Amusing problem about DC polarity

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 pyrotechnics!

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

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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.

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 negative. 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.

Jason.

I'll ask the cat

martin

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

Or

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

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

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!

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.

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

"Dr. Schroedinger! What's wrong with your cat? It looks half dead!"

The cat must be dead by now. Nobody's fed or watered the thing for over

70 years! ;-)

Cheers! Rich

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

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.

silic> Imagine a black plastic box that takes in DC power but has no polarity

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

I wonder why this makes me think of: A: Pandora and her ever-popular Box B: Maxwell's Demon (Daemon?) ;-)

Thanks! Rich

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

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

Mark

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.

DNA

Obviously we went back and dragged him out.

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 it.. 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 symmetrical. 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......