# How to tell DC polarity



## Ironton (Jan 2, 2008)

Hope this is the correct place to post this question.

I have an accessory that requires DC input. I also have a DC power source. However, is there a cheap and/or easy way to tell which side of the power source is positive? The manufacturer did not mark it for some reason.

Thanks.


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## Treeman (Jan 6, 2008)

You could pick up a cheap muti meter at Raido Shack. You may find many uses for it.


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## Jerry Barnes (Jan 2, 2008)

Get a digital one, lots nicer to use! In the window it will tell you if you have pos or neg. Read the book also!


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## sheepdog (Jan 2, 2008)

I think he knows the polarity of the power source but not the accessory. Many DC devices such as lamps, motors, coils don't care which way you hook them up and are not marked. Having said that, one should not assume if it is not marked, it's ok to hook it up either way. Lots of smoke generated with that assumption. 

What is the accessory? 

Craig


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## krs (Feb 29, 2008)

The way I read it, the OP's question is actually about the power source, not the accessory.

But in either case, an inexpensive digital multimeter is really the way to go. They sell for $10.00 or even less and have many, many uses if you're into anything electrical at all.


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## Bill Swindell (Jan 2, 2008)

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899
Here is an inexpensive meter. They work fine. 


http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899


This next one is for a nicer meeter. I use mone every day.


http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=37772


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## Schlosser (Jan 2, 2008)

If it's the source that needs labeling and if it provides about an amp, any loco, especially LGB, can be used for a polarity checker. In forward motion, the left rail is positive. Newer Bachman locos have that stupid switch to so you can match HO, but I've never run my G gauge locos on HO track, - - yet. Maybe make a model of the Super Train that was on a TV Series away back that ran on double track.


Art


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## Dan Pierce (Jan 2, 2008)

There is a multicolor LED that has 3 colors. One color for positive flow, one for negative flow and a third color for AC. Use a 820 ohm resistor and it will work for 6 to 20 volts with no problem. 

Part of the kiss program.


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## Ironton (Jan 2, 2008)

Thanks guys.

I have meters, use them mostly for checking continuity or resistance. Can't find the book.

The power supply is putting out about 25 volts, so I needed something that would hadle that voltage. Thus a simple LED check was out. Will try the LGB loco test. Yes the accessory is labeled for input of positive and negative so I am worried about blowing it if I hook up things wrong.


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## hcampbell (Jan 2, 2008)

Rich 
It's simple and cheap. Put both leads in a glass of slightly salted water, the one with the bubbles is negative. 
Would have been here sooner but I had to try it as I couldn't remember which was negative. 

Harvey C. 
SA1838


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## Michael Glavin (Jan 2, 2009)

If your volt meter is a DMM or DVM (digital multi meter or digital volt meter) it should have an LCD display, accodingly it should indicate a negative "--" sign if the polarity is reversed. For example select DC voltage, hook up the probes to your power source, if the display indicates -24vDC, then the test point on the red probe is the negative... To validate same reverse the probes, if the "--" is no longer displayed you have the probes attached properly, red +, black --. 

Michael


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