LED’s log V/I curve

How do you use an LED’s log V/I curve to force an exponential current change in the LED for a linear voltage change?

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The info needed to make log and exponential converters with silicon is widely available. The saturation current, intrinsic resistances, and the Ebers-Moll equation are sufficient. A few other numbers help extend the functional range.

Does similar information exist for LEDs? If so, can someone point me to it?

How much voltage increase does it take to double LED current? Is it the same for different LED colors and semiconductor formulations? How much does it vary with temperature? How much current can you push through one before the internal resistance contributes too much error? How little current can you push through one before nonlinearities and errors overwhelm the signal? Do they need to be blacked out so that ambient light can’t affect them?ledconverterebers-mollShareEditFollowFlagasked Jan 3 at 8:52KFW13155 bronze badges

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LEDs are specified, as far as they are, to allow you to design power supplies to get a reasonable current through them. They are not intended to have any particular V/I relation, or be judged on their goodness of fit to any particular theoretical relation. This means if you want to use LEDs in your V/I converter, you are on your own to characterise them.

To give you a leg up, here are some curves I plotted for my own interest a few years ago. Sorry that the colours of the traces don’t correspond to the colours of the LEDs. This plot was intended to be comparative, so I have not made a note of the temperature, and IIRC the illumination was typical lab lighting levels.

See how bad a 3 V zener diode is, compared to a LED. Notice the 1N4006 is a ‘better’ diode at both high and low currents than a 1N4148, so now you know which to use to put in shunt with your 200 mV input meter to protect it from accident.

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ShareEditFollowFlagedited Jan 3 at 14:21answered Jan 3 at 12:22Neil_UK113k33 gold badges123123 silver badges272272 bronze badges

  • From the looks of those curves, red and green ones show a similar curve to silicon diodes, but with a higher voltage. It looks like maybe a 100mv voltage change will multiply current by ten, and it looks like intrinsic resistance doesn’t come into play until above a milliamp. For the LEDs I’m using (Perkins-Elmer Vactrols), those are workable numbers. And given that the LED is controlling a LDR with 50% variability, it seems plenty accurate enough. Now all I need is what tempco resistor to compensate the scale change with temperature. If it’s small enough I can ignore it – KFW Jan 5 at 1:43

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