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?
If I read you correctly, that's four supplies: +3V, +3.3V, +5V, and -5V.
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
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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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:
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).
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.
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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Here's a cheap and quick, potentially noisy solution:
Buy two 5VDC "wall wort" power supplies.
connect the (-) from one to the (+) from the other. This is your common ground lead..
The + lead will give you +5VDC
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.
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.
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
: 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.
The bricks I've used have always isolation, but I live a relatively sheltered life. Where have you seen them, mainly?
-- Regards, Bob Monsen
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