| Author |
Message |
Grant Edwards
Guest
|
 Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
On 2005-12-02, Dave <dspear99ca@yahoo.delete.com> wrote:
| Quote: | 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! |
|
| Back to top |
|
 |
Jerry Avins
Guest
|
| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
Dave Pollum wrote:
| Quote: | 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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
|
| Back to top |
|
|
Bob Monsen
Guest
|
| Posted:
Mon Dec 05, 2005 7:48 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
On Fri, 02 Dec 2005 11:53:05 -0500, Jerry Avins wrote:
| Quote: | 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 |
|
| Back to top |
|
|
Jerry Avins
Guest
|
| Posted:
Mon Dec 05, 2005 8:05 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
Bob Monsen wrote:
| Quote: | 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.
|
It's not hard to get op amps that handle a few tens of milliamps, and
that represents the difference between the drains on the two rails. The
quiescent drains can be approximately balanced by a single resistor.
I don't recommend this except for emergencies. It seems penny wise and
pound foolish.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
|
| Back to top |
|
|
Bob Monsen
Guest
|
| Posted:
Mon Dec 05, 2005 9:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
On Sun, 04 Dec 2005 21:05:43 -0500, Jerry Avins wrote:
| Quote: | It's not hard to get op amps that handle a few tens of milliamps, and
that represents the difference between the drains on the two rails. The
quiescent drains can be approximately balanced by a single resistor.
|
You can get opamps that handle far more current than that, but the op said
a 'big card', so I was thinking maybe a PC motherboard or something like
that. A 300W ATX power supply can supply 30A through the +5 rail, but only
300mA through the -5V rail. Hook up a load that is designed for one of
those, and you'll be in for quite a surprise.
---
Regards,
Bob Monsen
What good your beautiful proof on (the transcendence of) PI. Why
investigate such problems, given that irrational numbers do not even exist?
- Leopold Kronecker |
|
| Back to top |
|
|
c d saunter
Guest
|
| Posted:
Mon Dec 05, 2005 10:22 pm Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
Frank (Francis.invalid@hotmail.com) 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 |
|
| Back to top |
|
|
c d saunter
Guest
|
| Posted:
Mon Dec 05, 2005 11:16 pm Post subject:
Re: 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 |
|
| Back to top |
|
|
Bob Monsen
Guest
|
| Posted:
Tue Dec 06, 2005 1:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:
| Quote: | 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. |
|
| Back to top |
|
|
Jerry Avins
Guest
|
| Posted:
Tue Dec 06, 2005 1:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
c d saunter wrote:
| Quote: | 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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
|
| Back to top |
|
|
Symon
Guest
|
| Posted:
Tue Dec 06, 2005 8:06 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
"Jerry Avins" <jya@ieee.org> wrote in message
news:Db6dnTyYHMr6TgnenZ2dnUVZ_smdnZ2d@rcn.net...
| Quote: | 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. |
|
| Back to top |
|
|
Guest
|
| Posted:
Tue Dec 06, 2005 8:11 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
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 |
|
| Back to top |
|
|
Meindert Sprang
Guest
|
| Posted:
Tue Dec 06, 2005 9:15 am Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
"c d saunter" <christopher.saunter@durham.ac.uk> wrote in message
news:dn2ho4$qhn$1@heffalump.dur.ac.uk...
| Quote: | Bob Monsen (rcsurname@comcast.net) wrote:
: On Mon, 05 Dec 2005 22:22:54 +0000, c d saunter wrote:
: The bricks I've used have always isolation, but I live a
: relatively sheltered life. Where have you seen them, mainly?
Some of the transformer based ones I have at work are - mind you I'm
in the UK where almost everything has an earth (mechanical interlocks
stop you plugging anything in without the third pin...) - I'm guessing
this isn't an issue in places like the USA as the various American bricks
I've accumulated don't have an earth pin...
|
I have quite a lot of experience with power supply modules (the "open frame
type") and I dear say that for instance NONE of the types offered in the
Farnell catalog have their - or 0 connected to the PE. From a manufacturer's
point of view this would be absolutely stupid because it means that he would
have to have two types of each model, one with the + to PE and another with
the - to PE. Also in lab supplies you will always see an extra ground
terminal, often between the + and - and possibly with a supplied bracket
between the ground and -.
Besides, many applications need a complete separation from mains, even from
ground. So that is another reason for manufacturers not to connect the - to
PE(ground).
Meindert |
|
| Back to top |
|
|
Peter
Guest
|
| Posted:
Tue Dec 06, 2005 3:32 pm Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
"c d saunter" <christopher.saunter@durham.ac.uk> wrote in message
news:dn2eju$p0p$1@heffalump.dur.ac.uk...
| Quote: | Frank (Francis.invalid@hotmail.com) 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
|
You are quite right. We had to pull our power supplies apart and disconnect
the mains earth connection or they would get very hot. They have been in
service for 5 years now and are still running OK.
Peter |
|
| Back to top |
|
|
Paul Keinanen
Guest
|
| Posted:
Tue Dec 06, 2005 3:34 pm Post subject:
Re: Quick question, how do I supply +-5V? |
|
|
On Tue, 6 Dec 2005 08:06:58 +0100, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:
| Quote: | 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 |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in