less hum: switching power supply?

[Heiko van der Linden]

Hi Jay,

What do you mean with the statement that these chips don't have short
circuit protection and overheat protection.

I think you know this already but they do have on-chip protection and I have
never had any problems with these little buggers.

Check:
http://www.national.com/ds/LM/LM7512C.pdf

What I'm interested in are what problems you have had with these IC's.
Where the problems oscillations due to the fact that the regulators were not
adequately decoupled for HF oscillations?

Greets,
Heiko

---
-----Original Message-----
From: Jay Martin <jmar at intface.com>
To: Tim Ressel <Tim_R1 at verifone.com>; 'Roel Das'
<Roel.Das at student.groept.be>; Synth DIY <synth-diy at node12b53.a2000.nl>
Date: Monday, June 05, 2000 6:30 PM
Subject: RE: less hum: switching power supply?


>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
>>
>