basic power-supply design question.

hello,

I want to build a ps wit lm317 and lm337 regulators.

the needed voltages are +/- 18V and +/- 15V

the transformer has 2 secondaries with 22V~

but I'm just aksing me where to connect the regulators for the +/-15V

in series after the +/-18V regulators??

or like the other regulators directly after the bridge rectifier??


thanks,
mat

Directly at the bridge - as 3V is too little for 317 to regulate predictably..

You will probably be better off with an 18+18V transformer rather than the 22V+22V. You'll have plenty of voltage to get +/-18VDC out of your regulators, even during a mild brown-out in Japan, and your heat budget will be much better.
And yes, you should run the 15V regulators in parallel with the 18V regulators, not in series. If you're worried about the 15V regulators running hot, then you especially want to follow my advice about the power transformer.

you *can* run a lower voltage trafo given that the trafo outputs a current that is overly sufficient. Many try this and pull too much current for the trafo to keep the rails nice and stiff, and the voltage droops. make sure that your VA rating is about 2 times what you actually need. this is cheap insurance.

Run the regs in parallel with the sec.

[quote author="Svart"]you *can* run a lower voltage trafo given that the trafo outputs a current that is overly sufficient. Many try this and pull too much current for the trafo to keep the rails nice and stiff, and the voltage droops. make sure that your VA rating is about 2 times what you actually need. this is cheap insurance.[/quote]

The other side of this is that you'll have more current capacity in a lower-voltage transformer of the same size (VA rating).

thank you !!

true, and an overspec'd VA rating will help with some current related noise too.