Rich said:
With enough capacitation, 16V is quite comfortable headroom for a 12V
regulator, if it's designed right.
I must point out, however, that spec'ing it at 1.2-12V would make the
design considerably simpller, if you're going to use an LM317.
Good point. To get 1V out, he could adjust it to
around 1.6 and put a diode in series. And that ~16.9
volts is AC peak before rectification, not the voltage
across the cap, so the 16 volts you mentioned is the
correct approximation for a single diode drop. (A
typical 1N540x will drop about .9V at 3A)
But he's not out of the woods. The LM317 won't handle
3A. He could use an LM350 instead. Still, the 16 volts
won't be there if he uses a bridge. It will be ~ 15.1
volts at 3A. Add in line voltage variation, transformer
sag, and ripple, and he'll have problems.
I think for most cases the supply would work - but most cases
are at well under the max current and with the line voltage
closer to 120 than to 110. For a solid 12V design, he needs
a different transformer.
Ed