How To Tell Polarity

[Boxer]

I mixed up the polarity on my little XM radio power supply a few weeks ago and apparently this is what fried it. If there are no markings on the wires, and both wires are silver colored, how does one tell which is positive and which is negative?

I have a new one now and the wires are properly marked, so I'm just asking this out of curiosity in case I screw up again sometime and lose the markings and have no way to know which wire to hook up to the power and which to hook up to ground.

See this photo here for what I'm referring to.

[mad1150]

One possibility...

Since it's a DC to DC converter the input and output Negatives "may" be connected together internally.

If you have a good Ohm meter, you can measure from the known negative on the output side to each wire on the input side. The one with the lowest resistance is the Negative input lead.

My gut is also telling me there may not me much difference between the two, so it may not work, but' it's worth a shot!

[R4R&R]

If there are no markings on the wires, and both wires are silver colored, how does one tell which is positive and which is negative?

I think you figured out already - you hook it up and if it works, then you got it right!

Seriously, the only real way to tell is to get one of those inexpensive voltmeters from Radio Shack and test the leads out. The voltmeter has lots of other good uses, so to me, it's money well spent.

[Boxer]

John-
Back in the good ole days when I was working in a vault in intel up in North Carolina, we received a new shop supervisor who was a Lt. Col. and had just come to us from some other office. I think he was a flight officer from another base. He was a really nice guy but he didn't know squat about our particular job and asked me to give him a couple of lessons on what I do. Well, I began by showing him some of the most basic stuff, assuming he wanted to start there. The basic stuff included reading the scale on the charts, measuring with a protractor type tool, finding coordinates etc. After about 30 seconds into his lesson, he stopped me abruptly and said, "I may not look so smart but I'm ahead of YOU. Could we move on to the stuff I don't already know."
Everyone in the shop got a real laugh at my expense.

The reason I relate this little story is your post reminded me of it. I already have a voltmeter and know how to use it for measuring voltage etc. I just don't know what to do with it with a device that has no power coming out on its on, that I know of. The two naked wires on the Power Supply hook to my battery (or in my case to the Centech fuseblock) and the other end plugs into the Radio. I should have been more specific in my description of what I don't know. How can I use the voltmeter to check the two wires that have no juice in them, other than the first method you outlined.

The voltmeter I have has several scales on it and you have to set the switch to a particular volt range and AC or DC or Ohms. I assume this is standard, since this is the first one I've ever owned. What would I set it on and what would I be measuring?

[mad1150]

Phil,

Try this:

1. Power supply not hooked up (either end)
2. Set the Volt Meter on Ohms
3. Connect one lead of the Ohm meter to the negative lead of the output that goes to the Radio
4. Connect the other lead to the two input power leads one at a time.
5. The one with the least resistance (in ohms) is the negative of the input leads.

If the meter is not auto-ranging then you way have to select ranges until you find one that shows a difference.

(PM'd you too)

[mcooperstein]

To elaborate a bit on mad1150's post... if you can crack the cover on the box, you should be able to visually tell which wire going to the pc board is negative. Well, I suppose I could tell by inspection, maybe not everybody. Be that as it may, once you've cracked the case, look for a ground point. Put one probe lead on it, then with your meter set to OHMS (lowest setting if it's not autoranging), put the other lead on one wire. It will deflect to Zero when you're on the ground lead, and either high resistance or infinite resistance (depending on the circuit) when your on the hot lead.

Mark

[MattPie]

I mixed up the polarity on my little XM radio power supply a few weeks ago and apparently this is what fried it. If there are no markings on the wires, and both wires are silver colored, how does one tell which is positive and which is negative?

I have a new one now and the wires are properly marked, so I'm just asking this out of curiosity in case I screw up again sometime and lose the markings and have no way to know which wire to hook up to the power and which to hook up to ground.

See this photo here for what I'm referring to.

I'm surprised the PS doesn't have a diode on the input so you can't fry it. I guess that would cost an extra $0.001.

If you don't have a multimeter (although if you're doing this kind of stuff, you probably should), I guess you could use a flashlight bulb and a AA battery. The little graphic with 3 circles on in the picture (lower right corner) looks like says that positive is the center (tip) of the radio-side connector, the outside would be negative. Make a connection from the outside (barrel) portion of the radio-side connector to the flat side of the battery, hold the center post of the bulb against the post (+) on the AA, and tap one of the leads against the outside part of the light bulb. The lead that lights the battery is directly connected the negative side on the output, so it's also almost surely negative.

This assumes a lot, like that a AA and bulb won't blow what's ever in the power supply. It shouldn't be able to, but no guarantees.

[Doug]

I just re-read your second post more carefully, so the info below is not really appropriate for detecting polarity w/out power. Why dont you just hook it to a battery on the input side to check?
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Boxer, there is a really easy way. Hook your XM adaper's wires to the black and red terminals on your volt meter. Set the range for 0-30V or something similar. Actually, it doesnt really matter in this case, just make sure its on volts. Attach the adapter to battery. Now, if you see the volt reading on the meter go to a positive # (i.e. 5.5V), then you've hooked it up right (the red wire is positive). If you see the value go negative (i.e. -5.5V), then you need to swap the wires. This should also work with a needle-based meter, it will just throw the needle to the left if its negative (or wont move if there is a stop), and swing right if its hooked up correctly. Hope this helps!