I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.
First two can be easily satisfied with two HP power supplies (each has its
own
GND pin also). Regarding +-5V, can I replace with an HP power supply set to
10V, -5V pin connect to HP's ground and +5V to power?

Frank wrote:

I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.

If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and -5V.

First two can be easily satisfied with two HP power supplies (each has its own
GND pin also). Regarding +-5V, can I replace with an HP power supply set to
10V, -5V pin connect to HP's ground and +5V to power?

No. Doing so would provide zero volts to the board's -5V rail and +10V to
its +5 rail.

You need four supplies.

--
================================================== ======================
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
mrkesti at comcast dot net | - The Who, Bargain

Michael R. Kesti wrote:
Frank wrote:

I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.

If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and -5V.

First two can be easily satisfied with two HP power supplies (each has its own
GND pin also). Regarding +-5V, can I replace with an HP power supply set to
10V, -5V pin connect to HP's ground and +5V to power?

No. Doing so would provide zero volts to the board's -5V rail and +10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw

Ryan Weihl wrote:
Michael R. Kesti wrote:

Frank wrote:

I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.


If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and -5V.

First two can be easily satisfied with two HP power supplies (each
has its own
GND pin also). Regarding +-5V, can I replace with an HP power supply
set to
10V, -5V pin connect to HP's ground and +5V to power?


No. Doing so would provide zero volts to the board's -5V rail and
+10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw

Maybe not. Compound switchers need a load on the main supply in order to
come up, and the auxiliary supplies aren't always well regulated.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

Ryan Weihl wrote:

Michael R. Kesti wrote:

Frank wrote:

I have a large board which asks for three power supplies, +3V,
+3.3V and
+-5V.



If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and
-5V.

First two can be easily satisfied with two HP power supplies (each
has its own
GND pin also). Regarding +-5V, can I replace with an HP power
supply set to
10V, -5V pin connect to HP's ground and +5V to power?



No. Doing so would provide zero volts to the board's -5V rail and
+10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw


Maybe not. Compound switchers need a load on the main supply in order
to come up, and the auxiliary supplies aren't always well regulated.

Jerry

They also make more noise than a neo-natal nursery. :-)

Regards,
Steve

Steve Underwood wrote:
Jerry Avins wrote:

Ryan Weihl wrote:

Michael R. Kesti wrote:

Frank wrote:

I have a large board which asks for three power supplies, +3V,
+3.3V and
+-5V.




If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and
-5V.

First two can be easily satisfied with two HP power supplies (each
has its own
GND pin also). Regarding +-5V, can I replace with an HP power
supply set to
10V, -5V pin connect to HP's ground and +5V to power?




No. Doing so would provide zero volts to the board's -5V rail and
+10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw



Maybe not. Compound switchers need a load on the main supply in order
to come up, and the auxiliary supplies aren't always well regulated.

Jerry


They also make more noise than a neo-natal nursery. :-)

If he knows what he's doing, ha can load the 10V supply with a beefy
op-amp connected as a follower to a divider across the rails and ground
its output. If something goes wrong, it can blow the board unless he
uses Zener-cum-fuse protection. There are DC-DC power-supply bricks and
chips that can probably supply all the -5 needed from a +5 supply.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

"Jerry Avins" wrote in message
...
Steve Underwood wrote:
Jerry Avins wrote:

Ryan Weihl wrote:

Michael R. Kesti wrote:

Frank wrote:

I have a large board which asks for three power supplies, +3V,
+3.3V and
+-5V.




If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and
-5V.

First two can be easily satisfied with two HP power supplies (each
has its own
GND pin also). Regarding +-5V, can I replace with an HP power
supply set to
10V, -5V pin connect to HP's ground and +5V to power?




No. Doing so would provide zero volts to the board's -5V rail and
+10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw



Maybe not. Compound switchers need a load on the main supply in order
to come up, and the auxiliary supplies aren't always well regulated.

Jerry


They also make more noise than a neo-natal nursery. :-)

If he knows what he's doing, ha can load the 10V supply with a beefy
op-amp connected as a follower to a divider across the rails and ground
its output. If something goes wrong, it can blow the board unless he
uses Zener-cum-fuse protection. There are DC-DC power-supply bricks and
chips that can probably supply all the -5 needed from a +5 supply.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I don't know what you people are talking about.
Back to my question, how do I make a +-5V?

"Frank" wrote in message
...
I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.
First two can be easily satisfied with two HP power supplies (each has its
own
GND pin also). Regarding +-5V, can I replace with an HP power supply set
to
10V, -5V pin connect to HP's ground and +5V to power?

You might with some caveats.
First, the intended +-5v supply needs to be floating with respect to the
other supplies.
Then the 10v voltage difference can be reference wherever you want in theory
and often in practice.

