: The bricks I've used have always isolation, but I live a : relatively sheltered life. Where have you seen them, mainly?
Some of the transformer based ones I have at work are - mind you I'm in the UK where almost everything has an earth (mechanical interlocks stop you plugging anything in without the third pin...) - I'm guessing this isn't an issue in places like the USA as the various American bricks I've accumulated don't have an earth pin...
Cheers Chris
I know that you Brits have a very positive outlook, but there surely must be a way to supply a negative voltage?
Jerry
-- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
So, wall warts almost never have an earth, even in the UK! Our wall warts usually have a plastic 'earth' pin to overcome the interlock. Most power supplies that are a separate 'brick' connect with a two wire connection to the mains, live and neutral, as in the US. Here's a photo of a UK mains lead for this:-
In the UK, such things are called 'double insulated', see :-
That said, some bricks do have an earth connection as Chris says. These often use a IEC connector like this:-
All the bench DC power supplies I've used, both in the UK and in the US, have separate isolated 0V and earth, usually connectable with a bit of metal.
FWIW, Syms.
My first reply would be "buy one". Cost of a PSU is a lot less than the cost of an average board...
Otherwise, and assuming your load is less than ~40mA, you can use the circuit in
Simon
I have quite a lot of experience with power supply modules (the "open frame type") and I dear say that for instance NONE of the types offered in the Farnell catalog have their - or 0 connected to the PE. From a manufacturer's point of view this would be absolutely stupid because it means that he would have to have two types of each model, one with the + to PE and another with the - to PE. Also in lab supplies you will always see an extra ground terminal, often between the + and - and possibly with a supplied bracket between the ground and -.
Besides, many applications need a complete separation from mains, even from ground. So that is another reason for manufacturers not to connect the - to PE(ground).
Meindert
For any low level measurements, the PE is badly polluted by the noise from switching mode power supply EMC filters etc. For this reason, a separate technical earth (TE) network is often used with only a single contact point between the neutral, grounding electrode and PE and TE earths.
A single power supply with the DC side connected to PE would pollute the whole TE network and you would very quickly get rid of such power supplies.
Paul
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