12 volt dc plug polarity

[Ivan Vegvary]

Need to make a power supply for my CPAP breathing machine. ( CPAP = Continuous 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.
BTW, I would simply buy the adapter but am going on a trip and can't wait for the mailman.

Thanks for any and all comments/advice.

Ivan Vegvary

 

[Tim Wescott]

On Thu, 15 Sep 2011 22:44:37 -0700, Ivan Vegvary wrote:

Need to make a power supply for my CPAP breathing machine. ( CPAP =
Continuous 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. BTW, I
would simply buy the adapter but am going on a trip and can't wait for
the mailman.

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

 

[Jasen Betts]

On 2011-09-16, Ivan Vegvary <ivanvegvary@gmail.com> wrote:

Need to make a power supply for my CPAP breathing machine. ( CPAP =
Continuous 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?

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

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.

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.

BTW, I would simply buy the adapter but am going on a trip and can't
wait for the mailman.

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


--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

 

On 16 Sep 2011 11:17:35 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

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.

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

 

[John Larkin]

On Thu, 15 Sep 2011 22:44:37 -0700 (PDT), Ivan Vegvary
<ivanvegvary@gmail.com> wrote:

Need to make a power supply for my CPAP breathing machine. ( CPAP = Continuous 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.
BTW, I would simply buy the adapter but am going on a trip and can't wait for the mailman.

Thanks for any and all comments/advice.

Ivan Vegvary

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

 

[John Fields]

On Fri, 16 Sep 2011 15:18:00 +0000 (UTC), Kaz Kylheku
<kaz@kylheku.com> wrote:

On 2011-09-16, Ivan Vegvary <ivanvegvary@gmail.com> wrote:
not sure of the polarity [ ... ] Is there any way to check?

You're messing around with electricity without a multimeter?

---
From the OP: "I do have a DMM."

--
JF

 

[Kaz Kylheku]

On 2011-09-16, Ivan Vegvary <ivanvegvary@gmail.com> wrote:

Is there any way to check?

You're messing around with electricity without a multimeter?

 

[Peter Bennett]

On Thu, 15 Sep 2011 22:44:37 -0700 (PDT), Ivan Vegvary
<ivanvegvary@gmail.com> wrote:

Need to make a power supply for my CPAP breathing machine. ( CPAP = Continuous
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.

BTW, I would simply buy the adapter but am going on a trip and can't wait for the mailman.

Thanks for any and all comments/advice.

Ivan Vegvary

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

 

[Kaz Kylheku]

On 2011-09-16, Ivan Vegvary <ivanvegvary@gmail.com> wrote:

have the device open and am looking at the electronics? I do have a
DMM.

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.