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I want to connect two IR LED to the 3.3v output on the PI in series. I need to reduce the 3.3v to 3.0v. What kind and measure should my resistor be?

I swear this is not a word problem from school....

Emitted Color : Infrared IR 940nm Viewing Angle: 135 degrees DC Forward Voltage (VF): 1.4V DC Forward Current (IF): 700mA Maximum Pulse Voltage: 1.6V

  • 3V3 from a gpio or from the 3V3 rail? A link to the IR LEDs would be useful. What are you trying to do? Illuminate a scene? If so the IR LEDs would use more power than the Pi could probably supply. – joan Nov 21 '14 at 12:04
  • Thank you for the foresight, but I really only need to know the resistor. It doesnot matter which 3.3v output I am using, only that the LEDs need 3v power and not 3.3 :) – RootWannaBe Nov 21 '14 at 12:09
  • Emitted Color : Infrared IR 940nm Viewing Angle: 135 degrees DC Forward Voltage (VF): 1.4V DC Forward Current (IF): 700mA Maximum Pulse Voltage: 1.6V – RootWannaBe Nov 21 '14 at 12:16
  • A gpio may be able to supply 20mA. The 3V3 rail on a B may be able to supply 50mA. Both far short of the 700mA needed per LED. A B+ may supply a lot more on the 3V3 rail. I'm not sure if anyone has tried. You might get a sustained 500mA from the 3V3 rail on a B+, or you might blow it up. – joan Nov 21 '14 at 12:46
  • I was thinking the same thing. What if I use the 5v rail? What should be the current and then what would I need for a resistor if it is even possible? – RootWannaBe Nov 21 '14 at 16:59
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Germanium diodes have voltage dropout of about 0.3V, so you can use it in series with wanted IR diode. Or you can use 1 Ohm/! Watt resistor for this task.

  • Great. So if my two LED are connected in series and use 3v and 700mA then this 1 Ohm resistor will help when connected to a 3.3v power supply on the PI..... – RootWannaBe Nov 21 '14 at 12:32
  • Actually it looks like two LED in series will be 3,2v but what does that do to the current? and if in series, will the pi be able to power these leds? – RootWannaBe Nov 21 '14 at 12:36
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I use two similar IR LEDs to send short RC code bursts without resistor without problems. If that#s your application then you should be fine. You may run into burned out LEDs, though, if the pin stays high for too long due to some issue.

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