The challenge is: now that you've floated the supply, how will you reference
it to the ground or 0v point on the board?

Think of the +-5v supply as a 10v battery. A battery "floats" with no
problem.
Unless you do more, the result looks like this:


+------------------------------+5v
|
|
+----+ +---------------+3.3v
| 10v| +----+
+----+ |3.3v| +----------+3v
| +-+--++----+
| | |3.3v|
| | +-+--+
| | |
| | |
| +-----+--------- 0v: the reference for +3.3v, +3v
|
|
|
+-------------------------------5v

With the 10v battery floating, there is no reference to the
other batteries. Current flowing through the circuit board will cause
the +/-5v terminals to go almost anywhere relative to 0v. Depending on
what's on the board,
the +5v terminal could end up at -6v and the =5v terminal at -15v (both
relative to 0v of course).



+------+-----------------------+5v
| |
| | +---------------+3.3v
+----+ | +----+
| 10v| | |3.3v| +----------+3v
+----+ ++-+ +-+--++----+
| |R1| | |3.3v|
| ++-+ | +-+--+
| | | |
| | | |
| +-------+-----+--------- 0v: the reference for +3.3v, +3v
| +--+
| |R1|
| ++-+
| |
+------+------------------------5v

A resistor divider with current much higher than the +/-5v
loads and connected to the 0v reference will refer the +/-5v
to the rest of the batteries.
It's not a very elegant or even practical solution but it makes the
point to address your question.

Fred

http://www.jameco.com/wcsstore/Jamec...dDS/212311.pdf

http://www.jameco.com/wcsstore/Jamec...dDS/212311.pdf

On Fri, 02 Dec 2005 12:27:10 +0800, Frank wrote:



¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I don't know what you people are talking about.
Back to my question, how do I make a +-5V?


+ - 5volts is actually 2 supplies, each giving out 5 volts, but one supply
has it's negative output connected to the 0 volt (common), the other
supply has its positive output connected to the 0 volt (common).

The effect is if you measure from the 0 volt (common) to one supply rail,
you will measure positive 5 volts, measuring from the 0 volt to the other
5 volt supply rail will be negative 5 volts.

Your original assumption that if you measured from the +5 rail
to the -5 rail you would find 10 volts is correct, but don't forget that
BOTH of the 5 volt supplies need to reference to the 0 volt as the current
drawn in the -5 circuitry may not balance the current drawn by the +5 volt
circuits..... without a 0 volt reference, you could end up with +3 volts
and -7 being supplied (the ratio between the + and - would depend on the
current being drawn from each of the 2 supplies)


Now, as for the +3 volt and +3.3 volts you also mentioned in your original
post, these would also have their negative rail tied to the 0 volt (common).

This is the 'normal' way that these type of multi voltage computer power
supplies are hooked up.

So your supply would have 5 output connections....
1, 0 volts (common to all 4 of the supplies!)
2, +5 volts
3, -5 volts
4, +3 volts
5, +3.3 volts

Pip

Frank wrote:

...

I don't know what you people are talking about.
Back to my question, how do I make a +-5V?

If you don't know what we're referring to, you're better off buying what
you need. Power supplies are cheap compares to your board. If you post
your current requirements, we can suggest specific hardware.

Using supplies with adjustable voltage but without knob locks is a
fairly common way to smoke parts. Be careful.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Fred Marshall wrote:
"Frank" wrote in message
...

I have a large board which asks for three power supplies, +3V, +3.3V and
+-5V.
First two can be easily satisfied with two HP power supplies (each has its
own
GND pin also). Regarding +-5V, can I replace with an HP power supply set
to
10V, -5V pin connect to HP's ground and +5V to power?


You might with some caveats.
First, the intended +-5v supply needs to be floating with respect to the
other supplies.
Then the 10v voltage difference can be reference wherever you want in theory
and often in practice.

The challenge is: now that you've floated the supply, how will you reference
it to the ground or 0v point on the board?

Think of the +-5v supply as a 10v battery. A battery "floats" with no
problem.
Unless you do more, the result looks like this:


+------------------------------+5v
|
|
+----+ +---------------+3.3v
| 10v| +----+
+----+ |3.3v| +----------+3v
| +-+--++----+
| | |3.3v|
| | +-+--+
| | |
| | |
| +-----+--------- 0v: the reference for +3.3v, +3v
|
|
|
+-------------------------------5v

With the 10v battery floating, there is no reference to the
other batteries. Current flowing through the circuit board will cause
the +/-5v terminals to go almost anywhere relative to 0v. Depending on
what's on the board,
the +5v terminal could end up at -6v and the =5v terminal at -15v (both
relative to 0v of course).



