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Old 16th November 2005, 06:58 AM   #1
Nordic is offline Nordic  South Africa
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Default Quick LED question

I am not sure if I remember correctly (knew this kind of stuff as a kid), but is the short leg on the LED the cathode (which goes to negative power rail).

Lol we had this crazy teacher, he explained to us the diffirence between anabolic and catabolic drugs, so he said, to remember which is which, think of a cat which claws things, as a breaking down process (sorry badly translated it all happened in my home language)... this image stuck till today so I know cathode is negative cause it has a cat in. LOL

No point knowing this if you still don't know which leg is supposed to signify which polarity.
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Old 16th November 2005, 11:42 AM   #2
SY is offline SY  United States
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The short leg is more negative when the diode is conducting. That may or may not be because of a connection to a negative rail, just so long as the longer leg is more positive than the shorter.

I never remember this, so I just use an ohmmeter before installing the diode.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 16th November 2005, 12:34 PM   #3
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The short leg side (cathode) of 3mm and 5 mm LED packages has a flat molded into the package. It is hard to see unless you look closely, but you can feel it with your finger. Think of the diode schematic symbol, and that flat as the cathode.

If you have trouble remembering which end should be more positive, use the diode symbol. The diode symbol has a triangle as an arrow-head to point in the direction of current flow. Current flows from a more positive point to a more negative point, in the direction of the arrow.

It took me a long time to get used to engineers thinking current flows from anode to cathode, when the majority of carriers in conductors are electrons and the flow of electrons is what constitutes current- in the opposite direction! If you think about vacuum tubes, the electrons flow from the cathode to the anode- they can't go the other way!

For some reason, a long time ago, someone (Ampere?) decided that current flow went from + to - even though that is opposite the physical process, except for the microscopic region in semiconductors where "hole" flow is the majority carrier (in which case current does move in the same direction as the carriers).

Electrons don't actually flow, either, the way water goes through a pipe. They are displaced by other electrons. One electron in a wire doesn't typically move down the length of the wire at nearly the speed of light. It moves a short distance (a few inches), at the drift velocity, which varies with the material, and is considerably slower than the speed of light. If the drift velocity is so slow, how can electric current move at (or near) the speed of light? When the power switch is closed, an EM field is established at the speed of light, moving all the electrons in the circuit very nearly simultaneously.

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Old 16th November 2005, 01:04 PM   #4
sam9 is offline sam9  United States
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For some reason, a long time ago, someone (Ampere?) decided that current flow went from + to - even though that is opposite the physical process, except for the microscopic region in semiconductors where "hole" flow is the majority carrier (in which case current does move in the same direction as the carriers).
I understood it to be Ben Franklin. His contibutipon wasn't really the lightning & kite story. Prior to his work on the subject the prevailong theory was that there were two kinds on electicity, one you got bu rubbing a cat with a glass rod and the other was obtained when you used a hard rubber rodn(or something like that). He realized that there was onlt one kind and that it wanteed to flow from high potential to low unyil the potentials were equal. It is for this very fundamental insight that some have opined that he would have neen a sure fire winner of the Nobel in physics had there been such a prise at the time.

Lacking any means to measure ther direction of the flow he guessed. The chances of getting it right were 50-50 and he got it wrong. If he had gotten it write historians of science would write learned essays trying to determine how he did it.
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