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

[ASSI]

On Donnerstag, 22. Februar 2007 07:07, Mike Beauchamp wrote:
> My VCO is "stable" in that it doesn't drift, however the frequency
> changes if I add more load (like an led or something) to the power
> supply.
>
> Maybe I'm wrong assuming that using power from voltage regulator
> chips gives me a "regulated power supply"?

The regulation is either not "good enough" or you are adding the load in 
the wrong place  - for instance after some resistance that is also in 
series with the VCO, while the reference point for regulation is before 
that resistance.  Your really want the supply for the VCO go directly 
to the regulator and all the other knick-knack that may introduce 
varying load on a seperate supply wire that joins the one from the VCO 
at the regulator (star topology).  If that is still not good enough, 
the oscillator core(s) need seperate regulation.

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.


Achim.
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