[sdiy] power supplies

[Colin Hinz]

On Wed, 25 Feb 2004, gregory zifcak wrote:

> i found this one, which apparently takes 16 or 12vac:
> http://sound.westhost.com/project05.htm
> would starting with 18vac give me higher output amperage? i'm not sure what
> my circuit needs. if i use 18vac, should i still use the 7815 and 7915? are
> there any other components that would have to be changed?

The main things to be concerned about are:

1) The electrolytic capacitors may need to be a higher voltage rating.
I would use 35V parts to be on the safe side. Looks like there's
ample room on the board for the higher-voltage parts. Go with even
larger electrolytics on the input side, if you can -- the AC is
merely half-rectified so you'll get *much* more ripple in the DC
which goes into the regulators.

2) If you're using a lot of current (hundreds of milliamps), you
may need beefier heatsinks. You need two separate heatsinks as the
heat conductive tabs will be at different voltages.

I've used this type of circuit with great success, though hand-wired
with solder lugs -- but having a nice PCB makes the construction
so much easier.

I disagree with the use of a 3-pin XLR connector for the power input,
though. One day, someone will pick up the power cable and plug it
into a microphone by mistake......*bzzzzt*. Reminds me of the
(apocryphal?) story of the guy who used regular two-prong mains
plugs to connect his speakers to his home-built amplifier. This
scheme worked well until someone unaccustomed to his wiring
scheme plugged the speaker cables into the house mains. I wonder
how far the cone moved before the coil melted......:=(

- Colin Hinz
  Toronto, Canada