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By esklar81
#107538
@leon_heller:
Do you know the negative sides of the three supplies in that box to be tied to each other and to Earth? Or, was that just another way of saying "RTFM"?

@waltr:
I'm not at all sure that "dual adjustable outputs" is what's needed here. It appears to me that what's needed is two DC supplies that have isolated (from each other and from Earth) grounds. If leon_heller is correct, this box has all three of its supplies (one of which is adjustable) tied to Earth ground. If one had a box with two, fixed-voltage, isolated, 5 V supplies, one could tie the high side of one to the low side of the other and to Earth and get a +5 V supply and a -5 V supply.

@60amp_relay:
That will work if either
  1. the apparatus being powered will tolerate floating 5 V above Earth or
  2. the adjustable and +5 V supplies are isolated from Earth


@diodedan:
You might be able to get what you need from something as simple as a couple of 5 V wall warts, if you can find at least one that is isolated from ground. You might also want to consider salvaging a power supply from a PC and regulating the -12 down (up?) to -5.


Eric

By smbaker
#107539
diodedan wrote: Are there other power supplies that I should be looking at?
You might want to look for a power supply with two variable outputs that can be bridged together. For example, something like a Mastech GPC-3030D (a little pricier than the one you were looking at, around $170). The advantage of this type is that the two outputs can be used independently, or can be put into series (up to -30/+30), or put in parallel for increased amperage. There are lots of manufacturers of similar power supplies.

It all depends if you're looking only to do +5/-5 or if you want something more versatile. If only +5/-5 is what you need then another option is to build your own as it can normally be done with little more than a transformer, some caps, and a couple of three terminal regulators. I once bought a kit somewhere and used it to build a +/- 15V supply for around $20 - $30 total.
By diodedan
#107589
Thanks for the quick replies everyone.

I'm looking at doing dual supply opamp design, and I don't want to kill myself doing switching power supply design. All I need to do is get stable/low ripple +/- voltages from the system for testing.

I was looking at a PSU because it would save me a lot of time. I could afford a $150 investment. I have just been overwhelmed with choices.

My understanding is that I could switch the + and - inputs on a PSU to get a negative voltage. I figured that this might not work with all units. The SFE unit I originally posted about should be able to do this, but I haven't been able to verify it.

Diodedan
User avatar
By leon_heller
#107596
You can buy little modules that generate -5V from a +5V supply, or +/- 5V from +12V. They aren't expensive and are ideal for powering op-amps. I've used them in the past for that sort of application.
By diodedan
#107774
Leon,

I think you're talking about DC-DC converters. Is there a specific module that you have had success with? Could you point me towards it?

Thanks,
Diodedan
By anhingus
#107976
things getting murky here. power supplies tied to earth ground are a 1950's amateur radio thing - today, couldn't get UL approval and nice lawsuit material.

you want +/- 5 volts? buy 2 5v power supplies. plug the + lead of the 2nd into the ground of the first. use that post as circuit ground.

put the common [-] lead of your voltmeter to this point. touch v+ of first supply with meter + probe. then touch v- of second supply with meter plus probe. voila! +/- 5 volts.

the plus/minus voltage for a circuit depends on what you are using for a ground reference point. want cheap? get 2 6v lantern batteries. connect one plus terminal to the other's negative terminal. that's your ground. you got +/- 6 volts at the other ends.

if you go the wallwart route, make sure they are regulated linear, not switching supplies. again, you have to tie them as if they were batteries.

the idea of dc-dc convertors for op-amp supplies is pure bull-goose looney!

if using a published circuit for your project, be aware that if any precision is to be accomplished that the chip manufacturer or the chip manufacturer's batch number may cause disappointment for out of the box expectations.

if using a found circuit for your project, considering going to national, ti, etc., and check their application notes on single-ended supplies, especially the rail-to-rail stuff.

on the power supply you are considering: me, i would call or email manufacturer tech support and find out if it will do a +/- 5v.

my expectation is that it would using the 5v fixed to the variable output set at 5v.

happy hunting!
By diodedan
#111377
This question is solved.

I purchased the analog version of the power supply and tried the common ground +/-5V configuration. I am happy to report that it works very well, and with the proper filter caps produces a very low noise output!