diode basics

When the anode is more positive than the cathode, (by some small amount that varies with diode material) current passes through the diode, and it is said to be forward biased. The end of the symbol that looks like a triangle is the anode, and the end with the bar is the cathode. Most small cylindrical diodes have a band around the cathode end. Larger diodes often have a diode symbol painted on to show the polarity.

That is correct provided the voltage is not beyond the breakdown voltage in which case it will flow as well in the opposite direction.

Cimarron

That is correct provided the voltage is not beyond the breakdown voltage in which case it will flow as well in the opposite direction.

In general, assuming conventional current flow.

A lot depends on how precise you want to get.

A sampling - the list goes on quite a ways:

A zener (or avalanche) diode is normally expected to operate in reverse (beyond the breakover knee) [pretty decent noise source]. All diodes will continue to conduct even when reverse biased with a fast transient (see reverse recovery time) and there's always some reverse leakage when the device is statically reverse biased.

What matters is what affects the circuit ;)

Cheers

PeteS

Correct:

But now someone will come along and argue that since electrons are deemed to pass from cathode (-ve) to anode (+ve) it should be the other way round!!!!!!!!

Ignore that; conventionally 'current' flows from anode to cathode. i.e. from positive to negative. Circuit analysis works fine that way. It IS the convention and since we can't see electrons any way it isn't important!

For most practical purposes a diode can be considered a one way current flow device. There are many different types, ranging fro those so small they are part of a computer chip to huge electron tubes that glow.

Oddly enough, I wanted to know about this too...

Is the conventional current flow really describing the phenomena of the 'flow' of the 'holes' left my moving electrons??

James

in

Not really. Conventional current describes the movement of hypothetical positive charges. When it was arrived at as a convention, the actual charges that make up current were not understood. Read up on Ben Franklin.

Actual current can be made of electrons, holes, positrons, anti holes protons, anti protons or larger ions, depending on the medium.

If electrons carry the charge in a circuit then surely the directions of the arrows of these diode symbols should be reversed to make interpretation more intuitive;

You agree? Yours thoughts please

James

"James" wrote in news:BM2sh.88042$ snipped-for-privacy@newsfe4-gui.ntli.net:

It really doesn't matter which way the current goes. You can assign it either direction and have all calculations work out just fine. The diode symbol was designated using conventional current flow, to do what you want there would need to be one symbol for conventional current and one for electron flow current. There is nothing really "intuitive" about electronics (despite what text books may say!), just learn the symbol and move on...

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The flow of charge from positive to negative is just as easily modeled as the flow of charge from negative to positive. What is important is consistency in the model.

It is convention to show electrical current as flowing from positive to negative, and indeed all symbols point that way. It is also used in datasheets to specify source (from a positive point) and sink (towards a less positive point) current capabilities.

Using a common method keeps things straight. Analysing a circuit using either method will yield *identical* results as it really doesn't matter what direction the charge carriers are moving for the purpose of analysis of a circuit. What matters is the delta between the various points in the circuit.

Having taught this subject, I know it's one a lot of students get hung up on.

Cheers

PeteS

It does matter which way the current flows as if it is flowing one way and your diode is facing the other you will not have a circuit.

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Early scientists studying electricity didn't know about electrons and other sub-atomic particles, and had no means to determine what was moving in an electric current, or which way it was going, so arbitrarily declared that current could be represented as a movement of positive charge, moving from the positive terminal of the supply, through the circuit, returning to the negative terminal of the supply.

Scientists and engineers have always used this convention, but during the vacuum tube era, many technicians were taught to think of current as a flow of negative charge, because it is very difficult, if not impossible, to explain the operation of vacuum tubes using positive charge.

It doesn't really matter whether you think of positive or negative charges moving - all the calculations work fine either way, except that the sign-obsessed will get lots more negative signs when working with negative charges. However, since most texts, data sheets, etc. use conventional (positive) current, it is best to stick with that.

Please learn to trim and bottom-post.

The arrow points in the direction of "conventional current" flow. This is from positive to negative.

This is what's taught in colleges and universities. In the real world, we know it's electrons that carry the current, especially those of us who learned about electronics by studying vacuum tubes; but it was the frat boys who made up the symbols for semiconductors - if they had their way, the symbol for a tube would be an arrowhead for the plate and a straight line for the filament. ;-)

The bottom line is, it doesn't matter. As long as you're consistent throughout your design, the equations are exactly the same.

I used to really annoy the "conventional current" gang by asking, "OK, so, in a CRT, how do the positive charges know exactly which pixel to leap off of so that they'll follow the exact right path through the deflection coils to the cathode?" and give them a coy smile. ;-)

Hope This Helps! Rich

They don't. CRTs don't have pixels....;-)

Bob M.