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12 volt dc plug polarity

Need to make a power supply for my CPAP breathing machine. ( CPAP =3D Conti= nuous Positive Airway Pressure). It runs on both 120v AC and 12v DC. It h= as a 12 volt socket and I picked up an appropriate connector (plug) at Radi= o Shack. Need to wire the plug to a cigarette lighter socket plug and not = sure of the polarity. Can I safely assume that the outside (not the centra= l pin) of the connector is the negative pole? Is there any way to check? = Are there any visual clues when I have the device open and am looking at th= e electronics? I do have a DMM. BTW, I would simply buy the adapter but am going on a trip and can't wait f= or the mailman.

Thanks for any and all comments/advice.

Ivan Vegvary

Reply to
Ivan Vegvary
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You can't count on it being one way or the other.

Sometimes the polarity is marked: there'll be a little picture like a dot surrounded by a 'C', with '+' and '-' signs showing which polarity belongs to the middle (the dot) and the outside (the 'C').

Or go back to Radio Shack and get a 12 to 120V inverter sufficient to run the machine.

I hope that you just need the thing to be comfortable, not to stay alive.

--
www.wescottdesign.com
Reply to
Tim Wescott

usually it is. (but I know of several exceptions)

some devices will present voltage on the DC connector when powered from the mains, especially when there is no plug in the DC socket. you could try measuing it.

It's your medical device you're taking risks with.

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Reply to
Jasen Betts

And you could trace the PCB traces and look at capacitor polarity markings.

Reply to
default

Positive Airway Pressure). It runs on both 120v AC and 12v DC. It has a 12 volt socket and I picked up an appropriate connector (plug) at Radio Shack. Need to wire the plug to a cigarette lighter socket plug and not sure of the polarity. Can I safely assume that the outside (not the central pin) of the connector is the negative pole? Is there any way to check? Are there any visual clues when I have the device open and am looking at the electronics? I do have a DMM.

the mailman.

Can you power the CRAP from AC and measure the center pin of its 12 volt connector with the DVM? It may have a reverse protection diode, but that may leak enough for you to at least see the polarity.

Or, if it does seem to have a diode, kill power and use the ohm-meter range of the DVM to figure out the diode polarity.

Or, power it from AC and then apply, say, 24 volts to the DC connector through a big resistor, 100K maybe, and measure stuff.

Stuff like that.

John

Reply to
John Larkin

You're messing around with electricity without a multimeter?

Reply to
Kaz Kylheku 

--
From the OP: "I do have a DMM."
Reply to
John Fields

Positive Airway Pressure). It runs on both 120v AC and 12v DC. It has a 12 volt socket and I picked up an appropriate connector (plug) at Radio Shack. Need to wire the plug to a cigarette lighter socket plug and not sure of the polarity. Can I safely assume that the outside (not the central pin) of the connector is the negative pole? Is there any way to check? Are there any visual clues when I have the device open and am looking at the electronics? I do have a DMM.

the mailman.

When it is running from 120 VAC, does it use a "wall wart" power supply that connects to the DC input connector? If so, just measure the output of the wall wart connector with your DVM, and make your DC cable the same polarity. (and check the polarity of the lighter socket in your car!)

--
Peter Bennett, VE7CEI  
peterbb (at) telus.net
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
Reply to
Peter Bennett

Ah, I missed the comment that you do have a DMM.

In that case, you can just measure the voltage, thereby obtaining the polarity.

Make sure your black DMM probe is connected to the jack labeled with the ground symbol, or "COM" or some such marking. Red goes in the other one.

If you put the red probe on terminal X and the black one on terminal Y, and get a positive reading, then X is positive with respect to Y.

If you get a negative reading, then X is negative with respect to Y.

Make sure your meter is configured for DC voltage measurement, and that the range is adequate. E.g. a range of +/- 20V would be adequate if you're expecting a voltage of 12V.

Try it on a battery first, both ways.

Reply to
Kaz Kylheku

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