Quick question, how do I supply +-5V? - Page 2

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

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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



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

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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



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

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


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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

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

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


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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

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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


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