Why is the Anode of an LED Connected to Positive?

[Alex Hughes]

So, I'm new to electronics and I started to build some circuits with LEDs. I read up on how LEDs work and how they consist of a doped semiconductor material etc. But when I actually went to wire the LED in, it said the anode should be connected to the positive terminal of the power source. I'm confused, isn't the anode the source of electrons and the cathode is where the electrons flow? If that is true, shouldn't the anode be connected to the negative terminal? Also, if the anode is the source of the electrons, why is it referred to as the positive side almost everywhere I read? I would think if a bunch of electrons were accumulated there, it would be negatively charged. I'm really confused on this entire topic, would love somebody to clear things up. Thanks.

 

[.Scott]

So, I'm new to electronics and I started to build some circuits with LEDs. I read up on how LEDs work and how they consist of a doped semiconductor material etc. But when I actually went to wire the LED in, it said the anode should be connected to the positive terminal of the power source. I'm confused, isn't the anode the source of electrons and the cathode is where the electrons flow? If that is true, shouldn't the anode be connected to the negative terminal? Also, if the anode is the source of the electrons, why is it referred to as the positive side almost everywhere I read? I would think if a bunch of electrons were accumulated there, it would be negatively charged. I'm really confused on this entire topic, would love somebody to clear things up. Thanks.

The electron surplus is at the cathode - and it is therefor negative.
Remember the old "cathode ray tube" - the cathode ray is a stream of electrons.

See: https://en.wikipedia.org/wiki/Cathode

 

[256bits]

I'm confused,and the cathode is where the electrons flow? If that is true, shouldn't the anode be connected to the negative terminal?

Anode/cathode is definitely confusing.
For an element in a circuit, the anode/ cathode can change, such as for a battery discharging or charging.
For discharge, the positive terminal is the cathode.
For charging a battery, the positive terminal becomes the anode.
One can see that, the cathode is where the CURRENT comes out of the device.

For an LED to work and emit light, one wants the current to come out of cathode, which is the pointy end in the figure.

To be more confusing,
For a regular diode, the pointy end is labeled the positive terminal in a general circuit diagram, but if biased in such a manner, no current flows through the diode device. Current will flow through the diode when the anode is more positive than the cathode, ie same as an LED working properly, and emitting light.

isn't the anode the source of electrons

Yes, from out of the device, not into the device.
Remember that electron flow is in the opposite direction from current flow.

Still confused.
I certainly am.

 

[anorlunda]

Blame Benjamin Franklin for your confusion. He was the one who first decided the sign convention for electric charge that makes the electron minus and the current flow in the opposite direction of electron flow. It was an arbitrary choice, and Franklin had no way to know what future confusion it would cause, but we're stuck with that convention for eternity.

One of the foremost experts on electricity in the 18th century was Benjamin Franklin, who argued in favour of a one-fluid theory of electricity. Franklin imagined electricity as being a type of invisible fluid present in all matter; for example, he believed that it was the glass in a Leyden jar that held the accumulated charge. He posited that rubbing insulating surfaces together caused this fluid to change location, and that a flow of this fluid constitutes an electric current. He also posited that when matter contained too little of the fluid it was "negatively" charged, and when it had an excess it was "positively" charged. For a reason that was not recorded, he identified the term "positive" with vitreous electricity and "negative" with resinous electricity. William Watson independently arrived at the same explanation at about the same time (1746).

 

[jim hardy]

256bits' symbol up above tells you , if you remember this 'memory aid': the arrow in a diode shows the direction of forward current , and that's textbook current not electrons.

 

[Alex Hughes]

Anode/cathode is definitely confusing.
For an element in a circuit, the anode/ cathode can change, such as for a battery discharging or charging.
For discharge, the positive terminal is the cathode.
For charging a battery, the positive terminal becomes the anode.
One can see that, the cathode is where the CURRENT comes out of the device.

For an LED to work and emit light, one wants the current to come out of cathode, which is the pointy end in the figure.
View attachment 220324

To be more confusing,
For a regular diode, the pointy end is labeled the positive terminal in a general circuit diagram, but if biased in such a manner, no current flows through the diode device. Current will flow through the diode when the anode is more positive than the cathode, ie same as an LED working properly, and emitting light.


Yes, from out of the device, not into the device.
Remember that electron flow is in the opposite direction from current flow.

Still confused.
I certainly am.

Thanks so much for your help, but let me try to summarize my understanding now. For conventional current, the cathode is where current flows out of and the anode is where current flows into. For electron flow, the anode is where electrons flow out of, and the cathode is where the electrons flow into. Also, I can't think of the anode as being positively or negatively charged since it changes depending on if the battery is charging or discharging. Is this right?

