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Discussion in 'Electronic Basics' started by Roger Dewhurst, Mar 17, 2007.

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  1. I have been given a large number of LEDs soldered into a board. Since the
    pins have been cut t the same length what is the best way of determining
    their polarity when removed from the board?

  2. If these are the cylindrical body with dome end type, they
    probably have a small flat at the base of the cylinder
    (usually indicating cathode (more negative lead). But I
    have seen exceptions. But which ever version they are, they
    should be consistent. You can also look through the epoxy
    and see which lead is the reflector base the die is mounted
    on. That is also usually the cathode. The anode lead
    generally has a wire bond to the top of the die.
  3. Thank you. I needed a magnifying lens to see the flat! Looking through the
    side I see two small triangles with the hypotenuses facing each other. The
    upper one is associated with the flat on the body.

  4. I suggest you pull one LED and find out by test, whether the
    flat is the cathode or the anode.
  5. For decades, I've always had a 9v battery lying around with about a 1K
    resistor soldered to one terminal, and sometimes a piece of wire soldered
    to the other. Since you know the polarity of the battery, when the LED
    lights up when you connect it, you know which terminal of the LED is which.
    If it doesn't light up, you reverse the LED. If it doesn't light up then,
    the LED is a dud (or your battery has finally died.

    This not only gives you polarity when it may not be clear, but if the flat
    of the package isn't visible (like when you've installed it with one of
    those LED holders) or whatever reasons, you can easily check before soldering.
    I got into the habit of checking LEDs this way before soldering because when
    I first started buying them, LEDs were relatively new and it was easy to
    get some that were not particularly bright, or for that matter I think
    some that were cheap because they had the terminals reversed.

  6. Ban

    Ban Guest

    Most LEDs have a max. reverse voltage rating of less than 7V and might be
    damaged or degraded by the full 9V battery, so a 4.7V Zener across the O/P
    should limit the reverse voltage to safe values. But beware of the
    continuous current draw.
  7. 6V screw-terminal carbon battery, then?

    As a side-bar, I've noted this site:

    Which appears to offer a nicely featured tester. I haven't purchased
    it (and I am definitely not affiliated in any way), but I was recently
    considering the idea of buying one.

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