Continuing on, we find, from Danny T:
<QUOTE>
http://www.kpsec.freeuk.com/components/diode.htm
says
"There is a small voltage across a conducting diode, it is called the
forward voltage drop and is about 0.7V for all normal diodes which are
made from silicon. The forward voltage drop of a diode is almost
constant whatever the current passing through the diode so they have a
very steep characteristic (current-voltage graph)."
--
Danny
<END QUOTE>
To which you replied:
The forward voltage drop is entirely dependent on temperature (the
junction temp. of the p/n junction; which is in turn dependent upon
the current passed.) Higher currents equals higher temp. equals lower
voltage drop. It's a well known effect which can eventually destroy
the diode altogether. The physics of diodes is actually more complex
than a lot of texts would have you believe.
---
Indeed, but if you think the forward voltage drop is _entirely_
dependent on temperature, you seem to have missed reading some of the
more fundamental ones.
For example, while it's certainly true that the voltage across the
junction can be described by:
kT / If \
Vf = ---- ln ( 1 + ---- )
q \ Ir /
and that when T is equal to zero at 0°K, Vf will be 0, you've
neglected to mention that current passing through the bulk resistance
of the diode, at any temperature, will cause a drop across the
junction which is dependent on the resistance and the charge flowing
through the diode.
More importantly, perhaps, you pooh-pooh'd Danny T's idea to use a
diode as a bad one merely because of your opinion, which was
erroneous. Diodes are _often_ used as voltage dropping elements in
the real world because of the small change in Vf caused by If.
Moreover, your example of the negative TC of a diode destroying it
would more closely describe a diode with a voltage source connected
across it allowing the diode to get into thermal runaway. Such a
condition would not happen with the load limiting the current through
the diode and the diode sized to carry the required current under the
required environmental conditions. Furthermore, depending on the
diode, above a certain current the tempco becomes positive, something
else you "neglected" to mention.
What's most disturbing, however, is that with Danny T admittedly being
a newbie and asking for information, you deliberately sidestepped the
issue when he presented you with the [valid] information he found
which supported Andrew Holmes' suggestion to use a diode in order to
keep from having to admit that you were wrong in stating that: "Diodes
are a crap way to drop voltage unless the load is light and
predictable!!!
For shame, sir! :-(
