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Comments on Select resistor for a diode

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Select resistor for a diode

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Suppose we have this circuit

see me

How do I pick a appropriate value for the resistor R1?

LED-driver
posted about 3 years ago
1y ago by Lorenzo Donati‭
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MissMulan‭
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A diode obeys the Shockley diode equation and for this circuit we have:hi

hi

The current through the diode must not pass a limit so for LTL-307EE we go to the datasheet to find the maximum current before the diode is destroyed:

For the LTL-307EE at 20C we have 10^4 A Isat and Vth=0.026V

hi

hi

(30mA).

Now in order to find the resistance we find the resistance of the resistor if max current was flowing through the diode

hi

If we solve this equation we get:

hi So in order for LTL-307EE to not be damaged the resistance of R1 > 295.05Ω

Hope this helps

posted about 3 years ago
3y ago
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2 comment threads

Don't assume you know the voltage to current function (1 comment)
Don't pick a value based on absolute maximum ratings. (3 comments)
Don't assume you know the voltage to current function
Olin Lathrop‭ wrote about 3 years ago
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The equation you use that implies the LED current as a function of the voltage across it is stated nowhere in the datasheet. You should only go by what it says in the datasheet. Also 5 digits is absurd in this case, especially due to the high temperature dependence of LEDs in general. "295.05 Ω" is actually wrong because it implies a precision that just doesn't exist here. Go back to high school math and look up "significant digits".