I've just got a very good deal on a ethernet media player/harddrive arrangement (£50 quid instead of around £200). According to the manual it should come with a 5v DC power adapter, unfortunately it actually comes with a 12V adapter with a large plug that won't fit the unit (probably handy as otherwise I'd have used it). Now I could take the whole thing back, but it's still a good deal.

I know nothing about electronics but presumably I could buy a cheap 5v DC adapter to use, would this be a problem? I also have an adapter lying around from an old CD player that looks like it would fit, it's marked 4.5v DC. Would this work or would I damage the unit (presumably there are warranty issues using someone else's adapter). I've emailed the manufacturer's (in France I think) to ask if they can supply the correct adapter, but was wondering whether the 4.5v unit would work in the meantime.

Your adapters have 4 key features you need to identify…

1) Polarity – which bit of the plug is +ve and which is -ve. and which does your system require (not easy to know)
2) The current capability of the adapter and the requirements of the unit you're driving – a 300mA adapter will go up in smoke if you ask it to provide 500 for more than a few minutes.
3) voltage – often 5v devices will cope with 4.5, but they may not – it might be unhappy doing so and not work to potential – depends what it is.
4) AC or DC? wall adapters are usually DC output, but sometimes AC (I have two AC ones, so not that uncommon) – AC may well trash your unit.

you need to a) find and match polarity, B) match or *slightly* exceed voltage, c) match or exceed current capability and d) make damn sure it's DC.

Ah, it may be best to talk to the manufacturers then. Simply plugging in something that fits might not be a great idea.

Since I don't have an adapter to compare against (I assume that the supplied 12V unit is completely wrong) and I only go from the manual which isn't very helpful but states that the unit requires 5V DC and talks about the 'supplied AC adapter', and in the specification that the power supply is AC 100-240v, 50-60Hz and that the power consumption is approx 12W. I'm sure that the manufacturers can supply the proper specs. but I'm hoping they'll send the correct unit anyway, perhaps I should wait.

Also need to check out the size of the plug. There are a million sizes of them, and one that appears to fit might be ever so slightly too small, causing dodgy connections and stuff.

5v is unlikely to kill it, there's probably a voltage regulator in the unit anyway if it's high tech electronics. But I'm not sure I'd risk £200 kit 🙂

As for the polarity, there should be a symbol on the label which looks like this:

It shows that the inside of the plug is positive and the outside negative, or vice versa: