forward voltage drop

Papabravo

Joined Feb 24, 2006
14,378
There are three progressively more complicated models of a diode.

1st Model(ideal diode) -- On a plot of current(I) vs. voltage(V) imagine a horizontal line at I=0 from minus infinity up to Vf = 0.7 Volts. At this point imagine a vertical line at Vf = 0.7 Volts from the horizontal axis off to infinity.

2nd Model(linear approximation) -- Same line from V = minus infinity to Vf = 0.7 Volts. Now instead of a vertical line draw one from Vf = 0.7 Volts with a very steep slope of to (V, I) = (inf, inf)

3rd Model(exponential approximation) -- Same line from V = minus infinity to V = 0.5 volts and now fit an exponetial curve at that point which goes off to infinity.

It is in this third model we can see that the diode begins to conduct some current at a voltage that is lower than the forward voltage in operation. If we try to raise the voltage a little bit the the diode will conduct much more current as the voltage goes from 0.5 to 0.7 volts.

This is why you often see a diode, especially an LED, in series with a resistor.

This story ignores the extreme conditions that will cause device failure from either too much reverse voltage or too much current while forward biased.
 

hgmjr

Joined Jan 28, 2005
9,029
It is probably worth noting that the modeling that papabravo has described is that of a silicon diode.

There are diodes constructed from the semiconductor material germanium that have the same basic modeling that papabravo has outlined but differ in the value of the voltage at which the knee occurs. I believe the generally accepted value for this voltage is around 0.2 volts. These diodes are not very widely used any longer as they tend to exhibit greater variations in their forward bias voltage due to temperature than the silicon based diodes.

hgmjr
 

Papabravo

Joined Feb 24, 2006
14,378
There are also Schottky diodes which have a threshold in 0.2 to 0.3 Volt range. There are many types of diodes which are customized for different applications. It is a very versatile component.
 

hgmjr

Joined Jan 28, 2005
9,029
There are also Schottky diodes which have a threshold in 0.2 to 0.3 Volt range. There are many types of diodes which are customized for different applications. It is a very versatile component.
That is a good point.

At the risk of dating myself, I recall the day when selenium rectifiers were all the rage. I believe the drawback to these devices were that selenium was toxic.

hgmjr
 

pebe

Joined Oct 11, 2004
626
That is a good point.

At the risk of dating myself, I recall the day when selenium rectifiers were all the rage. I believe the drawback to these devices were that selenium was toxic.

hgmjr
.....and smelled of old boiled cabbage! :)
 
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