Greg said:
I see so many circuits that want +/- 10 volts or 12 or 15. But when I
look at transformers, they're always around 6 or 12 volts. If I want a
+/- 15 volt supply, I'd think I'd want a 36 volt transformer, to put
three volts higher than the output on the regulator. But I rarely see
transformers so beastly, unless they're for high-power applications.
So where do these +/- 15 volt levels usually come from?
Hi, Greg. Newer analog electronics generally work on lower supply
voltages. In the '70s, analog op amps always used dual +/-12V or
+/-15V supplies. But since then, advances in electronics have led to
single supply op amps, "rail-to-rail" op amps (where the inputs can
extend all the way to the + and - rails of the power supply, and the
ouputs can go almost all the way to the supply rails), and special low
voltage op amps that will work with single supplies well below 5V. You
can do just about anything you would have done with a +/-15V supply in
1980 with a single +5V supply now, by choosing ICs that work on those
supply voltages and using single supply design techniques.
Most manufacturers of transformers will catalog their product by VA
rating, and you can usually find the 28VCT or 36VCT you'll need for a
small +/-12V or +/-15V supply if you look. They're not beastly -- they
just have smaller current ratings for a given size of transformer, and
they're not as commonly made as they used to be.
But you're right -- wall warts aren't commonly available in the higher
voltages. But they do exist, and you can take advantage of the ones
that are out there. You can also use an AC output wall wart to get the
split supplies, use a small AC-to-DC tabletop linear power supply to
give you canned, regualted +/-12V outputs, or use a small DC-to-DC
converter to get the higher analog split supply from a lower digital
level one.
Good luck
Chris