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This turned out to be several questions, though they're all closely related; I hope that's OK. Due to this, I'll create a small "summary" with just the questions at the end.

I'm looking at designing a dual-rail PSU build from a dual secondary transformer (2x12 V / 2x1.7 A, total 40.8 VA). The transformer will be the only part shared between the two rails, so I'll have two full-wave rectifiers, two big smoothing caps, two voltage regulators, etc.
To be clear, this is not a center-tapped transformer. It has 4 output pins, not 3, so each "rail" has two connections which are unshared, and as far as I understand, its own winding in the transformer as well.

Now, the goal here is to have a PSU which can power two things at once (not just tiny loads; more like 1 A each), at different voltages. Also, a secondary goal is to have a +/- capable supply. My understanding is that you can wire a dual supply such that you get a +/- voltage - IIRC by connecting + from one rail and ground from the other?
If so, does this mean that the current goes backwards through one supply? If it does, and I assume it does, then how can I measure output current from each rail with a µC? My home-hacked ammeter design (shunt resistor + opamp to amplify the tiny voltage across the resistor + ADC to measure it) won't like it very much, I take it? Not only is the opamp powered by positive and "ground" (not +/-), but the ADC doesn't support/tolerate negatives either.

So, to summarize the questions:

  • When connecting the rails together for a +/- voltage, will there be a reverse current through one supply? If yes, will a standard linear supply design (transformer/rectifier/LM317) handle that with no problems? Considering electrolytic caps and such.
  • Also, if yes above: how can I home-hack a µC based ammeter that can understand current in both directions?

  • For a transformer with this pinout: pin 1-2 = 12 V, pin 3-4 = 12 V, what happens if we measure voltage between pins 1 and 3, or 2 and 4, with a multimeter? Is a 0 volt reading guaranteed, or does something else happen? I don't really have a clue about which phase the voltages will have, in case it differs between transformers.

  • My current design has a 0.1 ohm shunt resistor (the supply is 1.2-13 volts or so, at up to 1.5 A). Is this a decent value? Lower is of course better for the output voltage, but the opamp has to amplify it more. I don't want to lose too much precision due to amplified noise!

Bonus question:

  • Assuming all voltages ultimately come from the same mains jack in one room, is it ever dangerous to put a multimeter set to voltage between ANY two places? For example, between the DC PSUs output and earth ground, or between earth gorund and a wall's live wire, etc? I'm not about to try this with the mains voltages, but I'm curious if I can poke around a bit and still feel (and be) relatively safe. I would almost assume this is safe due to the huge input impedance of the multimeter.
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Given this number of questions all in one blob, I'll only discuss each one briefly, but this could get you started:

When connecting the rails together for a +/- voltage, will there be a reverse current through one supply? If yes, will a standard linear supply design (transformer/rectifier/LM317) handle that with no problems? Considering electrolytic caps and such.

A negative supply will need to "sink" current rather than source it. But I don't consider this a "reverse" current, its the correct way for current to flow in a negative supply.

An LM317 is a positive linear regulator. It can only source current, not sink current. For your negative supply you will need a negative regulator, like an LM337.

You always need to connect your electrolytic capacitors so that their + terminals are connected to a more positive voltage than their - terminals.

Also, if yes above: how can I home-hack a µC based ammeter that can understand current in both directions?

You will need an analog circuit to translate the sense-resistor voltage into the range of your A/D input. This is a big enough topic for a whole question of its own.

For a transformer with this pinout: pin 1-2 = 12 V, pin 3-4 = 12 V, what happens if we measure voltage between pins 1 and 3, or 2 and 4, with a multimeter? Is a 0 volt reading guaranteed, or does something else happen?

If you are still talking about two unconnected secondary coils wound on the same core primary, there is no guarantee about the dc voltage between them except by what you wire up external to the transformer. The ac voltage will be in phase for the two secondaries because it just follows the input on the primary. Obviously, by observing the "dot-location" on the two coils, you could arrange for the two outputs to be 180 degrees out of phase.

My current design has a 0.1 ohm shunt resistor (the supply is 1.2-13 volts or so, at up to 1.5 A). Is this a decent value? Lower is of course better for the output voltage, but the opamp has to amplify it more. I don't want to lose too much precision due to amplified noise!

The resistor value won't much affect the output voltage, provided you take your feedback for your regulator after the shunt resistor.

For 1.5 A through 0.1 Ohms, your resistor will be burning 225 mW. This is somewhat excessive for this function, and will cause the resistor value to drift due to self-heating. So you can either lose precision due to noise by reducing the resistor or lose accuracy due to thermal effects by keeping it large. I'd expect you could drop the resistor to 0.01 Ohms and still get good precision in your current measurement (10 bit at least) but that will depend on good analog design.

If you really want exceptional accuracy in this application you may want to look into "bulk metal foil" resistors from Vishay, which are much more stable than carbon film and other types w.r.t. thermal drift (and a bunch of other effects).

Assuming all voltages ultimately come from the same mains jack in one room, is it ever dangerous to put a multimeter set to voltage between ANY two places? For example, between the DC PSUs output and earth ground, or between earth gorund and a wall's live wire, etc?

If you are using correctly-rated probes and a correctly-rated meter, you should be able to probe mains without damaging the instrument or injuring yourself. If you are using incorrect equipment you could start a fire or electrocute yourself. Even some meters (like hardware-store models) that are labelled for 100-200-400 V are not really designed safely, so stick to reputable brands (Fluke, Keithley, Agilent, ...).

Building an AC-DC power supply inherently means working with mains, so if you don't know how to keep yourself safe while doing that, you might want to consider alternative projects until you get more experience.

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  • \$\begingroup\$ Thanks, that answers quite a few things. So in other words, I have to choose between being able to output multple voltages and having +/- capability? The first being 2x LM317 or such, and the latter being a LM317/LM337 combo setup? Though if I go with the +/- combo, can I still hook up for example two separate, ordinary loads (one +/common and one common/-), so that both see an ordinary, positive voltage? As you might notice, the negative voltages are my biggest problem in all of this. \$\endgroup\$ – exscape Feb 13 '12 at 18:00
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    \$\begingroup\$ This is not an authoritative answer, but if your supply has strictly floating outputs (no internal common connection between the two outputs) I think you could use a positive regulator for both outputs and then externally connect them so as to give +/- output...but I may be forgetting some key problem. \$\endgroup\$ – The Photon Feb 13 '12 at 18:07
  • \$\begingroup\$ Here's the video that made me ask: youtube.com/watch?v=w99Q23mUEZg - I don't know how that supply works internally, though, but it certainly appears to have two separate outputs externally. \$\endgroup\$ – exscape Feb 13 '12 at 18:19

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