[urq] Electrical Experts - Question re. Capacitor installation

Huw Powell audi at humanspeakers.com
Wed Oct 31 11:14:09 PDT 2007

> 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



More information about the urq mailing list