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

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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?




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

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

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

You need four supplies.

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          Michael Kesti            |  "And like, one and one don't make
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Re: Quick question, how do I supply +-5V?
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will an ATX supply not do?
rw

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

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

Regards,
Steve

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

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



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

   ...

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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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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

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Re: Quick question, how do I supply +-5V?
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Only if you buy unregulated wall warts.  You can get regulated
5V wall-warts and cable-lumps.  They usually cost more of course.

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                                  at               here to buy 5000 cans of
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Re: Quick question, how do I supply +-5V?
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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


Re: Quick question, how do I supply +-5V?
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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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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

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Swing and a miss

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

...

: 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

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

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

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  Bob Monsen

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

: 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

Re: Quick question, how do I supply +-5V?
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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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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




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



Re: Quick question, how do I supply +-5V?
On Tue, 6 Dec 2005 08:06:58 +0100, "Meindert Sprang"

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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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