[urq] Electrical Experts - Question re. Capacitor installation

Mark R speedracer.mark at gmail.com
Wed Oct 31 13:58:09 PDT 2007

As always Huw is more eloquent than myself on these things.  =)

Let me rephrase.... adding the capacitor is adding more load to the supply
system (alternator and supply wiring).  Think of a bank of 4 batteries wired
in parallel.  Yes, resistance is lowered, voltage is the same, but the load
in increased on the charging/supply system.  Adding a capacitor adds to the
load (albeit a small load increase).  I suspect >IF< Ben is having high RPM
voltage stability problems, it's likely the +12V line is inadequate due to
being undersized, poor quality, corrosion, or poor connections.  Or his
engine ground(s) is(are) poor.  A capacitor won't "fix" this kind of issue.
Hopefully, this is more clear (and I feel stupid for using an incorrect term
earlier in explaining myself.).   =)

Not many lists have such posts!  Ben's questions have made us think and
discuss.  How cool is that?

Thanks Larry and Huw!

Mark Rosenkrantz

On 10/31/07, Huw Powell <audi at humanspeakers.com> wrote:
> > Something new to my realm of expertise.
> >
> > I would like to install an electrolytic capacitor to act as a buffer for
> an ignition
> > system – sort of like placing a second battery up close to the
> load..  Since the
> > distance of the battery to the coil is about 7-8' and my understanding
> that a circuit
> > may lag,  it seems installing a large capacitor might be a good idea.
> If so, it makes
> > sense that I'd install it in parallel with respect to feed wire and
> ground, however
> > diagrams for most stereo AMP installations show the cap to be inline,
> with no other
> > current path to the amp.
> >
> > Is it OK to install an electrolytic cap. as sort of a buffer or quick
> discharge
> > batteryin parallel with the circuit?  That is basically to connect the
> positive end to
> > battery post near load and ground as normal, leaving the original
> positive feed path
> > intact.  I have some 1 and 2 Farad caps  and this is mainly to prevent
> lag at high RPM,
> > similar to stereo clipping at high volume.
> Many others have commented, saying most of what I thought in response to
> this, but I figured I'd chime in anyway.
> 1. Any diagram showing a DC filter/stiffener cap in series with the load
> is simply wrong.
> 2. If the wire to your coil is too small, replace it.  If the wire to
> the coil is too small to keep battery voltage at the coil, a capacitor
> is not going to help.  The current to keep the cap charged has to come
> through that wire.
> 3. It might make things worse, but my head hurts when I try to look at
> the equations in my old college electronics class notes.  But basically,
> while the coil is "recharging", the capacitor might also be trying to do
> the same, using *more* current.  Also, the capacitor will have a
> "discharge rate" curve, that may not coincide well with the operating
> cycle speed.
> 4. So in conclusion, run a bigger wire, and don't do the cap unless you
> figure out a way to get the circuit exactly right for the application.
> --
> Huw Powell
> http://www.humanspeakers.com/audi
> http://www.humanthoughts.org/
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> ---
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