 

[dlgoff]

conventional current,

bold by me

That's the key word for the understanding.

To provide a definition of current independent of the type of charge carriers, conventional current is defined as moving in the same direction as the positive charge flow. So, in metals where the charge carriers (electrons) are negative, conventional current is in the opposite direction as the electrons.

 

[anorlunda]

Also, I can't think of the anode as being positively or negatively charged since it changes depending on if the battery is charging or discharging. Is this right?

The polarity of a battery doesn't flip when charging/discharging.

A voltmeter always requires two wire leads. It measures the difference between two points. In circuits it is the differences that change when flipping from charging to discharging.

To avoid confusion, stop trying to think of absolute potential as being the same as voltage. Think instead about the differences.

 

[sophiecentaur]

Blame Benjamin Franklin for your confusion. He was the one who first decided the sign convention for electric charge.

I think it is a bad idea to suggest that blame is involved here. The direction of electron flow is only a source of ‘confusion’ if electrons are introduced inappropriately early into teaching. Unfortunately this is how the subject is delivered these days. EE is a subject that’s basically Maths based and the negative sign of a charge is the very least of our worries.
So give BF a break. [emoji16]

 

[Tom.G]

Thanks so much for your help, but let me try to summarize my understanding now. For conventional current, the cathode is where current flows out of and the anode is where current flows into. For electron flow, the anode is where electrons flow out of, and the cathode is where the electrons flow into. Also, I can't think of the anode as being positively or negatively charged since it changes depending on if the battery is charging or discharging. Is this right?

This seems to be an endless source of confusion. The Anode is where the Oxidation reaction occurs (the reaction that frees up Electrons). However we are used to an Anode being the more Positive electrode for most of the components we use. There is a decent explanation at:
https://chemistry.stackexchange.com...e-anode-cathode-in-electrolytic-galvanic-cell

Cheers,
Tom

 

[sophiecentaur]

This seems to be an endless source of confusion. The Anode is where the Oxidation reaction occurs (the reaction that frees up Electrons). However we are used to an Anode being the more Positive electrode for most of the components we use. There is a decent explanation at:
https://chemistry.stackexchange.com...e-anode-cathode-in-electrolytic-galvanic-cell

Cheers,
Tom

PF is always getting questions about the Names of things and the confusion that results from insisting that the Name is more important than the Function of an item. What is called the Anode of a diode is the terminal that is positive when the diode is 'forward conducting'. That's all. It would be futile to say that, when a diode has broken down with excess reverse voltage, the Cathode has suddenly become the Anode. The name that is given to something is supposed to help people to have an idea how it will behave. If a device can work when connected up in different ways then that's another level of understanding and a better nomenclature may be called for.
When someone claims to be "confused" by a name, it is really up to them to take it on themselves to accept it rather than to look for a reason that the name is 'wrong'. It is essential to look at a number of sources for this sort of information because there are always sources that are just plain wrong. But that is all about developing a mature attitude to learning and figuring things out for yourself before just asking questions. (It is often a matter of developing confidence.)

 

[jim hardy]

What's in a name ? More than we think - it can initially lead one down a false path of logic.

We're so used to seeing the cathode of a rectifier on the positive supply rail with a + sign adjacent that as beginners we make a mistaken association.

I like Lavoisier's precautions in his preface to "Treatise on Chemistry" , a bit of a read but well worth it.
https://web.lemoyne.edu/giunta/ea/LAVPREFann.HTML

"A child," says the Abbé de Condillac, "is taught to give the name tree to the first one which is pointed out to him. The next one he sees presents the same idea, and he gives it the same name. This he does likewise to a third and a fourth, till at last the word tree, which he first applied to an individual, comes to be employed by him as the name of a class or a genus, an abstract idea, which comprehends all trees in general. But, when he learns that all trees serve not the same purpose, that they do not all produce the same kind of fruit, he will soon learn to distinguish them by specific and particular names." This is the logic of all the sciences, and is naturally applied of chemistry.

And electronics.

 

[gary350]

Anode is always positive. Cathode is always negative. It is designed and built to be like that.

 

[jim hardy]

Anode is always positive. Cathode is always negative. It is designed and built to be like that.

Except when it's reverse biased.

 

[Tom.G]

Anode is always positive. Cathode is always negative.

Absolutely true... for a passive device/load.

But I suspect this is another lovely case of...

...an endless source of confusion.

