[sdiy] zener regulators and negative supply voltage.

[Theo]

Well not really wrong, but your probably are making things hard for your
self. Killing the ripple with a zener and a RC or LC filter no longer makes
sense for most applications. Exceptions include high voltage supplies for
tubes and exotic things like that.

Did you look at linear regulator ICs like the LM317 or the 78XX and  79XX
series?
The 78xx and 79xx are for a fixed voltage and easiest to use.
The xx indicates the voltage and the 78 is for positive and 79 for negative
voltages.
The 317 and cousins can output any voltage set by the ratio of two
resistors.

Jim Patchell has a good example for the 78/79 and in depth explanations of
calculations on his site.
http://www.silcom.com/~patchell/powersupply/powersupply.pdf
http://www.silcom.com/~patchell/powersupply/powersupply.html
There are zillions of other examples of this on the web too.

However if your application does not drain a lot of current you may forget
Jim's calculations.
470uF or 1000uF for the input cap and 47uF or 100uF for the output cap
should work in ok. Smaller caps may be ok too, but 470uF or 1000uF are no
longer that large in size anyways.
Don't forget the100n decoupling caps though they are important!

Adding 22n to 47n caps in parallel with  your rectifier diodes (or bridge)
will help kill the switching transients from the diodes.
But this is a fine tuning detail and not needed.

Note that the regulator ICs need a input voltage that is at least 2V higher
than the output voltage to work!
Also note that the current rating for the ICs if only valid if you add a
cooling body to the IC.
Without cooling most ICs can handle a little over half the rated current,
depending on the input voltage.
Just to be on the save side I usually use the 1A rated ICs even if I only
need 300mA.

HTH
Theo


From: Arun Bohm <arun_bohm at hotmail.com>


> Yeah I am trying to build a power supply. Where am I going wrong? So far
> I've set up a bridge and capped it. Just trying to get the ripple out.
I've
> always only worked with breadboards at the work bench so this will be my
> first attempt at actual fabrication.
> Thanks.
>
>
>
>
>
> >From: "Theo" <t.hogers at home.nl>
> >To: "Arun Bohm" <arun_bohm at hotmail.com>, <synth-diy at dropmix.xs4all.nl>
> >Subject: Re: [sdiy] zener regulators and negative supply voltage.
> >Date: Sun, 22 Dec 2002 23:11:39 +0100
> >
> >Maybe I get you wrong, but I smell something burning.
> >What exactly do you want to build?
> >If it is a power supply, you might be on the wrong track.
> >
> >Theo
> >
> >From: Arun Bohm <arun_bohm at hotmail.com>
> >
> >
> > > I would just reverse the reverse bias for a negative supply voltage,
> >right?
> > > is it smart to use an inductor to elliminate ripple or would that
create
> > > transients at the power out.
> > >
>
>
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