[Elecraft] OT - troubleshooting NIR-12

[Ron D'Eau Claire]

So I should not have seen a different voltage on the 'plate' side of the
diode - if I understand correctly.

73, Jamie
WB4YDL

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Diodes have a forward voltage drop. It depends upon the material used in the
junction and how it is treated or 'doped' in manufacture. It can vary from
less than 0.2 volts to about 0.6 volts for a common silicon diode.

So if you measure 13.8 volts between the positive (anode or arrow) side of a
diode and ground, you'll see something between 13.6 and 13.2 volts between
the cathode side of the diode and ground. This drop is largely independent
of the current being drawn, within limits. It's called the "barrier voltage"
and acts as though a small battery with a voltage between 0.2 and 0.6 volts
is inside the diode, bucking the voltage applied. There's always some small
value of ohmic losses in any component too, so the drop will vary slightly
when there's a large change in the current.

Of course when you reverse the diode, current almost stops. There's some
leakage, usually in the range of a few microamperes, but it's usually
negligible. That is, until you reach the reverse breakdown voltage. At that
point an avalanche of current will occur melting the junction and turning
the diode into a resistor. So in a circuit design it's important to respect
the reverse breakdown voltage specification of a diode.

Some special types of diodes use that effect in a controlled way that avoids
damaging the junction. One common one we see all the time are
voltage-regulating "Zener" diodes. The Zener effect is just that: when
reverse-biased (positive to the cathode, or line side on the schematic and
negative to the arrow side) very little current flows until a specific
voltage is reached. Beyond that the diode begins to conduct heavily,
maintaining a fixed voltage across its junction. Such circuits always
include a current-limiting resistor. Without it, the diode would be
destroyed by the current flow. As long as the current is limited to less
than the maximum rating for the diode, the voltage across the diode will
remain close to the initial zener value. That's why they make quite good
voltage regulators. 

Of course, those diodes are used with the connections reversed from normal
diodes: the cathode (straight line on the symbol) goes to the positive
voltage and the arrow goes to the negative voltage. Such diodes are shown
schematically with little extra "wings" on the straight cathode line to
indicate that they are Zener diodes. 

Ron AC7AC