Diode polarity

[Murphyv310]

Very true comment Ed.
Sometimes I feel quite rusty on my theory. The odd thing is I'm going through the intermediate paperwork for radio amateur exam. When I do the mock mock tests I just about get all the electronics questions correct, I don't have to think about the calculations etc it's like second nature. I just look at a circuit and am able to see what to expect, ask me to calculate something though as often as not I trip up on the maths or get the decimal point wrong.
Talking about diodes one question involved a bulb and Diode in series and a battery supply, you were given four answers to choose which one of which was zero across the bulb, the Diode in fact is reversed biased. Allegedly 95% of candidates give the answer as the supply less 0.6v as they mostly think the Diode is forward biased and not 0v. Again lack of practical experience.

[Diabolical Artificer]

That PCB legend is asking for trouble, it doesnt make sense as the tant's minus/ground connection is in the same vicinity, so that's probably gnd. Diodes are prone to this confusion as they can be used in rectification, the anode goes to AC if you want + out or it could be the cathode if you want - out. If used as reverse polarity protection the cathode goes to +, if used to protect a mosfet different again, if used in an audio OP stage or CCS different again. Chuck zeners into the mix for more confusion.

Why not print a diode symbol on the PCB and be done with it, a rectangle with line is too wishy washy, someone unfamiliar with discreets might fit a resistor or radial cap.

Andy.

[Craig]

Of course there is the perennial issue of "conventional" current flow, which is from + to - , and yet the actual electrons flow in the opposite direction.

To add to the confusion, the old AVO8 (certainly for the I, II and III) reversed the polarity of the terminals for resistance measurement...

[Kalee20]

The whole idea of teaching that diodes, you make the anode positive, the cathode negative, seems strange to me... An ideal diode, after all, you would never BE able to do that because an ideal diode turns on with zero volts when passing forward current. You could only ever impose a voltage across it in the reverse direction.

If you connect a DC voltmeter across a diode in a rectifying circuit, you'd find the average voltage across it is anode negative, cathode positive.

Yes, there is a huge anomaly with Zener diodes, in normal use you make the cathode positive and the anode negative (why?? After all, a gas-filled voltage stabiliser you make the anode positive, the cathode negative). Even more with the TL431, which is an IC but the manufacturers call it a 'programmable Zener' and thus again, what they call the anode, you have to use as negative terminal.

Andy, yes agree completely (though the legend does have D9 beside it, they'd know to fit the diode because of a parts list showing what D9 is). Unfortunately, we can't change the legend, we're stuck with it.

[peter scott]

ask me to calculate something though as often as not I trip up on the maths or get the decimal point wrong.

Yes, I'm the same. You'd think that someone who spent many years using a sliderule would at least manage to put the point in the right place.

Peter