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!
- 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.