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

Started by December 1, 2005
```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.
&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;
```
```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.
&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;
```
```"Jerry Avins" <jya@ieee.org> wrote in message
news:gpKdnbflleKAIhLeRVn-vg@rcn.net...
> 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.
> &#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;

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

```
```"Frank" <Francis.invalid@hotmail.com> wrote in message
news:438fa5a4@news.starhub.net.sg...
>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
```http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/212311.pdf
```http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/212311.pdf