difference between anode and cathode?

Discussion in 'Electronic Basics' started by Michael Noone, May 28, 2005.

1. Michael NooneGuest

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

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

2. Michael NooneGuest

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

3. John PopelishGuest

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.

4. Randy DayGuest

When the cathode is negative, current flows.
When the anode is negative, current stops.
Not really. Diodes (incl. LED's) usually have a band
or other marking on the cathode. With caps, one lead
usually has an indelible marking for pos or neg.

5. ImpmonGuest

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)

6. Bob MonsenGuest

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.

7. Z80Guest

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

8. Tom BiasiGuest

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

9. kellGuest

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.

10. Michael A. TerrellGuest

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.

11. Bob MastaGuest

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

Hope this helps!

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

12. John LarkinGuest

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

John

13. PeteSGuest

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