So I should not have seen a different voltage on the 'plate' side of the diode - if I understand correctly.
73, Jamie WB4YDL --------------------------------------------- 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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
