wallwarts and regulation..

Jay Martin jmar at intface.com
Thu Mar 5 22:31:14 CET 1998

Better solution would be to use a 7805 with a diode forward biased in
the ground connection.  Should give you 12.7v with decent ripple
rejection (with the proper cap's off course)  I doubt that the .3v will
give you a problem.  If it ABSOLUTELY has to be 13v you could add a very
small trimpot to the ground or add 2 diodes forward biased..one silcon
(1N914) and a Germanium diode (germaniums have about a .3v forward bias
drop and silicon have about .7v).  Something like this:

IN -------7812------OUT

Again the diode would be FORWARD biased,,not REVERSED.  We are not
trying to block voltage, just trying to cause a slight voltage drop.
And don't forget filtering.  If you want me to CAD up a sample
schematic..let me know and I'll whip something up.


> -----Original Message-----
> From:	media at mail1.nai.net [SMTP:media at mail1.nai.net]
> Sent:	Thursday, March 05, 1998 1:09 PM
> To:	cyborg0 at GlobalEyes.net
> Cc:	synth-diy at mailhost.bpa.nl
> Subject:	Re: wallwarts and regulation..
> >But, i have never had to use a wallwart to power anything before that
> >needed 13v supply...I dont even know how to go about regulating it to
> >13v.. Maybe if i just build a low-pass passive filter just to filter
> out
> >the 120hz ripple..Im really in a quandry, since i never had to do
> this..
> >anyone know of maybe a one chip solution to this, such as perhaps a
> >programmable 7805 type regulator?? I need something ultra low power,
> so
> >i can get it into the Oddy once its working properly..
> I don't know about low power (you'll have to ask someone skating
> around
> Australia :) but 317 and 723 are very popular adjustable regulators.
> A 723
> has 14 pins, requires quite a few extra parts and is usually used to
> build
> more elaborate power supplies -- switching, high current, regulated
> current, etc.  I've used the 3-legged 317 to ape 9-volt batteries --
> all it
> requires is two resistors.  It has a 65db (??) ripple rejection that
> can be
> increased if you throw in an extra cap or two (one bybassing the
> resistors
> and an .1uF as an LPF at the output).
> There are dozens and dozens of voltage regulator chips available.
> Still,
> no one will tell me why a 7815/7915 isn't good enough for a modular .
> . .

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