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I bought a small DIY infrared emitter today, but two of the LED's are missing. The data sheet for the LED's is also missing.

The schematic is like this (a bit bigger):

schematic

Power source is 12V.

There are 3 LED in row followed by a 100Ω resistor.

Lets say L5 and L6 are missing. Is there a way to calculate a new R2 without knowing the U and I of the LED's?

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  • \$\begingroup\$ Vf for IR LEDs is usually about 1.8V. If you put that into Ohm's law, you get about 66mA of current through each LED. You should be able to replace the missing LEDs with any common IR LED which can withstand at least those 66mA. If, as I assume, the current through the LEDs is pulsed this can be taken into account, because most LEDs can be driven at higher current in pulsed mode, so that a normal 20mA IR LEDs may be used if its datasheet says that at least those 66mA are supported for the pulse waveform produced. \$\endgroup\$
    – JimmyB
    Commented Nov 28, 2014 at 15:51
  • \$\begingroup\$ Seems a bit odd that they would leave out a resistor (almost free) but populate an LED (not free). \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 28, 2014 at 18:17

2 Answers 2

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Since you have the device in your hands, you can determine all of the needed information.

  1. Measure the voltage across R1.
  2. This voltage divided by R1 (100 ohms, but you should measure it) will give you the current. Application of Ohm's law I=E/R
  3. Take the difference of the voltage across R1 and 12V, divide that difference by three to get the voltage of the LED.
  4. Subtract the voltage for one LED from 12Volts and divide that by the current you calculated for R1, and you have the needed value for R2. Application of Ohm's law R=E/I
  5. Put jumpers in for L5 and L6, and all four LEDs should light up the same.

Alternatively, purchase new IR LEDs that match the calculated current and the measured voltage.

You may also find some marking on the existing LEDs that will give you the model number, then you could order exact replacements (unlikely, as most LEDs with legs don't have markings.)

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  • \$\begingroup\$ "The difference of the voltage across R1 and the voltage across L1 to L3" seems to be misleading. I propose rephrasing to "The difference of 12V and the voltage across R1 is the same as the voltage across L1 to L3, which divided by three is Vf." \$\endgroup\$
    – JimmyB
    Commented Nov 28, 2014 at 15:45
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You can measure U (forward voltage) of one of the existing LEDs (with a multimeter). If you disconnect one of the 100 Ohm resistors from a working chain, you can measure I (current), too. In fact, you need not disconnect -- simply measure the voltage across the resistor, and use Ohm's Law to work out the current. With that information, you can either buy new LEDs to replace the missing ones, or alter the resistors and bridge the missing LEDs.

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    \$\begingroup\$ For extra accuracy, measure the resistor too, it certainly isn't 100 exactly \$\endgroup\$
    – PlasmaHH
    Commented Nov 28, 2014 at 15:24
  • \$\begingroup\$ @PlasmaHH Yes, very good point! \$\endgroup\$
    – John Honniball
    Commented Nov 28, 2014 at 15:33

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