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Old 11-07-2007, 12:47 AM #1
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Default Considerations for driving LEDs/LDs

I posted this as a response in another thread but I think the issue merits a stand alone thread.

The discussion of driving a LED or LD with a voltage source regularly arises. *Due to the electrical nature of LEDs and LDs the preferred method of driving them is with a constant current source. * There are two reasons for this:

1. The Voltage-Current (VI) Curve is very steep (i.e. a very small change in voltage results in a very large change in current)
2. The VI curve shifts with the temperature of the device.

I will use an image I snagged off the net and marked up. *It cannot be applied to specific situations but it is representative of the concept.

1. Check out the Green markings on the drawing. *Using the 25C curve and tracing the corresponding current for voltages V1 and V2 you get:

V1= 1.17V, I1 = 4mA
V2= 1.34V, I2 = 20mA

So for a 0.17V change in drive voltage the current went from 4 to 20mA. *
Expressed in percentages that's a 500% increase in current for a 15% increase in voltage!.


2. There are three Voltage-Current (VI) curves for the same LED for three different temperatures (-40C, 25C, 100C).
*
Let's pretend that the LED is driven by a regulated voltage (indicated by the red arrow). *

At -40C the current through the LED would be about 1.5mA.
At 25C the same voltage would result in a LED current of about 4mA.
At 100C the same voltage would result in a LED current of 15mA.

Note that the 10x difference in current between a cold (really cold) LED and a really warm LED. *

In practice, the temperature will not swing that much but it should be extra warning for people using TECs. *A room temperature LED will heat up in proportion to the power pumped into it and the amount of cooling it has. *

Also recall that the LED/LD die will be much hotter than the module it is housed in. *

You can extrapolate from this graph that the current for a LED/LD supplied by a fixed voltage could easily double as it heats up.

To check the change in resistance vs temperature use R=V/I
For -40C curve, V=1.17V, I=1.5mA, therefore R=780 ohms
For 25C curve, V=1.17V, I=4mA, therefore R=292.5 ohms
For 100C curve, V=1.17V, I=15mA, therefore R=78 ohms




I hope this example clarifies things for people with questions. *Good luck and happy modding. *


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Old 11-07-2007, 04:39 AM #2
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Default Re: Considerations for driving LEDs/LDs

Chimo, very nice explanation. Thanks for the easy to understand graph.
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Old 11-07-2007, 07:19 AM #3
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Default Re: Considerations for driving LEDs/LDs

i dunno, is it just me, or is that red font really hard to read??? :-? anyway apart from that, nice explanation!
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Old 11-07-2007, 09:56 AM #4
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Default Re: Considerations for driving LEDs/LDs

I've got my screen on the light background - forgot about the default black. How's it now?

Quote:
Originally Posted by things
i dunno, is it just me, or is that red font really hard to read??? :-? anyway apart from that, nice explanation!
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