Wrong 12V plug on PSU for computer monitor?

[Lobster]

I have a flatscreen monitor whose (separate) PSU has died and needs replacing. It's a 12V 3A unit, and as luck would have it I happen to have a similar one going spare from a netbook which I no longer have.

Q1: is there any reason why this putative PSU wouldn't be fit for purpose - ie is there any other characteristic other than 12V/3A that I need to consider here?

Q2: Assuming we've made it past Q1 - unfortunately the (LV) plug which connects to the monitor is slightly but crucially differently sized to the original, so it won't fit - both are standard-looking concentric plugs (ie this sort of thing:

but the central pin is evidently larger on the monitor than it was on my netbook (though the external diameter looks the same). What are the specs for these things - how do I work out what plug I need to buy? I'm assuming it's possible to buy a solderable version; otherwise everything I have here is welded together - is there a neat way of cutting and rejoining the LV flex so I could recycle what I have? (I wouldn't want to bodge it as I'll probably pass this monitor on to someone else once it's repaired...)

Thanks for any thoughts

[Chris Bartram]

The polarity of the plug- sometimes the centre is +ve, sometimes -ve. Sometimes marked on the PSU or the socket on the device it powers.

Maplin, CPC, or Farnell all sell plugs; they are specified by the length of the metal connector, it's inside diameter, and inside diameter. It can be tricky to measure, so you may have to buy a selection.

Probably the neatest way would be solder and heatshrink, but you should be able to get a solderable plug.

[Martin Brown]

And sometimes annoying not marked at all on either part. It would have been nice if there had been a manufacturers convention on polarity and external diameter used for a given voltage, but it is pretty much random. Centre positive is more common but there are exceptions :(

Allowing for delivery charges Maplin is probably easier for a one off.

The ID is best measured by finding something like insulated wire or thin rod that just fits inside and then measuring it that way.

You might need a couple of guesses to get the exact match, some universal PSUs usually come with a selection of swapable connectors.

[John Rumm]

Sounds like it should be fine...

Solder and individual + overall heatshrink would do a neat job.

[Scott M]

A soldered on plug should be considered the worse of the two options as they're a damn sight flimsier than splicing the two cables together. It matters less for a monitor which doesn't move much but, having done various laptops, joining the cables is the far more robust solution.

Also, IMO, if you find a solder plug the correct size, they tend not to be as good a fitment to the socket.

To join the cables, two bits of heatshrink, one large, one small. Cut the conductors 1cm apart (ie stagger the joins.) Large heatshrink pushed over both conductors, small on one only - not doing both reduces the bulge at the join. Big bit of heatshrink covers both joins.

[damduck-egg]

Only one thing I'll add to that is I have known a conductor in a flex break within the insulation where it enters the LV plug sleeve which may be the cause of an apparent power supply failure. The OP will probably know more but if there is any doubt it may pay to check the plug by testing its barrel and socket with the flex conductors with a meter or even a torch lamp and battery and giving the flex entry a good wiggle. It would be a shame to go through the work of connecting the old plug to the other power supply and find it still doesn't work. and don't cut the flex too near the plug,leave plenty to make further joining attempts with if the first one doesn't go well.

G.Harman

[fred]

In article , Scott M writes

Solder jointing would be my approach too, yes to the staggering although use of heatshrink on the inner cores can be problematic for the inexperienced (shrinking the sleeve whilst soldering in close proximity). A plain sleeve on the inner may be easier and it will be well held by the outer heatshrink or no inner sleeve at all if the stagger is long enough to avoid any risk of shorting. A spare bit of outer cable sheath or a few turns of tape (my pref self amalg) will match the bulk of the join to the outer of the original before applying the outer heatshrink.

Also, best to make the join at least 6" from the plug as flexibility will be reduced at the join.

Alternatively, split the cases on the psus and swap the leads over. Take some care in rebuilding the strain relief on the cable, they're usually moulded on the cable and sod's law dictates that they will be different.

[Mike Tomlinson]

In article , Lobster writes

It's a 3.5mm jack plug, but they come with internal diameters of 2.1 and

2.5mm. Your 'new' PSU has the 2.1mm 'ole and you're trying to get it into a socket with the 2.5mm pin.

Cut the cable off the dead PSU and splice it onto the working one. Use heatshrink sleeving to make a neat joint.

And DOUBLE CHECK you have the polarity right if you don't want to let the magic smoke out.

[Roger Mills]

Be very careful! If the two power supplies have different IDs, it's for a good reason. One will be stabilised and the other won't. An unstabilised supply will produce a much higher voltage under no load conditions, and relies on the load to bring the voltage down to the required level. The plugs are designed to be different in order to make them non-interchangeable. If you use an unstabilised supply with something which requires the supply to be stabilised, damage may occur!

[Sam Plusnet]

And one of those good reasons is if they were made by different manufacturers.

Or even by the same manufacturer, and built on the same production line

- but supplied to different OEMs who want their part numbers printed on it.

Things are never that straightforward.

[Scott M]

Cor, where you been living for 10 years? They're all switched mode these days and churn out the whatever voltage it says on the label. Different sized plugs is manufacturers playing silly buggers.

[Scott M]

Cor, where you been living for 10 years? There's about 4 different diameters of centre pin and 5 different outer diameters.

[Michael Chare]

When my Dell laptop PSU failed, the fault turned out to be where the low voltage cable came out. So a repair was just possible although the original case had to be cut open.

[Scott M]

Cor, where you been living for 10 years? (I'm going to get bored of this but not before you lot are.) Remembering the days of them being a hotch-potch of polarities, I always check any switched mode wallwarts that come my way amd I've not found a PSU with a negative centre pin for years, probably not since the days of having actual transformers in them.

[Lobster]

Roger Mills grunted in news: [e-mail]:

Thanks to all for the replies.

How would I know if one PSU was stabilised? There's nothing relevant written on either of them AFAICS. The working PSU was a laptop charger - is that likely to mean it might be stabilised and that the purpose- supplied item for a monitor might not have been?

[Harry Bloomfield]

If there is a good staggering of the joints, one large heat-shrink will cover it and be neater/slimmer.

[Mike Tomlinson]

En el artículo , Scott M escribió:

Bet you that what the OP has is a 3.5 mil jack plug though.

[John Rumm]

Its not a jack plug... they don't have a hole in the end!

[John Rumm]

Can't say I have ever seen enough standardisation of plug specs to draw that conclusion.

Its is almost guaranteed that both will be switched mode supplies (and hence regulated / stabilised)

(a 12V 3A linear supply would be quite a hefty brick and probably weigh as much as the laptop!)

[Lobster]

John Rumm grunted in news: [e-mail]:

Thanks John. Looks like the next step is to try and get inside the welded faulty PSU without damaging it, and work out whether it's the flex that's broken or the electronics; if the flex is OK I'll transfer it to the new PSU via solder/heatshrink.