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[Reply #15 on:]

I do think its good we rely on fuses so much, just a pity so many people don't think about the value they stick in the plugs!

Back to my project, I have just thought... why don't I use a 6v transformer http://uk.farnell.com/myrra/44085/transformer-1-5va-6v/dp/1689047

same footprint, almost same current output and surely will leave less work to do for the Nano's regulator?

[Reply #16 on:]

Can you get the equivalent of this?
http://www.dipmicro.com/store/DCA-0510
Lot less fuss than messing  with a transformer.
Output is also short circuit protected.

[Reply #17 on:]

If you can use a transformer closer to your target voltage, all the better.  A volt or two extra is nice to allow the regulator to regulate when the line voltage isn't quite perfect.  However, many transformers are rated at high load (for their target capacity) so light loads will be higher still.  Furthermore, RMS AC rectified to DC will result in a slightly higher DC voltage.

Aiming for a 6v AC transformer will give you some cushion if you use a 5v regulator.  If you go higher, then using a 9v regulator will shed some of the excess as heat, and then your Nano's onboard regulator will shed the excess from 9v to 5v as heat.  This is better for each of the regulators than going straight from (say) 12v to 5v, as the waste is dissipated via two regulators instead of one.

Re: fuses.  I would always, always put a fuse on any mains-connected PCB.  Just in case.  There is all manner of excrement that can occur.  Sure, your UK power cord may have a 3A or 5A fuse in it.  But if you have (for e.g.) a transformer rated for 250mA, how's it going to feel about passing 3A until the fuse blows?  Circuit breakers and even power cord fuses are rated high enough to not trip as a nuisance.  A product's own fuse is rated for the expected load of the product, and will tend to be a much lower value.  I don't know what the laws are, or even what common practice dictates, but if it were me, I'd pony up that $1 or so for a 250mA fuse.  It's cheap peace of mind.  Your house, your life, so it's entirely up to you, and I won't arm-wrestle anyone that wants to do otherwise.  It's just my not-so-humble opinion. 

[Reply #18 on:]

Thanks again all.

@CrossRoads - I could probably get similar but that would not fit in a socket flush mount back box, although I could mount it remotely but that would mean more cables running to the device because I still need to run mains to my project (its a thermostat). Thanks for the idea though its one I haven't yet considered.

@SirNickity - Thanks that's some really useful info there. I think I might pick up a 6v transformer and test it out using only the nano regulator before I finalise a PCB. Also good point about the fuses!