please explain

[richerdick]

how can i predict the volt drop across a diode for a given current through it whilst forward biased. i ask the question because i am working with a drive that has an analogue reference that has a diode in series with it the refernce signal varies from 0-5v.h measurment i can see that the volt drop across the diode varies between 0.4 and 0.2v depending on the applied voltage

 

[ScottyUK]

The diode datasheet might have a plot of typical I_f against V_f if you are lucky. You could do the same experimentally too. Otherwise why not use a current source as the reference instead of a voltage source? Then the diode volt-drop doesn't matter and you gain other advantages like noise immunity and ability to run much longer distances. 4-20mA neatly gives 1-5V when terminated at a 250[Ω] resistor or 2-10V with 500[Ω].


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Sometimes I only open my mouth to swap feet...

 

[IRstuff]

You can, carefully, measure the Is vs. Vs curve of the diode and extrapolate what the lower current diode drive would be, or measure it directly.

The basic equation looks something:

I = Is*exp(Vs/Vt) where Vt = kt/q

If you already have a couple of points, you can fit the equation.

Earlier, I said "carefully," since the current flow affects the temperature of the diode, which affects Vt.

TTFN

FAQ731-376

 

[logbook]

Some people use diodes or emitter-base junctions to measure temperature since the voltage changes by around 2mV/degC. If this is too much of an error for you then these is no point in continuing with this diode measurement exercise.