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Polarity of DC input (mains adapter)


 
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Coniferman

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PostPosted: Mon Feb 04, 2008 11:17 pm Reply with quote

I use a universal power adapter for various things - the one where you can set the voltage 1.5, 3, 4.5, 6v etc. and set the polarity. In the past the devices would state the polarity required i.e. centre pin negative or positive. However I have bought a couple of items recently with a dc input which states the required voltage but not the polarity. Is this because there is a standard or is it more likely to be one than the other?
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333rocky333

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PostPosted: Mon Feb 04, 2008 11:39 pm Reply with quote

If you have the adaptor that comes with the unit you buy,test that lead with a multimeter, and that will be the polarity required.

Most things now just have a sort of three quarter circle with a dot in the centre, with + and - next to each.

Usually this is on the charger and also by the small input socket on the unit or on the label,possibly in the tech spec in the manual as well

Positive centre pin and negative outer , seems the logical , but not always the case.
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KingboyD

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PostPosted: Mon Feb 04, 2008 11:56 pm Reply with quote

333rocky333 wrote:
If you have the adaptor that comes with the unit you buy,test that lead with a multimeter, and that will be the polarity required.


Doesnt always work as most modern plug in transformers are load sensing, a multimeter usualy shows 0V on modern PSU`s.
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Softus

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PostPosted: Tue Feb 05, 2008 1:21 am Reply with quote

Are you sure there's no symbol such as this: embossed on the appliance casing?
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Coniferman

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PostPosted: Tue Feb 05, 2008 9:42 am Reply with quote

Softus wrote:
Are you sure there's no symbol such as this: embossed on the appliance casing?


That is what I am looking for and usually find except for a couple of recent purchases. They do not come with a power supply and are supplied with batteries which I am using for now but they do have a dc input with the volatage marking but no polarity symbol.
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NamelessRob

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PostPosted: Tue Feb 05, 2008 11:30 am Reply with quote

Most products should have built in protection against putting the batteries in the wrong way round - in which case you should be able to just try the adapter with each polarity & see which way works.
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tim west

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PostPosted: Tue Feb 05, 2008 11:53 am Reply with quote

Do a continuity test on the battery terminals to see which two end up at the socket it is then obvious which is positive and negative.
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Coniferman

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PostPosted: Tue Feb 05, 2008 1:17 pm Reply with quote

tim west wrote:
Do a continuity test on the battery terminals to see which two end up at the socket it is then obvious which is positive and negative.


That sounds like a plan. Will give it a try.

Thanks for the replies.
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ColJack

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PostPosted: Tue Feb 05, 2008 1:40 pm Reply with quote

some things don't matter what polarity.. they have bridge rectifiers in them that correct the polarity and even allow the use of AC...
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ban-all-sheds

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PostPosted: Tue Feb 05, 2008 5:32 pm Reply with quote

NamelessRob wrote:
Most products should have built in protection against putting the batteries in the wrong way round - in which case you should be able to just try the adapter with each polarity & see which way works.

Sod's Law says that (a) any generic adapter you have will be the wrong polarity, and (b) reverse polarity will destroy what you plug it into...


Last edited by ban-all-sheds on Tue Feb 05, 2008 6:19 pm, edited 1 time in total
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tim west

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PostPosted: Tue Feb 05, 2008 6:16 pm Reply with quote

Quote:
some things don't matter what polarity.. they have bridge rectifiers in them that correct the polarity and even allow the use of AC...


Be careful with trying an AC input as there is an assumption here that the equipment has a built in rectification circuit which it may not have and the equipment could be destroyed.

Also with the assumption that there is no smoothing after the bridge rectifier then the resultant rippled DC may also cause problem especially with PIC chips and brownout.
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333rocky333

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PostPosted: Tue Feb 05, 2008 9:08 pm Reply with quote

tim west wrote:
Do a continuity test on the battery terminals to see which two end up at the socket it is then obvious which is positive and negative.


If there was a connection would you not then get the same battery voltage back to the socket, with the plug out
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Coniferman

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PostPosted: Wed Feb 06, 2008 3:25 pm Reply with quote

I did a continuity test between the battery terminals and the socket and the centre pin is positive.

Problem solved icon_biggrin.gif
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