Filter caps WAS: bypass caps?

[J. Larry Hendry]

Us mind?  Why heck no !!  Any light shed on this issue on the load side of
the power transformer would be enlightening.  Bill's well thought out
comments got me to thinking about things I never before considered. 
However, I have changed the subject line since this thread has somewhat
broken loose from the original post concerning bypass caps  Go for it Art !

My area of capacitor expertise lies in their high voltage AC application. 
Since that is OT to this list, I shall leave any questions concerning that
to private e-mail.

Thanks to all for making this such a great list.
Larry

----------
> From: Arthur Harrison <theremin1 at worldnet.att.net>
> Bill Layer had a though-provoking post last month in this regard, in
which
> he responded to a comment from J. Larry Hendry. I've been meaning to
> do some tests to investigate the relationship of PS capacitance to
> line-frequency noise, and specifically see if a real-world example of
> "more is worse" could be created.  Keep in mind that the capacitor
> at issue in the following post is the pre-regulator "bulk" filter, not
the
> capacitor at the regulator output.
> 
> Anyway, I am taking the liberty of reposting Larry's question and Bill's
> response.  (I hope y'all don't mind):
> 
> Bill Layer:
> 
> Hi Larry, All,
> 
> J. Larry Hendry:
> 
> >I was having trouble following the logic that a linear power supply
using
> >solid state diodes would cause distortion of this type.  Then the
obvious
> >hit me like a brick --  the diode does not start conducting until the
> >voltage magnitude of the sine wave overcomes the voltage required to
cause
> >the diode to connect.  So, at each zero crossing, you have a shut off
> >period on both sides where neither diode is connecting.
> 
> Bill Layer:
> 
> This is true, but the zero-crossing is not always the ugliest hour for
the
> SS rectfier. Imagine a simple single-wave rectified supply, with a single
> diode feeding a single filter cap. Assume that this supply is unloaded,
the
> voltage across the capacitor now rises to the peak of the AC waveform,
> about 1.414 x RMS. Assuming that no current is drawn (not the case, for a
> real cap) the voltage remains constant AND the diode never conducts
again.
> 
> But when current is drawn, something else ocurrs. The voltage across the
> cap falls, and now the diode wants to conduct, BUT ONLY at the peak of
the
> waveform. Now, the filter cap tends oppose that change in voltage (the
> bigger the cap - the bigger the opposition... read on), and when that
diode
> does switch on - look out. The AC waveform is at the near peak of it's
> power cycle, and when the diode switches on, the instantanious current is
> large. If the supply voltage now falls to 1.2 X peak, this means that for
> over 10% of the waveform, the transformer and rectifier are pushing
against
> a brick wall.
> 
> To an extent, this also relates to the fact that given two otherwise
> identical power supplies, more current may be drawn from an inductive 1st
> filter than a capacative 1st filter design. Peak currents are lower in
the
> inductive filter.
> 
> What this means to me, is that the guidelines for filter cap size in
power
> design may need to be re-thougt, on the basis of 'bigger is not better'.
> How would you like to find out that that ultra-quiet 10X overfiltered
> supply you built is doing more harm to the overall sound of your system,
> than good for the device it is powering! Obviously the extreme case, but
> the logical end nonetheless.
> 
> Any thoughts?
> 
> Bill Layer
> Sales Technician
> <b.layer at vikingelectronics.com>
> 
> 
> --------end archived post---------
> 
> Steve Varner:
> 
> >It seems that a lot of other people on this list are using this size cap
on
> >the power lines and smaller ones into each IC. I was under the
impression
> >that the caps on the ICs were usually only necessary for digital
circuits
> >and for op-amps that handle low-level signals.
> 
> Art Harrison:
> 
> Ironically, some digital circuits built on power/ground plane boards need
no
> bypass
> caps at all.  I was informed that the stored energy exists due to the
> topology of
> the planes.  Something about the "H" field, but I'm no expert in these
> matters, so
> perhaps a lurking physicist may care to comment.
> 
> In some instances, the use of bypass capacitors for linear integrated
> circuits is
> desirable for stability, regardless of power supply noise.
> 
> Last thought:  In many instances, large output capacitors can actually
cause
> instabilities
> in a regulator.  !00uF of tantalum loading a 7812, for example, has cause
> oscillations,
> in my experience.
> 
> 
> 
>