"John Williamson" wrote in message
...
On 05/01/2017 09:32, Tim Watts wrote:
As for mains - unless you are planning on pouring coffee over it,
there's nothing dangerous about it. It's more dangerous to have a bulky
wall wart in a socket down the side of the bed where it's covered by
bedding or liable to get hit by the bed frame (on a smooth floor).

Also, I place more faith in a nice solid iron transformer with provable
isolation than some random cheap switch mode PSU.

The SMPS will have the same level of isolation as a heavy transformer
based one, it will be more efficient and a lot smaller. They also run
cooler, so are less likely to overheat if covered by bedding.

Don't forget, your heavy iron transformer based supply is under the same
cost constraints as the SMPS, so the same corners are likely to be cut.

It should be easy enough to make one work from a 5v USB supply.
(not like you need massive power or anything in an alarm)


That would be a theory - doesn't seem to stop the shop ones comming with
massive wallwarts sadly.

Part of the size of the wallwarts is to provide enough space inside to
safely isolate the mains from the low voltage side of things. Air gaps are
cheap, while space saving plastic mouldings cost money, which is why the
plugtop sized USB chargers cost more than the larger ones. If you are
buying one, then the space saved may be worth the extra, when you are
buying thousands, and are not the end user, the cost savings of buying the
larger ones are considerable.

I'm not convinced that there are any real cost savings.
The bigger case will cost more and likely more than a
bit of insulation used instead of an air gap for isolation.