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Need help with Tantalum polarity

Discussion in 'Electronic Basics' started by royalmp2001, Feb 21, 2005.

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  1. royalmp2001

    royalmp2001 Guest

    This is my first project using tantulum capacitors.
    I cannot tell from the markings what the polarity is.

    I see a "+" that is halfway between the leads (maybe slightly nearer
    the left lead) and there is a bold upside down "L" to the right of that
    which is still between the leads but now slightly nearer the right
    lead.

    Which is lead is which please?
     

  2. I cannot tell from this distance. The devices must
    be misprinted if the polarity indicator is ambiguous.
    I would look at the collection you have, (of the same
    make and type) and see whether some of them have
    a less ambiguous indication. Then use that polarity
    for all of the similar set.

    Be sure to stand back or wear goggles when you
    first apply power. Backwards tantalum caps can
    spew molten blobs of tantalum in a random direction.
    You don't want to rely on the blink reflex to limit the
    damage to merely superficial scars.

    You could measure leakage with a cap in series with
    a large resistor, biasing the string at the rated voltage.
    If the cap is backwards, it will probably leak a lot
    more than when properly biased. This can be done
    without the danger of red hot streamers mentioned
    above, if the resistor is at least a few K Ohms.
     
  3. royalmp2001

    royalmp2001 Guest

    I have 19 of these tantalums and some are the same and the rest have
    the "+" smack bang in the middle and the inverted "L" above the right
    leg.
    What does the inverted L mean? Is it "-ve" or is it meant to point to
    where the "+" lead is?

    I cannot test these at the rated voltage as I do not have a 35V supply.
    Any ideas anyone?
     
  4. Sad so say that sometimes tantalum capacitors get printed with the text
    off-center, and therefore you get this ambiguous marking occasionally.
    Tantalums do NOT like being connected backward, so I recommend trying to
    attach it to a DC power supply at its rated voltage. Mark which way you've
    connected it; and if it blows up, then you know that's the wrong way to do
    it. Yes, one must be sacrificed for the good of the rest.
     
  5. Bob

    Bob Guest

    Try this (at your own risk):

    Connect two to a power supply. Connect them in parallel, but one must be
    reversed from the other). Note their hookup (i.e., their polarity with
    respect to the polarity of the applied voltage). Anything up to the rated
    voltage is okay.

    Put the two caps in a protective enclosure (like a strong box). Wear eye and
    ear protection.

    Turn on the supply. The one that either blows up, or gets hot (check after
    you've turned off the power) is the one that was hooked up incorrectly.

    Throw out the one that got hot (or blew up).

    Bob
     
  6. Rich Grise

    Rich Grise Guest

    A reasonable guess would be that, if they're all from the same batch,
    they're probably the same, polarity-wise. Look at one that is clearly
    marked, and then look at the rest with the same orientation - if the
    marking is merely displaced, that won't change the polarity of the part.

    Good Luck!
    Rich
     
  7. royalmp2001

    royalmp2001 Guest

    Thank you everyone for your input. I went with gut instinct and
    connected one of these critters to 20VDC and stood far back. I had the
    positive going to the right leg under the inverted "L" or bar sign, and
    it didn't explode or get hot.
     
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