cascaded voltage regulators?

[tempus fugit]

Hey all;

The project I'm currently working on calls for 3 different voltages: 12v (at
about 300mA), 9v (<100mA) and 5v (50mA). I was thinking about using 2
xformers (one for the 12v as it will power relays, and one for the remaining
9 and 5v). Either way, I'm going to need more than one regulator on a
xformer. My plan was to connect the 9v to the rectified out from the
xformer, and then cascade the 5v regulator by connecting it to the out of
the 9v regulator (giving an extra regulated voltage?). I know that this is
done, but is there any reason to do it this way as opposed to just
connecting the relays in parallel (i.e., the rectified out going to each of
the regulator ins)?

Also, is there any advantage to using 2 xformers instead of just one
(isolation from the relay switching for instance)? The 5v will power logic
circuitry, the 9v will be for effects pedals.

Thanks

[cpemma]

"tempus fugit" <tocatta...@ciaccess.com> wrote in message
news:1051498686.165669@arakis...

> Hey all;
>
> The project I'm currently working on calls for 3 different voltages: 12v
(at
> about 300mA), 9v (<100mA) and 5v (50mA). I was thinking about using 2
> xformers (one for the 12v as it will power relays, and one for the
remaining
> 9 and 5v). Either way, I'm going to need more than one regulator on a
> xformer. My plan was to connect the 9v to the rectified out from the
> xformer, and then cascade the 5v regulator by connecting it to the out of
> the 9v regulator (giving an extra regulated voltage?). I know that this is
> done, but is there any reason to do it this way as opposed to just
> connecting the relays in parallel (i.e., the rectified out going to each
of
> the regulator ins)?
>

You'd need to do the maths for your circumstances, but the spreading the
waste power over two series regs could mean smaller heatsinks. Though with
your currents that's not so important.

[Rich Grise]

You really should need only the one transformer, i.e. for the
12V supply. Then you could run the 9V reg. either off the same
supply or off the regulated 12v, and run the 5V reg off any of
the three - at the current levels you indicate, you won't have
to dissipate more than a watt or two (I'm too lazy to do the
arithmetic for you), and you were going to heatsink them
anyway, right?

No matter how you do it, the regulators are going to be
dissipating some power, so it's basically a matter of figuring
out where you want the majority of that power to be dissipated.
Then there's switchers, but they're _WAY_ out of my league!

Good Luck!
Rich

"tempus fugit" <tocatta...@ciaccess.com> wrote in message news:<1051498686.165669@arakis>...