[urq] Electrical Experts - Question re. Capacitor installation

[Ben Swann]

Folks – don’t start thinking I got bad wirin’ – just the opposite – most of it is new.
I pose these questions sometimes as a theoretical question to see if improvements can be
made over what is there already, or for some anticipated change in the systems -
Improvements on already massive overkill wiring and redundant grounds and power leads
and likely not necessary for the ignition part, but definitely applicable to the stereo.
The questions applied to two or three cars I’m dealing with, so just trying to gain some
addition insight on 12V power supplies.  My ignition in the UrQ is so far good through
7000 RPM, I just want to make sure I’m doing everything right, or if improvements can be
made.

 

Anyhow, my questions were answered quite sufficiently.  Enough to know what to do with
capacitors – some of information I previously got via internet search was entirely
incorrect or misleading.  Once again, the folks on these lists prove we are smarter than
a lot of other sources that claim to know their stuff.

 

Ben

 

  _____  

From: LL - NY [mailto:larrycleung at gmail.com] 
Sent: Wednesday, October 31, 2007 6:21 PM
To: Mark R
Cc: audi at humanspeakers.com; Ben Swann; URQ; quattro at audifans.com
Subject: Re: Electrical Experts - Question re. Capacitor installation

 

Hah! We're all right!  

One thing, IF the original power lead is inadequate and it's hard to thread the lead,
one other
option would be to run a parallel lead for power. I had missed the original point of
this, which was
to add the cap filter to an ignition system that was unstable. As Huw, Mark and others
stated, it's 
big no-no, you're building a RCL (resistance, capacitance, inductor (coil)) circuit, and
unless you 
like solving diffy Q's (differential equations), not a lot of fun, I think even Audi
does a better job at
that then we do. Add a bigger ignition lead. 

As for using a parallel cap for the stereo, go for it. It'll either help, or not. No
loss but time. 

Thanks to Huw, Mark, et al.

LL-NY - get to exercise my RCL muscles, which doesn't occur too often. Well, I flexed
them a bit. 

On 10/31/07, Mark R < speedracer.mark at gmail.com <mailto:speedracer.mark at gmail.com> >
wrote:

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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