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difference between anode and cathode?

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

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  1. 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
     
  2. 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. 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 Day

    Randy Day Guest

    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. Impmon

    Impmon Guest

    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 Monsen

    Bob Monsen Guest

    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. Z80

    Z80 Guest

    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 Biasi

    Tom Biasi Guest

    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. kell

    kell Guest

    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. 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 Masta

    Bob Masta Guest

    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
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  12. John Larkin

    John Larkin Guest

    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. PeteS

    PeteS Guest

    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
     
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