Quick question, how do I supply +-5V?

Bob Monsen (rcsurname@comcast.net) wrote:
: On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:

: 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

c d saunter wrote:
> Bob Monsen (rcsurname@comcast.net) wrote:
> : On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:
> 
> : 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...

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.
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"Jerry Avins" <jya@ieee.org> wrote in message
news:Db6dnTyYHMr6TgnenZ2dnUVZ_smdnZ2d@rcn.net...
>c d saunter wrote:
>> Bob Monsen (rcsurname@comcast.net) wrote:
>> : On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:
>>
>> : 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...
>
> I know that you Brits have a very positive outlook, but there surely must
> be a way to supply a negative voltage?
>
> Jerry
>
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:-
http://www.maplin.co.uk/images/Full/1168i0.jpg

In the UK, such things are called 'double insulated', see :-
http://en.wikipedia.org/wiki/Double-insulated

That said, some bricks do have an earth connection as Chris says. These
often use a IEC connector like this:-
http://en.wikipedia.org/wiki/Power_connector
Dunno if the output 0V is connected to this earth. I suspect not in most
cases.

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 http://sound.westhost.com/project43.htm with a 10v supply.
Be careful to separate GND's (since the GND of your +/- 5V is really at
5V....)

Simon

"c d saunter" <christopher.saunter@durham.ac.uk> wrote in message
news:dn2ho4$qhn$1@heffalump.dur.ac.uk...
> Bob Monsen (rcsurname@comcast.net) wrote:
> : On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:
>
> : 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...

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

On Tue, 6 Dec 2005 08:06:58 +0100, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:

>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).

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