Positive and negative DC off of the same AC source?

[bancika]

Is this possible or I need a DPDT switch to toggle between the two rectifiers and have only one active at any given moment?

[R.G.]

Is this possible or I need a DPDT switch to toggle between the two rectifiers and have only one active at any given moment?

This is a very tempting idea that shows up from time to time. There is a problem with it, though.

As drawn, you can get two **isolated** supplies. If you ever connect what might be the putative ground between them, half of each of the full wave rectifier forms a full wave rectifier directly across the input AC, effectively shorting the AC with two diodes in each direction. Smoke and flames (OK, well, just Bad Things) ensue.

It's possible if the two outputs are never, ever connected through one ground. If you are OK with only having one output active at any time, you can use only one of the rectifier/filter sections and use a DPDT to reverse its polarity with respect to ground, and save half the parts.

It's simpler to do two half-wave rectified supplies and double up on the filter caps. See "Power Supplies Basics" at geofex.com for how to do this.

[bancika]

tnx RG

so if I have GND and +9 isolated from everything else and I just flip the polarity, I can use it as -9V and 0?

[ashcat_lt]

Don't know exactly what you're trying to accomplish, but all voltage is relative.  If you look at a typical 9V battery, you can call either terminal 0V.  We typically assign that to the - terminal, in which case the + terminal is called +9V.  No reason you can't call the + 0 and the other -9, though.  As long as you're never shorting the - and the + directly, it makes no difference.

Does that apply at all to what you're considering?

[bancika]

I think it does. I got a custom 5 secondary transformer to make a supply for my rig. I need 18V, 12V, 9V and negative voltage along with AC for future use. Something like this. If I can just flip 0 and 9 I can get away without one of the "columns" and just switch the output.