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gzimmerPosted - 17 September 2013 1:50  Show Profile  Email Poster  Edit Message
I think I've found the reason for the Diode Threshold myth:

Because the diode's forward V/I curve is essentially Exponential, it is difficult to deal with using simple maths.
A student trying to calculate an exponential bias current will run into trouble using Ohm's Law.

One way of handling this is to break the curve into a number of separate straight-line segments. This is known as piece-wise approximation.

Each segment consists of a resistor in series with a voltage source. With many of these segments in parallel you can get a good approximation of an Exponential curve.
As you move up the curve, each succeeding voltage source cuts in and causes the graph to bend.

For a simple calculation, just one segment is often sufficient. This would be one resistor in series with a voltage source.
Different diode types need a different threshold voltage to handle the different curve shape.

So students are taught to calculate the diode current by putting a voltage source in series with the existing circuit resistance.

Unfortunately they then take the single voltage offset as gospel. I know that I did, way back when...

for graph example see...
https://en.wikipedia.org/wiki/File:PWL_diode_characteristic.PNG


........... Zim


Edited by - gzimmer on 9/18/2013 5:56:39 AM

_J_Posted - 23 September 2013 0:26  Show Profile  Email Poster  Edit Message
To me, I suspect it may be the graph.
If you graph a 1N34 on a 1:1 scale it really looks like it has a knee at .2 volts. The curve is fairly straight to the negative side of that. It goes straight up to the positive side, so straight it is hard to imagine there is anything going on in that "straight" line.

It helped me to see the same knee-looking bend in the plain e**x curve. Having studied calculus, I know e**x has no point of max curvature, no knee. Without the perspective of calculus I can see why people prefer to believe "their eyes," I probably would do the same?

John Davidson

gzimmerPosted - 23 September 2013 2:9  Show Profile  Email Poster  Edit Message
My perspective is that every technician is taught that there is a threshold voltage with diode (or transistor) junctions.

For instance read the Wickipedia article at

https://en.wikipedia.org/wiki/Diode

"Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction (a state in which the diode is said to be forward-biased). The voltage drop across a forward-biased diode varies only a little with the current"

and
https://en.wikipedia.org/wiki/Diode_modelling#Variation_of_Forward_Voltage_with_Temperature

It's something students are taught from day one...

........... Zim

golfguruPosted - 23 September 2013 6:25  Show Profile  Email Poster  Edit Message  
http://www.doe.carleton.ca/~smcgarry/ELEC3908/Slides/ELEC3908_Lect_1.pdf

.....................

gzimmerPosted - 25 September 2013 7:41  Show Profile  Email Poster  Edit Message
Another text which discusses whether the Threshold Voltage exists or not....

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/chars/chars.htm

They refer to it as the "Corner Model"

It seems to support my post above..

........... Zim

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