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My nephew and I have been working on a circuit that uses +/- 4.5V. Up until now we have been using 2 3-AAA battery holders to supply each leg and now wish to move to mains power.

My question is, discounting the more unusual topographies(auto transformers, cap dividers), is it safe to assume that 2 2-prong 5V(12V) power supplies are isolated from each, and therefore I can use 2 positive regulators to regulate the output. As noted in some of the comments, I have metered inside/outside of the barrel to each prong, with all of them showing open.

I know I can use 2 separate taps from a transformer for this.

In the past I've used 2 supplies like this in a audio application(sans regulators), and it worked fine.

If it comes down to it, I can dig out a multi-tap transformer(not a center-tap) and use that, but that's a lot of work, and the result is much heavier, hotter and less efficient compared to using 2 wall-warts.

And as far as I know, using an inverted buck DC/DC still requires a negative regulator.

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  • \$\begingroup\$ If isolated then superposition applies, otherwise NOT \$\endgroup\$ Commented Nov 16, 2019 at 15:48
  • \$\begingroup\$ As long as you are satisfied with isolation, they can be put in series in any polarity. \$\endgroup\$ Commented Nov 16, 2019 at 16:53
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 Short of taking a Megger to them(which to be truthful, would not really show me what I needed), I'm not seeing any connection between any of the outside connections. My question is basically confirming what I thought(assumed?), that is, if I don't see a connection, it's safe to use positive regulators to regulate them both. It would be sort of an embarrassment if I told my nephew we could do this, and the power supply went up in sparks(or worse!) afterwards. \$\endgroup\$
    – GB - AE7OO
    Commented Nov 16, 2019 at 17:07

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If you view the output of a black box that has two wires coming out from a power source inside, one ++ and the other -- you would never know if it was a negative or positive regulator. In addition any current flowing out the ++ has to flow back in the -- wire when a load is connected between the two. All just depends on what you bring out as GND.

So as long as the wall warts that you use have:

  1. 100% isolation between the input AC terminals and the DC outputs
  2. No connections to the AC side safety GND

You can use the two wall warts along side each other, one as the +V supply and the other as the -V supply. Just tie the opposite sides of each together to become the DC circuit GND.

enter image description here

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  • \$\begingroup\$ That is why I specified TWO prong power supplies and no unusual types. As far as I know, the only connection to ground is via neutral at the breaker box. And from what I've seen, that is such a tenuous connection that the regulators would have no problem. \$\endgroup\$
    – GB - AE7OO
    Commented Nov 16, 2019 at 12:26
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    \$\begingroup\$ If the wall wart happens to have any low resistance connection from one of the outputs back to either one of the AC input prongs it cannot be paired with another to make a bipolar power supply. If, as I said, they are 100% isolated it is safe to use as shown. \$\endgroup\$ Commented Nov 16, 2019 at 12:32
  • \$\begingroup\$ I do meter them(inside/outside of barrel to each prong) and the ones I'm using show open. It's possible that there is leakage, but if takes a Megger to find it, I've always just ignored it...:) \$\endgroup\$
    – GB - AE7OO
    Commented Nov 16, 2019 at 12:36

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