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Finding the right PSU (power supply unit) for your portable device

Can I run THIS electronic item off THAT power supply transformer? Will it work? How about a Universal Power Supply adaptor? etc.

Many small electronic devices don't plug directly into the mains but instead have a small box that plugs into the wall power outlet, transforming the electricity supply down to a lower voltage. If your laptop, mobile phone, music keyboard, CD player, or other battery / mains portable device needs a power supply unit but you can't get the right one, here's some useful advice on how to get something which will work.

Obviously it's best to get the correct power unit, as recommended by the manufacturers of the machine, but if you can't get such a thing easily and without paying too much, you might be wondering if one of those Universal Power Supply Units will do. Surely, if the PSU can supply any voltage it might just work? Well, it's not quite as simple as that. There are a few things to understand about using a non-standard power supply. Using a power supply for something different from that for which it was designed is a matter which requires understanding a few things about voltage and current, and the kinds of things that work.

low voltage mains adaptorI'm talking about electronic devices which have a little box that plugs in the power outlet and then have a thin cable going to a little plug that goes into the machine. The plug is usually a round metal cylinder and usually has two connectors, one inside and one outside. Some manufacturers have used strange proprietary plugs which is a bit antisocial of them, but most manufacturers use a standard co-axial power plug or a jack plug, or something similar.

The first thing to do when eyeing up what kind of power supply you need, is to look on the device you're going to power, somewhere near the hole where the power is supposed to plug in, and find out what the machine says it needs to run it.

The information should include something like "9v" , or "12volts" or "19v" etc. This is the voltage. Make a note of this. It should also say something like "850mA" or "2A" or "3 Amps" etc. This is the current, measured in amps or milliamps (1A = 1000mA). It should also either say "AC" or "DC" or should have a line and a dotted line, or a curvy AC symbol. Sometimes there's a tiny diagram of the socket, with the centre and the outer and one of them with "+" (plus) and the other with "-" (minus).

The meaning of this information gives you the important facts about the type of power supply required:

Having found out this information, you can now specify the power supply! This means that even if you couldn't buy a PSU off-the-shelf, you could (in theory at least) have one made, engineered to your specification.

Generally you won't need to get a power supply custom made, and will be able to recycle one off another machine, but you have to get a few things right:

* If you can't get exactly the right voltage, you may still be ok if the voltage is slightly less than the proper spec. Voltages between about 60% and 110% of the correct number are usually ok, but let's be sensible; you don't want to damage the device by overloading it by risking too high a voltage. If the voltage is too low (for example running a 9 volt radio off 6 volts) it will probably work. This is especially likely if it's got a battery option, as such machines are designed to work if the batteries are low, and therefore will also work on an under voltage mains adaptor.

* The current doesn't need to be an exact match, but the current should be at least as much as is required. If the voltage is about right, and you use a power supply that can supply a higher current than is required, it should still be ok, as the machine will only take what it needs in terms of current. So, a 12 volt 1 amp appliance will run perfectly happily off a 12 volt 2 amp power supply, as it will only take 1 amp. However, if the power supply gives a lower current than is required, then it's not going to be up to the job, and if you try it, it'll probably burn out the power supply. So, for example, if you've got a computer printer rated at 30 volts 3.4 amps, it's not going to run off a 1 amp power supply, and you can forget those little 3v/4.5v/9v/12v bargain power supplies rated at 500mA (half an amp). It would be like replacing your car engine with one off a motor scooter and still expect to climb hills.

So to summarise the story so far: You need a power supply that's got about the right voltage, but failing that, a LOWER voltage, and you need it to have a current which is preferably MORE than enough.

a different low voltage mains adaptorNow the subject of POLARITY: If the device is AC (alternating current) then it needs an AC power supply, and as AC has no polarity you don't need to worry about which way round the two wires go. However, if it's DC (direct current) then it has a distinct +PLUS and -MINUS and needs to be connected the right way round. If you rashly decide to ignore this and risk destroying the device, it's worth knowing that some devices can survive a few seconds of being connected to reverse polarity, and some can't. However, most can't survive being left plugged in with their + and - the wrong way round all night. More about AC and DC here. I'd advise against trying to run a small portable DC device off AC. There are some AC machines that will work off DC, but this is a compromise to avoid if at all possible, as it may burn out the bridge rectifier after a while.

The type of power plug to use depends on the type of socket, and these can sometimes look the same and yet be subtly different. Co-axial power adaptors tend to have different diameters of inners and outers. However they are made in various standard types, and you can usually get the right plug from an electronics shop if you can show what it's supposed to go into. If you are replacing a dead power supply with a new alternative, you can sometimes recycle the plug off the old cable, but if you do, you should leave plenty of wire and avoid cutting it off too short.

It's also possible to replace a DC power supply with a set of batteries which add up to be the right voltage, provided they are strong enough to supply the current. Use rechargeables, or it could end up being a bit expensive!

As a final word of warning: Use commonsense in any of these experimental adventures in electrics! At your own risk be it!

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