### Author Topic: Diode knee voltage.  (Read 2259 times)

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#### Jimbz

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##### Diode knee voltage.
« on: March 26, 2015, 12:18:28 pm »
Hello everybody, small question:
What is a diode knee voltage ? And what is diode forward drop ?
As far as I figure, the knee voltage is the minimum voltage required for the diode to allow conducting the smallest bit of current ?
And forward voltage drop is the value to fully make the depletion zone disappear ?

#### IanB

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##### Re: Diode knee voltage.
« Reply #1 on: March 26, 2015, 12:22:55 pm »
I'm not an EE--what am I doing here?

#### Dave

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##### Re: Diode knee voltage.
« Reply #2 on: March 26, 2015, 02:29:47 pm »
As far as I figure, the knee voltage is the minimum voltage required for the diode to allow conducting the smallest bit of current ?
Nope. Diodes conduct anywhere above 0 volts (and below for that matter). It's just that the function of current with respect to voltage is exponential, so the current below the knee voltage will be very small.
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it

#### TimFox

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##### Re: Diode knee voltage.
« Reply #3 on: March 26, 2015, 11:30:40 pm »
As the previous poster stated, diodes conduct at all voltages.  The I vs. V curve goes continuously through the origin (0,0) with a finite slope.
In the forward bias direction, the current increases exponentially (very fast) with voltage, so there is no well-defined "knee".
In the reverse bias direction, as the voltage goes negative the negative current saturates at a low "leakage" level, but increases abruptly at a well-defined knee voltage, becoming a Zener diode.
For on-purpose "Zener" diodes with knees greater than about 5 V, the diodes are actually "avalanche" diodes and the current increases much faster above the knee than with true Zener diodes at, say, 3.3 V knee.
By the way, if you are measuring current for a diode in a glass case (e.g., 1N4148), you may see current at zero voltage due to photocurrent induced by room lighting.
« Last Edit: March 27, 2015, 12:23:39 am by TimFox »

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