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Amusing problem about DC polarity

Discussion in 'Electronic Design' started by siliconmike, Jun 9, 2006.

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  1. siliconmike

    siliconmike Guest

    Imagine a black plastic box that takes in DC power but has no polarity
    markings on its power socket.

    The problem is to experimentally determine the correct DC polarity
    without opening the box and without letting the box die.

    How close can we come?
  2. Tim Williams

    Tim Williams Guest

    So what is it, a bare resistor with a sense circuit across it that ignites a
    charge when you apply a certain negative voltage to the terminals? Or does
    it have a specific transfer curve, such as an exponential or negative
    resistance or unidirectional behavior at any point before it explodes? Not
    only the electrical characteristics, but how am I to know if I have hooked
    it up right? If it explodes in one direction and does nothing in the other,
    I personally would be more inclined to plug it into the wall and watch the

    Sorry...It could be an interesting question...but it is, at best, poorly
    worded when talking to an engineer. ;-)

  3. Roger

    Roger Guest

    The diode tester feature on a DMM uses low current and probably will
    not cause damage.

    If you get 1.2V either polarity it probably has a bridge on the input,
    and does not care.

    If you get a low voltage in one direction and offscale in the other
    (perhaps after a few seconds) then the correct polarity is probably the
    high voltage one.

    More than that is difficult to say without having some idea of what is
    in the black box.
  4. Jason S

    Jason S Guest

    Based on experience - assuming it's a standard chassis DC Socket, *usually*
    (like 90% of the time) the inner/centre is positive, and the outer is
    No guarantees of course. Bit silly of them not to mark the casing!
    There may be a way of testing it properly somehow without opening it (or
    breaking it), but sorry, I can't help with that one.


  5. I'll ask the cat

  6. mc

    mc Guest

    (1) Puncture the wires going in and use a voltmeter. That's why Fluke probe
    tips are so sharp.


    (2) Separate the 2 wires going in and check the magnetic field.

    Did you envision a situation in which neither of these was possible?
  7. mc

    mc Guest

    Based on experience - assuming it's a standard chassis DC Socket,
    For 12 volts. Lower voltages are very commonly the other way around.

    Meade telescopes and SBIG astronomical cameras (commonly used together) use
    the same power supply connector, both 12 volts at about 1 amp, with opposite
    polarities. I *hate* these infinitely variable DC coaxial connectors!
  8. Fred Bloggs

    Fred Bloggs Guest

    If your only criteria for operation is that "it takes in DC power" then
    any connection that delivers DC power is the right one. This includes
    anything other than a dead short or open circuit, since the ideal short
    or open circuits consume no power, in which case a simple current
    limited supply for test is adequate.
  9. James Beck

    James Beck Guest

    What if it's dead?
    Who are you gonna' ask then?
  10. mc

    mc Guest

    I'll ask the cat
    "Dr. Schroedinger! What's wrong with your cat? It looks half dead!"
  11. Rich Grise

    Rich Grise Guest

    The cat must be dead by now. Nobody's fed or watered the thing for over
    70 years! ;-)

  12. I used to work in a lab that made portable x-ray machines.
  13. almo

    almo Guest

    I do that all the time. My eyes are not what they were, and if I pick
    up something with two pins, it might be a black box, or a little black
    diode, or multicolored. If it's multicolored, like stripes, I'm
    thinking resistor. Or, one stripe, a diode. 3 pins, transistor.
    Although, a bipolar transistor is just 2 diodes. And a phototransistor
    only has two pins (sometimes.) Bottom line, stick it on an ohm meter.
    First one way, then the other way. If it's DC, you will get 1) a very
    high resistance one way, and a not so high resistance the other way, 2)
    the same resistance both ways, could be zero, infinity, or other 3) you
    get a fairly high resistance both ways, but the harder you look at it,
    you can't quite make up your mind which way has the higher resistance
    4) your analog ohm meter acts a little funny, but your digital meter
    makes no sense at all

    1) when you read the lower resistance, look at the red probe. That's
    ground. If that answer is wrong, then I'll correct myself right now
    and say the red probe is positive.
    2) it's a resistor, or open circuit, or closed circuit.
    3) you've got your sweaty fingers squeezing the probe leads too hard
    and shunting your body resistance across the ohm meter. Don't do that.
    4) probably a capacitor
    5) your ohm meter doesn't work now. You should have checked it it
    with a volt meter first because it might be a battery. And if the box
    is black, about the size of a bread box, and weighs about 20 pounds,
    it's a 12v car battery, and you shouldn't have had to test it in the
    first place.
  14. James Beck

    James Beck Guest

    But you still can't be sure until you open the box......
  15. almo

    almo Guest

    then again, if it looks like a duck...and it quacks like a duck...
  16. I wonder why this makes me think of:
    A: Pandora and her ever-popular Box
    B: Maxwell's Demon (Daemon?) ;-)

  17. James Beck

    James Beck Guest

    I was thinking about a mummified cat. Hmmmmmm
    After 70 years it SHOULD be pretty dried out, even taking all 9 lives
    into account.
  18. redbelly

    redbelly Guest

    So, we don't even know what the voltage should be?

  19. Genome

    Genome Guest

    I think it was amusing when we came out of the Fat Cat after a fair few
    pints of Old Roger and Gary fell into the, more than deep, hole the road
    diggers had probably dug in the wrong place and we left him there.


    Obviously we went back and dragged him out.
  20. Robert Baer

    Robert Baer Guest

    Hmm...the "black boxes" we make are open in both directions and break
    down ("zener") at 1250V one way and about 8 times that the other way.
    If you did not know that, you would be severely challenged in testing
    I can think of the device these units replaced (a few Corotron(TM)
    still exist) and they also are open until break down - but probably are
    And i know of similar devices that take thousands of volts to break
    down that are definitely symmetrical (neon sign tubes).
    So the generic "device in black box" could be a tin whisker, some
    currently common electronic part, a neon bulb, neon sigh tube, cold
    cathode x-ray tube, a PMT with divider, etc etc etc......
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