2.5mm DC socket

[hicks]

I have a baby monitor which is powered by a 9V mains transformer. The cable has recently become unreliable, as I have to twist it at a particular angle in order for the power to stay on. I decided to replace the 2.5 mm plug on the end. I understand that the outer sleeve of the plug is earth, and the inner part is for live. Problem is, I don't know which is which for the two wires in the cable.

I think what I have to do is connect the probes of my multimeter to the wires. If I get a reading of +9V then the wire connected to the red probe is live? Can someone confirm this?

Thanks.

[Grunff]

Yes, this is correct. Make sure the multimeter is set to Volts, and to a range 20V +. It will likely read higher than 9V (if it is unregulated). This is perfectly ok.

[Paul-S8]

Some devices have plugs wired the opposite way so that the outer sleeve is positive.

If you have not already cut it off try your multimeter on that, positive probe in the middle and ground to the outside. if you get +9v or above you are correct.

You are correct with measuring the polarity of the wires.

Paul

[hicks]

Unfortunately I already cut the plug off. If I do get the polarity wrong, is this likely to break the device?

[The3rd Earl Of Derby]

Look at the device where the plug fits into it, usually there is an indication as to where the polarity is. ie center + outer - or reverse of this?

-- Sir Benjamin Middlethwaite

[Rumble]

Maybe, depending whether the baby monitor is reverse polarity protected.

If you still have the old plug (and you have left a bit of wire on it), if you look closely, you may find that one of the wires has a small ridge running down the edge or that there is printing on one wire only. This will allow you to identify how the plug was connected using the continuity test function of you multimeter.

HTH

[Dave Plowman (News)]

Unfortunately it could be either positive or negative tip on the plug - although positive is more common.

And wrong connection could wreck the device. If you can get inside it easily it should be possible to determine which it is - look for a large capacitor which should give the ground polarity.

[hicks]

Thanks, but there is simply no way I will be allowed to take this thing apart :) The other option I considered was to purchase one of these off-the-shelf multi-voltage power transformers. But I'm even more uncertain if this will work, or even damage the monitor, as they appear to be 100 mA or above. Whereas the monitor's transformer is 10mA. I'm guessing this is significant, but I could be wrong?

[Mike Harrison]

Yes - leads like these almost always have some sort of polarity marking on the cable - either a stripe or ridge is common, but I have seen cable where one conductor was tinned-copper, the other bare copper.

[The3rd Earl Of Derby]

Thats fine, the supply will only draw the current it needs.

-- Sir Benjamin Middlethwaite

[Kate]

1] Look at the power supply unit. It will tell you the voltage and current output and as a bonus it will tell you how the plug is polarised, by means of a little diagram. 2] Write this down on a piece of paper. 3] Go to a high-street electrical retailer and buy a replacement power supply unit with the same information on it. Less than £10 should do it. Argos also sell them. 4] Plug it in to your monitor, plug it in to the mains, and that's it. 5] This is a baby monitor, you need to be assured it is working and reliable. If you don't have the expertise to know what to do, don't do it for this critical application. 6] HTH

[hicks]

Thanks, but there's no need for the patronising tone. I'm not a total retard.

[Dave]

Every device I've seen has a symbol near the input socket to indicate the polarity; go to Google, click on Images and do a search for 'power symbol centre positive'. This gives two pictures; one for centre positive and one centre negative. Have a close look at your device and see if you can locate the symbol. Connect the new plug appropriately to the wires as identified by the procedure you described earlier.

Dave

[Sparks]

The fact you use the words "Live and Earth" not Positive and Negative The fact you cut off the plug before testing The fact you didn't tell us the make model (Someone may have the same unit, or may have a manual for it, The fact you have to ask how you use your meter makes me think you should retract that comment.

Kate was just trying to help here as far as I can see, especially with point

5 here (as you don't appear to be very competent in this field)

Ask for help, but please don't flame people if they answer the question, even if you feel they have over explained or simplified something.

Shall we start again here?

Make: Model: Voltage on adaptor: 9v Current on adaptor: 10mA AC or DC on the output side (It will be AC on the input of course) (AC will have a symbol like this ~ (That's a wavy line if it does not display correctly on here) DC will have a solid line with a broken line under it

If it an AC output, the polarity will not matter

Normally, you will see a symbol like this

either the adaptor or the device - do you have one?

Sparks...

[meow2222]

Whether you get a new plug on yours or a new wall wart makes no difference from the point of view of getting the polarity correct. Same problem either way.

Where it does make a difference is reliability if and only if you cant solder. If you can, fine, but if you cant then I'd go with Kate and get one ready made. Unsoldered dc pwoer plugs are just not reliable.

Either way you need to know the polarity for the unit, plugging it in and hoping is 40% likely to kill it. There is almost always a little diagram either on the wallwart or by the socket on the appliance, usually in raised plastic with no ink, so you have to hold it to the light to see it. If for any reason theres no marking, open it and see which wire goes to the + and - on the big capacitor. Opening it is far more likely to result in a working unit than guessing.

Fitting a new plug means there no voltage selector and polarity selector switches to get moved to the wrong place, with unit either not working or frying... so maybe diy is safer after all.

NT

[Dave Plowman (News)]

Not in my experience; the wall wart is more likely to be marked - but not always.

[Owain]

It's significant, but not from your point of view.

However purchasing another power supply does not solve your polarity problem, as most of those are switchable between +ve centre and -ve centre.

Owain

[mike ring]

[e-mail] wrote in news: [e-mail]:

I think it is most unlikely you will break the device by connecting it the wrong way round, with something not hugely greater than the marked voltage

- in any case, if all else fails this is your only option and it's a 50/50 shot.

But I've had to do this so many times with unknown equipment, of most cheapnesses, and always got away with it, that I've become pretty blase, and tend to try it istead of thinking about it.

So good luck, and I don't wish to hear from your lawyers,

[Kate]

Thanks for your kind words, Sparks.

Like you, I was worried that the OP has missed such obvious information as is certainly avaliable on the power supply, and then proceeded to ask how to use test equipment in order to find it out.

As I said, this is an application with some concerns: a failure of the power supply or alarm leading to smoke release from compromised circuit boards or plastic casings, and failure of the alarm at a point when the baby is at risk are just two concerns.

I'm sorry my point-by-point suggestion didn't go down too well, but this is a serious application that requires more of an input to put right properly than the OP has so far demonstrated.

[hicks]

My original posting was asking for confirmation that my understanding was correct. Thanks to the helpful responses, I've now solved the problem. The reason I jumped on your post was that to me it did have a patronising tone. I can't abide people taking this attitude, or belittling others when they ask a reasonable question. You may have a lot more knowledge on this subject than me, but we all have to start somewhere. Yes maybe I did overlook the information on the power supply, but I'm sure we have all missed the obvious at some time. But I apologise if I misunderstood your post.