less hum: switching power supply?

[Jay Martin]

I beg to differ,

The 78xx series of regulators do NOT have built in protection for shorting,
overheating or reverse-voltage damage (reverse voltage can happen when power
is removed but an output capacitor still has voltage in it.)  I have had
many circuits (especially EPROM and RAM types) destroyed by an LM7805 that
decided to start spiking at 8 volts.  I have never had the problem with a
Zener-Bypass transistor design or with the LM/SK309 or similar TO-3 based
regulators.  The LM309 has survived shorts, over-voltage spikes, reverse
voltage and will go into shutdown mode if overheated, protecting the
circuits being regulated.

Jay Martin
ISI/CLEO Technical Support.

-----Original Message-----
From: Tim Ressel [mailto:Tim_R1 at verifone.com]
Sent: Monday, June 05, 2000 10:16 AM
To: 'Roel Das'; Synth DIY
Subject: RE: less hum: switching power supply?


There is nothing wrong with using 7815/7915's. In fact, they are quite clean
on
the output. The standard trick to clean up the output of a noisy switching
power
supply is to use a low-dropout regulator on the output. Use plenty of
heatsink,
and make sure your filter caps follow the 2000uF/Amp rule. And keep in mind
they
are only good for 1 Amp.

There are lots of books that explain about power supplies, including Barry
Klein's.

I have gone with a Power-One linear supply +/-15V at 4A. Problem solved.

Tim Ressel--Compliance Engineer
Hewlett-Packard
Verifone Division
916-630-2541  
tim_r1 at verifone.com                     



-----Original Message-----
From: Roel Das [mailto:Roel.Das at student.groept.be]
Sent: Monday, June 05, 2000 7:20 AM
To: Synth DIY; Jay Martin
Subject: Re: less hum: switching power supply?


Hello Jay, others,

I am VERY interested in anything that has to do with power supplies. I'll
start building my modular in 2 weeks (after I graduate, woohooee, finally),
and the first thing you need is a powersupply, i guess... My midi cv
convertor still needs a decent psu too, it's now running on a very basic
7815/7915 design, even halfwave rectifier... Might be good enough, but I
want better.

Thanks,
Roel


----- Oorspronkelijk bericht -----
Van: "Jay Martin" <jmar at intface.com>
Aan: "chris" <espace at e-server.net>; <synth-diy at node12b53.a2000.nl>
Verzonden: vrijdag 2 juni 2000 20:23
Onderwerp: RE: less hum: switching power supply?


> OK, I have to put my $.02 in here.  It has been my experience that the
> biggest problem with linear power supplies is that most of them try to
make
> the regulator do too much work, and all they use for filtering is
> capacitance.  If you want to make a "clean" linear power supply, all that
is
> needed is a true "pi" type filter after the rectification using both
> capacitors for Voltage storage and inductors for Current storage (and to
> keep the current and voltage in phase).  The regulator would follow all of
> this and would have a very clean input so it would be able to deal with
the
> rest of the ripple.
>
> Trying to use a switching power supply is probably the wrong approach.  It
> is very difficult to get rid of the RF that is introduced, and the
circuits
> can be downright dangerous to work on (RF has much more transient
energy!!)
>
> Something like this approach is what I'm trying to convey:
>
> >From full wave bridge >-----+---^^^^^^---+---^^^^^^-----> To regulator
>                             |     L1     |     L2
>                             |            |
>                           C1=          C2=
>                             |            |
>                             |            |
>                Ground >-----+------------+------->
>
> L1,L2 are inductors, value is probably not critical.
> C1,C2 are capacitors, I prefer at least 3300mF ea.
>
> OK, So it's not a "pi" because of the extra L2, but I forget the name for
it
> with the extra inductor.  Also the values can be calculated so that the LC
> of the network produces a proper phase correction..but don't ask me what
the
> formula is...I'd have to dig out the reference manuals and they are at
home.
> It usually ends up not being too critical anyway.
>
> Also over design the thing.  It has also been my experience that the noise
> ripple will go up dramatically the closer you get to the tolerance of the
> parts.  For example, if you are trying to draw 1 amp from a 1 amp power
> supply transformer..there will probably be noise no matter how much
> filtering you do.  A good rule of thumb is to design 3 times the value
that
> you plan to draw.  If you are planning on drawing an amp, use a 3 amp
> transformer and the appropriate support components to match.
>
> One final note, use the TO-3 based regulators instead of TO-220 based (SK
> parts instead of the LM78xx). the regulation is much cleaner on them.
> Another idea is to use a zener-to-transistor regulator.  If done right, it
> will regulate better than any of the IC based ones and even has some
ripple
> compensation in the form of a "super capacitor" (the capacitance actually
> gets amplified by the bypass transistor).  If anyone is interested in this
> type of design, request through the group and I'll dig out the schematics
> that I did years ago.
>
> Jay Martin
> ISI/CLEO Technical Support
>
>
> -----Original Message-----
> From: chris [mailto:espace at e-server.net]
> Sent: Friday, June 02, 2000 11:20 AM
> To: synth-diy at node12b53.a2000.nl
> Subject: less hum: switching power supply?
>
>
>
> hello friends on Synth-Diy !
>
>    i'm usually lurking around here, sorry for the sudden surprise
entrance.
>    in a quest to totally eliminate the hum in the audio circuits...
>    i've decided that switching power supply is the way to go to get rid of
> the big 60-cycle EMF.
>    could anyone point me in the direction of a small, lightweight,
> onboard-style switching power module: input 120VAC, output +/-15V @ 500mA,
> +5V @ 1000mA.
>
> christo
>