[sdiy] "Load" on power supply changes pitch of VCO...

[Mike Beauchamp]

Thanks again for the replies! As I said earlier.. I seem to have
"fixed" the problem by increasing the filter capacitors after the
regulators from .01uF to the .1uF caps recommended in the regulator's
datasheets (and as I had specified on my schematic.. duh!). Datasheet
says to use .1uF caps for "stability".

Changing this seems to have stopped any "drop" in voltage by adding a
load, and it had the side-effect of completely cleaning up the bit of
irregular noise I was hearing from the really high-pitched output from
the VCO.

I DID think that my 12VAC adapter was the problem, however I did the
math before-hand and I checked everything with my meter, as that was
the first thing I figured was the problem too.. insufficient supply.

At the load I am putting on it, the 1A 12VAC wallwart is actually
putting out about 14-15VAC.  Once it gets rectified it "should" be
putting out + and - 1.414 times that 14-15VAC, minus the drop for the
rectifier. Real-world measurements I got after the rectifiers are
19.6VDC going into the 7815 and -20V going into the 7915. Which seems
to be well above the minimum required input for these regulators...
correct me if I'm wrong here.

If this problem somehow pops up again, then I will make sure to do
measurements and see if the voltage regulators are getting enough
juice. Since their max is 35V or so, there's no harm in me feeding
them more than the 20V they are getting now. I do have some pretty
good size heatsinks handy just incase. But for now, it seems as though
the lack of .1uF capacitor seems to have made a huge difference...
weird.

Mike

On 2/22/07, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> >
> > That all said, I'd think that the 12V AC wallwart is a bit on the low
> > side with regard to the minimum drop the 7815 and especially the 7915
> > needs to see to regulate the output well.  Depending on the exact make
> > you need around 3V or more drop across the regulator for good
> > regulation over the full load range - have a look at the datasheet.  As
> > more drop means more power dissipation, you should check the cooling as
> > well.
>
> I second that. Try changing it for a 18VAC transformer and see how
> things go. A bit of extra headroom on the power supply can prevent all
> kinds of weirdness.
>
>
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Mike Beauchamp
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