for six in series with adequate supply voltage. You can read about variations and binning here. \$\endgroup\$
    – Transistor
    Commented Oct 28, 2021 at 22:01

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