Old Library

The rule of thumb to follow for capacitor size is generally 1 farad for
every thousand watts.  A capacitor really isn't necessary unless you are
doing a competition system, and our VR6's have such a strong alternator that
you will have little to worry about.  If you notice your lights dimming, a
cap may help in that case, but at that point an alternator upgrade would be
best for the car.

Greg


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> Subject: [gti-vr6] re: capacitors/stereo setup
&gt; From: &#103;koliopoulos&#64;molex<img src=/i/dc.gif border=0 width=35 height=15>
> Date: Mon, 1 Jun 1998 15:59:12 -0500
>
> Sorry for the off topic post, but I figured some of you may have capacitors
> installed in your car audio setup.  My question is are these installed inline
> with your sub amp or can you install them before you distribution blocks so that
> the entire system benefits by using them?  Also, what is the formula used to
> determine what size capacitor is needed.

Let me preface my remarks with the comment that I don't "do" car stereo;
I find my stock system "adequate".  However, I do "do" power supplies.
Great big ones.  Up to a megawatt or so.  Since the principles are
identical, let me explain the purpose of capacitors in an application
like this:

Capacitors serve as a "local" source of current during transident
periods of peak demand. In effect, they are a "local" rechargeable
battery.  You should have little or no need for capacitors if you have
properly sized wiring feeding your amps from the battery, and the
battery is up to snuff.  OTOH, if the wiring between your battery and
your distribution block is marginal, or if your battery is getting weak,
caps can help quite a bit.

IMO, the best way to determine whether it's worth adding capacitors in
to take an oscilloscope and look for transient voltage droop at the
power input to your amps (while you're playing your system in a
demanding way).  If the power is nice and clean there, caps won't do
diddly.  If you get substantial transient droop, caps will help
attenuate it.

As far as sizing the caps, to do it scientifically, you need to measure
the droop and how much current your amps are drawing at the time of the
droop.  From this you can calculate how much energy storage you need.
Let me know if you get that far and I'll help you with the calculations.

BTW, "Rules of thumb" for sizing the caps are all well and good, but
there's a lot of things they fail to take into account.

-Uwe-

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