12V receptacle polarity

[Janss]

Just bought a 12v cord for CPAP machine use in RV. Instructions state "The Shielded DC Cord is intended for use on 12 volt positive hot electrical systems. These systems might also be referred to as negative ground."
Also:
"The connector end of the DC cord is constructed so that the outside of the barrel is ground and the inside of the barrel is positive."

So I used a multimeter to test the 12v outlet in my bedroom. (I know nothing about electrical stuff.) When I touched the red probe to the center plate at back of outlet and the black probe to the barrel wall of the outlet, the multimeter showed 13.71. Then I touched the black probe to center plate and red to barrel wall and multimeter showed -13.71.

So does this mean the outlet is "positive hot", as needed for this cord to work properly?

[red31]

keep the pontificating coming!

[Sam_Spade]

cavie wrote:

How did this become a thread on how to use a multi-meter. The OP used his meter correctly. Period. I'm out of this thread.

No need to get snippy.

UNLESS the test leads are PERMANENTLY attached to his meter, which is highly UNlikely, then you don't KNOW if he was doing it right or not.

The devil is in the details.

[MEXICOWANDERER]

This reminds me of a movie I saw a decade or so ago. A rerun.

[red31]

Janss wrote:
Ha ha ha! After the first and second response, I was outta this thread too! Thank goodness Dave5143 set me straight. I don't know what the heck the rest of you are chatting about, but sounds like you're having fun! I know this happens on alot of threads. One comment leads to another...and another...and pretty soon the respondents are talking amongst themselves.
Anyway, the leads on my multimeter are built in. I cannot change them.

thanks for sharing

[wa8yxm]

I have to agree with Mex on this.. I always check to insure leads properly inserted before doing anything... I have one melted meter where something went wrong.. Problem si.. It was turned OFF stored in its carry case when it melted.. So I have no clue as to what went wrong. it's a 200 dollar_+ meter I got on sale.. Oh well. Still works.

[MEXICOWANDERER]

it must have been a miracle then how the jack polarity got reversed. Sheesh. When testing assume nothing --- ever. Two seconds of verification can eliminate a costly error. This is basic troubleshooting 101. I can stow my crystal ball now.

[Janss]

Ha ha ha! After the first and second response, I was outta this thread too! Thank goodness Dave5143 set me straight. I don't know what the heck the rest of you are chatting about, but sounds like you're having fun! I know this happens on alot of threads. One comment leads to another...and another...and pretty soon the respondents are talking amongst themselves.
Anyway, the leads on my multimeter are built in. I cannot change them.

[cavie]

MEXICOWANDERER wrote:
I know nothing about electrical stuff

You are asssssssssssuming an awful lot after reading (those words) written by by the OP.

The hundred or so .375 ampere blown glass fuses and near equal number of fried black circuit boards I have encountered on 60+ years tells me people forget return test probes and dial settings far too often.

Cheers

How did this become a thread on how to use a multi-meter. The OP used his meter correctly. Period. I'm out of this thread.

[MEXICOWANDERER]

I know nothing about electrical stuff

You are asssssssssssuming an awful lot after reading (those words) written by by the OP.

The hundred or so .375 ampere blown glass fuses and near equal number of fried black circuit boards I have encountered on 60+ years tells me people forget return test probes and dial settings far too often.

Cheers

[cavie]

MEXICOWANDERER wrote:
SLOW DOWN

make sure the meter test leads are not reversed where they plug into the meter holes.

Then raise the hood and verify across the marked -- and + polarity battery polarity markings.

If everything checks OK but polarity us still reversed at the socket reverse the wires at the other end of the socket.

The op does not have a problem. He was asking if he diagnosed it correctly. He did. His problem was worrying about neg ground which I haven't heard for over 50 years.

[MEXICOWANDERER]

SLOW DOWN

make sure the meter test leads are not reversed where they plug into the meter holes.

Then raise the hood and verify across the marked -- and + polarity battery polarity markings.

If everything checks OK but polarity us still reversed at the socket reverse the wires at the other end of the socket.

[Dave5143]

Janss wrote:
Just bought a 12v cord for CPAP machine use in RV. Instructions state "The Shielded DC Cord is intended for use on 12 volt positive hot electrical systems. These systems might also be referred to as negative ground."
Also:
"The connector end of the DC cord is constructed so that the outside of the barrel is ground and the inside of the barrel is positive."

So I used a multimeter to test the 12v outlet in my bedroom. (I know nothing about electrical stuff.) When I touched the red probe to the center plate at back of outlet and the black probe to the barrel wall of the outlet, the multimeter showed 13.71. Then I touched the black probe to center plate and red to barrel wall and multimeter showed -13.71.

So does this mean the outlet is "positive hot", as needed for this cord to work properly?

Yes, you did exactly the right thing by using a multimeter to verify the polarity. Don't let the discussions about negative vs positive grounds confuse you.

[wa8yxm]

I will say again. I think it was in the 70s they started making the dash out of plastic. and often the wires on the lighter socket got "Swapped" So I built a tester... Today they usually do it right and due to the polifiration of 12 volt accessories.. I'd have to think a auto maker who wired one bass ackwards would soon be paying the lawyers big money.

[YC_1]

There were many positive ground trucks going into the early 70's.. It was very important to check them before installing two way radios. There were radios that had an internal plug to change them from negative to positive ground.

As mentioned, it is good to remember that the black wire of the RV is often the HOT wire and in todays units that is positive. Just don't mistake the black wire 12 volts for 110 volts AC.