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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 over 3 years ago
1y ago by Lorenzo Donati‭
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MissMulan‭
73 reputation 58 11 -52 61
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+0
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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 over 3 years ago
3y ago
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MissMulan‭
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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 pick a value based on absolute maximum ratings.
Lundin‭ wrote over 3 years ago · edited over 3 years ago
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Peak forward current listed below absolute maximum ratings is the stress value they promise that the diode can take for a short while. If you feed it that continuously it will eventually burn to a crisp. Don't design using absolute maximum ratings! You should be looking at electrical characteristics and the rated current for normal use. Also keep in mind that regular resistors have a 1% accuracy and that their resistance, as well as diode forward voltage, will vary with temperature. One should always design with margins.

MissMulan‭ wrote over 3 years ago
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You are correct I will edit

MissMulan‭ wrote over 3 years ago
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Done check it out now.

Skipping 1 deleted comment.