+------+-----------------------+5v
| |
| | +---------------+3.3v
+----+ | +----+
| 10v| | |3.3v| +----------+3v
+----+ ++-+ +-+--++----+
| |R1| | |3.3v|
| ++-+ | +-+--+
| | | |
| | | |
| +-------+-----+--------- 0v: the reference for +3.3v, +3v
| +--+
| |R1|
| ++-+
| |
+------+------------------------5v

A resistor divider with current much higher than the +/-5v
loads and connected to the 0v reference will refer the +/-5v
to the rest of the batteries.
It's not a very elegant or even practical solution but it makes the
point to address your question.

You can make Fred's solution more practical by adding an operational
amplifier that can deliver the difference between +5 and -5 currents.

+------+--------------------------------------------+5v
| |
| | +---------------+3.3v
+----+ | +----+
| 10v| | |3.3v| +---------+3v
+----+ ++-+ +-+--+ +----+
| |R1| +----------+ | |3.0v|
| ++-+ | |\ | | +-+--+
| | +---|-\ | | |
| | | +---+----------+------+--------- 0v
| +--------|+/
| +--+ |/
| |R1|
| ++-+
| |
+------+------------------------5v

Is it worth it?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Here's a cheap and quick, potentially noisy solution:

1. Buy two 5VDC "wall wort" power supplies.
2. connect the (-) from one to the (+) from the other. This is your common
ground lead..
3. The + lead will give you +5VDC
4. The - lead will give you -5VDC.

Make sure the wall worts are rated to deliver enough current otherwise
you'll get too much voltage drop for your app. Note that this supply is
UNREGULATED, that is, increasing load will decrease your voltage.

If you need a regulated supply, use 7-12VDC wall worts (to compensate for
the voltage drop across the regulators), buy a 7805 positive and a 7905
negative 5V regulator, two 3300uF electrolytics and two 1uF ceramic caps to
filter the output and a small PCB. There are tons of diagrams how to
arrange things to get what you want. Google is your friend. Total cost of
parts should be about $5 to build the regulator, plus some sort of enclosure
if you require.

If you happen to have two matching wall worts of 6VDC you can probably use
them if you regulate, the regulator chips can accept up to ~35VDC. note
that with larger input voltages you'll need to use heatsinks on the
regulators to dissipate the extra power/heat.

Dave

"Frank" wrote in message
...

"Jerry Avins" wrote in message
...
Steve Underwood wrote:
Jerry Avins wrote:

Ryan Weihl wrote:

Michael R. Kesti wrote:

Frank wrote:

I have a large board which asks for three power supplies, +3V,
+3.3V and
+-5V.




If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and
-5V.

First two can be easily satisfied with two HP power supplies (each
has its own
GND pin also). Regarding +-5V, can I replace with an HP power
supply set to
10V, -5V pin connect to HP's ground and +5V to power?




No. Doing so would provide zero volts to the board's -5V rail and
+10V to
its +5 rail.

You need four supplies.

will an ATX supply not do?
rw



Maybe not. Compound switchers need a load on the main supply in order
to come up, and the auxiliary supplies aren't always well regulated.

Jerry


They also make more noise than a neo-natal nursery. :-)

If he knows what he's doing, ha can load the 10V supply with a beefy
op-amp connected as a follower to a divider across the rails and ground
its output. If something goes wrong, it can blow the board unless he
uses Zener-cum-fuse protection. There are DC-DC power-supply bricks and
chips that can probably supply all the -5 needed from a +5 supply.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I don't know what you people are talking about.
Back to my question, how do I make a +-5V?

previous replies were snipped

Frank wrote:
I don't know what you people are talking about.
Back to my question, how do I make a +-5V?

We need answers to these basic questions:
1)You need -5 volts, +5 volts, +3.3 volts, and +3.0 volts - correct?
(can you use just +3.3 volts instead of both +3.0volts _and_
+3.3volts?)
2)How much current do you need for each voltage?
3)What voltages and currents do your 2 HP power supplies produce?

Assuming that the HP's can produce negative voltage (with respect to
the HP's GND), then set one HP for -5 volts and set the other one for
+5 volts. Then use Low-DropOut (LDO) voltage regulators to produce the
+3.0 volts and +3.3 volts.

Dave Pollum

Dave Pollum wrote:
previous replies were snipped

Frank wrote:

I don't know what you people are talking about.
Back to my question, how do I make a +-5V?


We need answers to these basic questions:
1)You need -5 volts, +5 volts, +3.3 volts, and +3.0 volts - correct?
(can you use just +3.3 volts instead of both +3.0volts _and_
+3.3volts?)
2)How much current do you need for each voltage?
3)What voltages and currents do your 2 HP power supplies produce?

