difference between anode and cathode?

Hi - I thing the single thing that causes me the most problems with electronic devices is polarities. Right now the thing that is really bothering me is the difference between an anode and a cathode. According to dictionary.com, an anode is:

1. A positively charged electrode, as of an electrolytic cell, storage battery, or electron tube. 2. The negatively charged terminal of a primary cell or of a storage battery that is supplying current.

What got me confused is that I had always thought cathodes were positive - but then I was reading that the end pointed to by the triangle in a diode is the cathode, and since the triangle denotes the current flow - that means current flows from the anode to the cathode in a diode. So - can somebody please help me out a bit here?

On a related note - is there any standard in which lead in a polarized component is positive and which is negative? (ie electrolytics, tantalums, leds, etc.)

Thanks, and sorry for such a simple question.

-Michael J. Noone

John Popelish wrote in news: snipped-for-privacy@adelphia.com:

I am though - current flows from + to -, so if the triangle is pointing to the cathode, that must mean it is the negative terminal of the diode, as current is flowing to it, not from it. But I could have sworn that anodes were negative, not cathodes - thus my confusion.

Oops I forgot to mention I meant lead length. With electrolytics it's normally easy to tell - the vertical stripe denotes the negative (anode?) end. With tantalums, I'm always confused. I've seen some that have little plusses on them, but besides that I'm normally really confused. So you say the short lead on an LED denotes the cathode normally - is the cathode the negative terminal? So - what I mean is does a shorter lead always denote the negative end?

It's simply amazing to me that I still haven't grasped this concept... I guess it's probabaly due to all of my classes (I'm a third year EE at UIUC) being all theory based with very minimal labs.

-Michael

Think conventional current (the flow of positive charge) not the movement of electrons.

There are several. ;-) Some are marked with a "+" and some are marked with a "-". Some film capacitors have a strip at one end, but this indicates the lead connected to the outer wrap of foil that acts as a shield for the rest of the capacitor. I think most polarized capacitors for through hole mounting also have a short lead, usually indicating the more negative terminal. LEDs usually have a small flat spot on a ring around the case and a short lead that indicates cathode, but I just found an exception to that. If in doubt, check the data sheet.

Cathode comes from the greek kathodos, meaning 'down path'. Anode is also from greek, meaning 'up path'. The cathode is generally the place where current leaves the device. The anode is the point where current enters the device.

Batteries, of course, have current leaving the device at the cathode as well, and entering at the anode. However, it's job is to boost the voltage in this direction, contrary to the effect on current of a resistance. Thus, in this case, V(cathode) > V(anode).

Also, this usage has been subverted by devices such as the zener diode, which is 'reverse biased' in normal operation. Thus, for these devices, current enters the cathode, and exits the anode.

Electrolytics usually have a stripe painted on the side, with a bunch of minus signs. Tantalums mostly have a little + on the positive lead. LEDs usually have a longer anode lead, and the cathode lead is often marked by a small flat spot on the epoxy case.

Anode always have been positive side and cathode negative side. They have been used in reverse bias in some circuit such as across coil of relay (to allow residual current to drain across diode rather than through the relay driving circuit) and in cheap voltage regulator circuit (zener diode is almost always used in reverse bias)

Hi I always think of a CRT - cathode ray tube - where negative charged = electrons are leaving the cathode and heading towards the positively = charged display screen.

Also working with transistors - I find it easier to imagine the flow of = electrons and not the so called flow of 'holes' - these are only = apparent because of the ripple effect of electrons flowing in the = opposite direction.

Just my 2 cents worth (convert to pounds as I am in the UK) Alan

According to=20

storage =20

positive -=20

diode=20

tantalums,=20

Hi Michael,

With the risk of possibly adding confusion to what others have said;

Anions are negatively charged atoms, radicals or groups of atoms which travel to the anode or positive pole during electrolysis.

Cations have a positive charge and travel to the cathode. Regards, Tom

Want to get even more confused? With a diode bridge, the positive output comes from the cathode (negative) end of the diodes. Think like this: for current to flow through a diode, it must travel in the direction of the arrow (talking about conventional positive current here). Like water, current flows "downhill" in a diode -- from the positive (anode) to the negative (cathode). So it comes out the cathode, making that the positive output of the bridge.

That's not confusing. You have two terminals marked AC and a single

• or - output. A full wave bridge has four terminals, so you have both

• and - outputs that connect directly to the filter capacitor, + to + and - to -. The electrons are pulled from the + line, leaving it with a positive charge and are sent to the - to give it a negative charge. Forget the water analogy. They will trip you up because they are only vaguely similar.

Skyworks makes some surface-mount schottky diodes with a bar marked on the cathode end, and others with a bar on the anode!

John

A handy way of remembering which are anions and which are cations is ANION = A Negative ION

Hope this helps!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

A number of manufacturers make diodes with 'reverse' style markings (notably IR) where the part number has a 'R' suffix. I have had occasions where these were bought by an unsuspecting (and electronically clueless) buyer [they thought the slight difference in part number was of no consequence) with spectacular results at initial test as this was a reverse input protector (this was before the product had reached it's ICT stage).

Sometimes, the only clue is the part number itself (LEDs in particular are notorious for this). I have a bunch of LEDs from Kingbright where the 'cathode mark' is actually on the anode.

Cheers

PeteS