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.
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