Assuming that the HP's can produce negative voltage (with respect to
the HP's GND), then set one HP for -5 volts and set the other one for
+5 volts. Then use Low-DropOut (LDO) voltage regulators to produce the
+3.0 volts and +3.3 volts.

If those hp supplies are like mine, either the positive or negative
terminals may be grounded by a link.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 2005-12-02, Dave wrote:
Here's a cheap and quick, potentially noisy solution:

1. Buy two 5VDC "wall wort" power supplies.
2. connect the (-) from one to the (+) from the other. This is your common
ground lead..
3. The + lead will give you +5VDC
4. The - lead will give you -5VDC.

Make sure the wall worts are rated to deliver enough current otherwise
you'll get too much voltage drop for your app. Note that this supply is
UNREGULATED, that is, increasing load will decrease your voltage.

Only if you buy unregulated wall warts. You can get regulated
5V wall-warts and cable-lumps. They usually cost more of course.

--
Grant Edwards grante Yow! There's enough money
at here to buy 5000 cans of
visi.com Noodle-Roni!

On Fri, 02 Dec 2005 11:53:05 -0500, Jerry Avins wrote:


You can make Fred's solution more practical by adding an operational
amplifier that can deliver the difference between +5 and -5 currents.

+------+--------------------------------------------+5v
| |
| | +---------------+3.3v
+----+ | +----+
| 10v| | |3.3v| +---------+3v
+----+ ++-+ +-+--+ +----+
| |R1| +----------+ | |3.0v|
| ++-+ | |\ | | +-+--+
| | +---|-\ | | |
| | | +---+----------+------+--------- 0v
| +--------|+/
| +--+ |/
| |R1|
| ++-+
| |
+------+------------------------5v


The main problem with this scheme is that if either of +5 or -5 volt rails
tries to draw more than a few mA, the opamp won't be able to source or
sink that much current, so that rail will droop. (The other rails will
just follow whatever they've been grounded to.) This isn't too bad, but
it'll cause the _other_ rail to suddenly spike by the same amount in the
opposite direction. So, a 5A spike on the +5 rail might cause the -5 rail
to go to -10V, possibly smoking something funny (like components.)

I'd say you need some kind of beefy totem pole output to drive it, meaning
a couple of power transistors in addition to the circuit above. Now, is it
worth it? Particularly when you can buy a +-5V regulated supply that will
source 20A for $50.

---
Regards,
Bob Monsen

Music is the pleasure the human soul experiences from counting without
being aware that it is counting.
- Gottfried Leibniz

Frank ) wrote:

....

: I don't know what you people are talking about.
: Back to my question, how do I make a +-5V?

'Buy one' is the most sensible suggestion I've seen elsewhere on the
thread.

*BEWARE*
Another one I've seen mentioned is to get 2 stock 5V supplies, A and B,
and connect the +5V of A to the 0V of B, and call pair this 0V, thus
making the 0V of A -5V and the 5V of B +5V.
*BEWARE*

This may or may not work - many lab power supplies and some bricks
conenct the 0V from the DC side to the mains earth, so doing this
with two such supplies will cause funny noises, bad smells and possibly
worse as the magic smoke escapes.

If you're not sure, don't try ths aproach!

---

cds

On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:

This may or may not work - many lab power supplies and some bricks
conenct the 0V from the DC side to the mains earth, so doing this
with two such supplies will cause funny noises, bad smells and possibly
worse as the magic smoke escapes.

If you're not sure, don't try ths aproach!

The bricks I've used have always isolation, but I live a
relatively sheltered life. Where have you seen them, mainly?

--
Regards,
Bob Monsen

"You told me, 'God made the World.'" "No, no!" Harshaw said hastily.
"I told you that, while all these many religions said many things,
most of them said, 'God made the World.' I told you that I did not
grok the fullness, but that 'God' was the word that was used." "Yes,
Jubal," Mike agreed. "Word is 'God'" He added. "You grok." "No, I must
admit I don't grok." "You grok," Smith repeated firmly. "I am explain.
I did not have the word. You grok. Anne groks. I grok. The grass under
my feet groks in happy beauty. But I needed the word. The word is
God." Jubal shook his head to clear it. "Go ahead." Mike pointed
triumphantly at Jubal. "Thou art God!" Jubal slapped a hand to his
face. "Oh, Jesus H. What have I done? Look, Mike, take it easy! Simmer
down! You didn't understand me. I'm sorry. I'm very sorry! Just forget
what I've been saying and we'll start over again on another day. But "
"Thou art God," Mike repeated serenely. "That which groks. Anne is
God. I am God. The happy grass are God, Jill groks in beauty always.
Jill is God. All shaping and making and creating together ." He
croaked something in Martian and smiled.

Bob Monsen ) 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 ) 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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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" wrote in message
...
Bob Monsen ) 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"
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