# Dual polarity vs dual rail power supply

My previous post turned out to contain too many questions, so I'll focus on the main one first, and figure the rest out as needed.

I'm looking at building a DC lab power supply, roughly 1.2-14 volts and up to about 1.5 A, based on the LM317 (and possibly also LM337) voltage regulators. However, I'm a bit torn between two choices when it comes to the basic design: a dual polarity design (+/- 1.2-14 V with LM317/LM337) or a dual rail (2x 1.2-14 V, completely individual rails, 2x LM317).

Example design for the dual polarity:

(Credit: Jason Neal)

The reason I can't decide right now is that I don't understand the following:

• If I go with dual polarity (e.g. +9/0/-9 V output), can I power two loads, perhaps with +9/0 for one and 0/-9 for the other? Both should act as if they each had a regular, +/ground supply. My understanding is that the current will go backwards in this case, so that the answer is no?

• If I go with the dual separate, 2x positive design, can I wire it up to give the positive/negative output? For example, I've seen people bridge + from one rail and ground on the other, then use the remaining two outputs as +/- and the bridged one as ground.
Will the example design be able to do this, without sacrificing output amperage or cause other problems?

As you can see, the goal is to get both features in a single supply, if possible (without quad rails or such madness).

In either case, I would have a single transformer; either a 12-0-12 V center tapped transformer, or a 2x12 V dual secondary transformer.

• See the schematic here (bb-elec.com/bb-elec/literature/12vdpsin.pdf) for an example of how to generate two isolated unregulated supplies from two secondary windings on the same transformer. If you add a separate LM317 circuit to each of these unregulated supplies you'll have dual regulated supplies at some voltage less than 12 V. If you tie the high side of one to the low of the other, you'll have a +/- supply. More detail later (probably a fix-up to my previous answer, which was overly-restrictive about what you can do) if I have time. Commented Feb 14, 2012 at 21:47