# How to read IR333-A infrared LED datasheet

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"

https://thepihut.com/products/super-bright-5mm-ir-led-25-pack

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.

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

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.

Figure 1. A constant current sink.

See my article for an explanation of operation.

• 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. Commented Oct 28, 2021 at 21:49
• The Figure 1 circuit would be suitable for six in series with adequate supply voltage. You can read about variations and binning here. Commented Oct 28, 2021 at 22:01