[urq] Electrical Experts - Question re. Capacitor installation

urq urq at pacbell.net
Thu Nov 1 06:51:57 PDT 2007

Another thing to consider is what can happen when the capacitor fails.  A
capacitor which has the characteristics you are looking for to filter the DC
will also be capable of providing a very low resistance path should it
develop an internal short circuit.  If such a device shorts out your stereo
it isn't going to be that big a deal, but for the ignition ... I think
that's probably one reason why you don't see it done.  

It sounds like you have already done what I would do were I concerned about
the power source ... run beefier wires.  That should be sufficient for you.
I did have questions about your "redundant" grounding scheme ... you do need
to be careful not to create ground loop situations.

Steve B
San José, CA (USA)

-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf
Of Ben Swann
Sent: Wednesday, October 31, 2007 8:07 AM
To: 'Mike Sylvester'
Cc: 'Ben Swann'; 'URQ'; quattro at audifans.com
Subject: Re: [urq] Electrical Experts - Question re. Capacitor installation

I don't think my wiring is inadequate or getting voltage drop.  I'm mainly
trying to be
proactive with my current supply since the electrical loadings are greater
than the
stock  wiring originally supported.  I have run extra current feeds, but
evaluating if a cap will add value, especially since there are going to be 2
drivers fully loaded, along with high RPM amplified ignition, the computers,
and then
add draw from lighting, fan, stereo, etc. that are not on this circuit.


I know I have power lag in the rear to my amp, even with 10 guage wiring.
That will
definitely get a cap, but I question the methods I've see to install the


I can always wire the cap to be hot all the time by putting directly off the
post, or I can install a slow charge resistor with a diode for when the
engine is off to
keep the cap charged and a switch to full current wire when ignition on.


The ignition is not something I'm doing today, but am thinking about doing
it if need
presents itself or as a safeguard.  I presently have at least one 6 guage
wire goine
from battery + to coil and injector feed.  That was added in addition to the
wiring that goes to the starter and alternator.  The #6 wire continues its
rund out to
the alternator and starter where it rejoins the factory wiring - I think is
a pair of #
6 wires, but was considered sufficient before, but I noticed a little too
much starter
draw, and the new run of wire definitely helped that.





From: Mike Sylvester [mailto:mike at urq20v.com] 
Sent: Wednesday, October 31, 2007 10:47 AM
To: Ben Swann
Subject: RE: [urq] Electrical Experts - Question re. Capacitor installation


I have not done this on any auto.

The did not read the link that you just sent, but from what you mentioned, I


Caps do not like to be changed and discharged rapidly.

For that reason, I will guess that the 1F cap you have has a high ESR.

That is fine if you are using it to stabilize the power supply of an audio


If you are trying to stabilize the power supply for the ignition coil, it
might not


It all depends on the rise time of the current draw by the coil.

It is probably fast.  If so, a cap with a high ESR can not supply current
quick enough
to keep the voltage from drooping.


If you get a 1F cap with a low ESR and connect it directly to +12V, you will
get a huge
instantaneous current draw when it has to charge up.  This can stress the
cap to
failure.  This type of application needs what is called a soft start
circuit.  This
circuit will limit the current draw at initial start-up to protect the cap
and anything
supplying the current.


The other issue I see is, I doubt that the +12V on the coil is hot all the
time.  It
must disconnect when you turn off the ignition.  With a cap on there, you
will be
keeping 12V at the coil after you turn off the key.


Have you measured the voltage at the coil with a scope while the car is
running?  Does
it sag when the coil fires?  If so, maybe a better solution is to run a
larger wire from
the source to the coil.


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