To summarize:

• A Cathode of a device is the terminal thru which Electrons enter the device.
• An Anode of a device is the terminal thru which Electrons leave the device.

Those are the definitions of Anode and Cathode. If Electrons are entering, that terminal is the Cathode. Electrochemically, that is the terminal at which reduction takes place.

Those definitions aren't so hard, it's just that we are so used to things like vacuum tubes and diodes being biased Anode positive and Cathode negative that we forget/ignore the definitions (or maybe never learned them), and we automatically extend the labelling to sources; where it doesn't quite work.

How about from the Encyclopedia Britannica, a generally reliable source: https://www.britannica.com/technology/cathode
Cathode, negative terminal or electrode through which electrons enter a direct current load, such as an electrolytic cell or an electron tube, and the positive terminal of a battery or other source of electrical energy through which they return. This terminal corresponds in electrochemistry to the terminal at which reduction occurs. Within a gas discharge tube, electrons travel away from the cathode, but positive ions (current carriers) travel toward the cathode. Compare anode.

https://www.britannica.com/technology/anode
Anode, the terminal or electrode from which electrons leave a system. In a battery or other source of direct current the anode is the negative terminal, but in a passive load it is the positive terminal. For example, in an electron tube electrons from the cathode travel across the tube toward the anode, and in an electroplating cell negative ions are deposited at the anode. Compare cathode.

Or from another usually reliable source: https://www.merriam-webster.com/dictionary/anode
(Also read the rest of the definitions on that page.)
Definition of anode
1 : the electrode of an electrochemical cell at which oxidation occurs: such as
a : the positive terminal of an electrolytic cell
b : the negative terminal of a galvanic cell

 

[sophiecentaur]

Except when it's reverse biased.

Yes. Half of the diodes in a bridge rectifier are reverse biased at anyone time.
But isn't this thread making a terrible fuss about what names to use? Anyone who can't work it out reliably just has to look at the symbols on the side of the device and then spend some time getting to understand the function of the device in its particular place in the circuit. You never need to use the A word or the C word if it confuses.

 

[jim hardy]

You never need to use the A word or the C word if it confuses.

On a schematic.
On a real diode the stripe is on the cathode end.
A wise guy once handed me a rectifier and asked "Which is the positive end of this thing?" .
To which i would now reply "Both ends. Just one end is more positive than the other."

old jim

 

[sophiecentaur]

To which i would now reply "Both ends. Just one end is more positive than the other."

I'd still have a problem with that. A catching diode in a relay circuit may spend most of its life connected 'backwards'. And what about an avalanche diode? They certainly don't follow that rule.
There is no substitute for the V/I graph in the spec sheet if you really want to know how a little piece of glass with two wires will behave, Call the ends "Pin No.1 and Pin No. 2".
Students hate rote learning - except when it suits them. Unfortunately they get a bit too much comfort from some of the cosy names they are told about. It's so much easier than learning the Functions.

 

[jim hardy]

I'd still have a problem with that. A catching diode in a relay circuit may spend most of its life connected 'backwards'. And what about an avalanche diode? They certainly don't follow that rule.

My train of thought is that voltage across a diode is determined by the external circuit in which it finds itself.
"Cathode" is marked by a stripe or the familiar barred arrowhead symbol printed on the case
and a voltmeter connected from cathode to anode will read positive when it's conducting and negative when it's blocking or avalanched.

old jim

 

[sophiecentaur]

a voltmeter connected from cathode to anode will read positive when it's conducting and negative when it's blocking or avalanched

Perfectly correct but what a mouthful for a student to try to remember by rote. How would they recognise the sign of the bias?
I realize you are more than capable of looking at these things every which way and getting the right answer but what about the poor beginner who wants a simple, four word rule? Basically, there isn't one.

 

[jim hardy]

Perfectly correct but what a mouthful for a student to try to remember by rote. How would they recognise the sign of the bias?
I realize you are more than capable of looking at these things every which way and getting the right answer but what about the poor beginner who wants a simple, four word rule? Basically, there isn't one.

Only thing i can suggest is hands-on tinkering.
"Feeling' inductive kick and 'seeing' a voltmeter report average voltage across a rectifier trains our thought processes toward same ones Mother Nature uses.
In my high school electronics class every Friday was "Project Day" . All students were required to build something of their choosing . Teacher had access to lots of surplus from Cape Canaveral so there was no financial burden on our parents. .
That experience influenced a lot of us boys' lives for the better.

To this day i maintain we learn faster and deeper by doing than by reading about doing.

old jim