I have been working on a project for a freetrack IR clip to use with some of my simulators, and I had a question regarding resistor values. The guide I am using is here, and it details the use of 30 Ohm resistors. However finding these has been a pain but I have found 33 Ohm resistors available near me.

I was wondering if there would be a significant difference between using 30 and 33 ohm resistors for this circuit, which has 3 IR leds powered by a 2xAA battery pack.

I look forward to your answers, and thank you for reading my question.

Cheers, Finn

  • \$\begingroup\$ If you'd show a schematic we could tell what they are used for and what value would be fine. \$\endgroup\$ – PlasmaHH May 29 '18 at 12:11
  • \$\begingroup\$ Hey, thanks for the quick response! The schematic is here: i52.tinypic.com/2wc2p8w.jpg \$\endgroup\$ – Finn May 29 '18 at 12:13
  • \$\begingroup\$ thats not a schematic. \$\endgroup\$ – PlasmaHH May 29 '18 at 12:21
  • \$\begingroup\$ Oh, sorry. I had a bit of a brainsnap and forgot what you meant by a schematic :) Unfortunately I don't have one, but luckily Andy was able to answer my question. Thanks though! \$\endgroup\$ – Finn May 29 '18 at 12:29

The SFH485P I.R. diode is capable of taking current continuously up to 100 mA. It also has a forward volt drop of about 1.4 volts when this current is flowing. If your power supply is 3 volts then you need a resistor than can drop 1.6 volts at 100 mA. Ohm's Law tells us the resistor should not be smaller than 16 ohms. Given the ones recommended are 30 ohms you will get slightly less light output using a 33 ohm resistor but this may not be a problem.

Alternatively use 27 ohm resistors as you will be a long way from exceeding the 100 mA limit current but, battery life will be less.


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