fury

I'm looking for a capacitor. I was looking at a Power Acoustik 2Farad:

http://cgi.ebay.com/NEW-POWER-ACOUSTIK-2-F...1713.m153.l1262

2 questions:

1. What do you guys think of this capacitor or any other types recommended?

2. I'm pretty new at this, so I don't know if this is normal, but I noticed that it says 24 DCV. How would that work given that we put out 12 DCV??

fury

Almost all of the capacitors I look at have this 24VDC, in their specs. People tell me that it can be used in a 12VDC system. But I really don't understand WHY! Does the 24VDC rating on caps represent the required input voltage for it to operate or what? Does anyone know what this spec represents?

Stocker

Everyone starts at complete ignorance. Welcome to the club.

if you operate a capacitor above its rating it will potentially leak acid or asplode. YARLY.

If you operate a capacitor below its rating it will not be operating at its rated level. Theoretically there are some problems associated with this, but practically, you will probably have 0 problems.

Cars use a 'nominal' 12V system. They operate at over up to 14.4V normally, and can see spikes of 16+ if something bad happens. Running a 12V or even 15V rated cap in that environment is a potential disaster.

radu_rd2

The voltage rating on a capacitor is the MAXIMUM voltage it was designed for.

Why do you need a capacitor? Usually that's just a crutch that hides the real problem..

fury

Stocker, nice explanation! A BIG THANK YOU!!!! I completely understand this now! smile.gif

I'm in the phase of working on my interior. Electrical is what I'm working on now (leds all over, gauge cluster, gps, alarm, vent halos, deck, amp, subwoofer, ...). So, to answer your question, radu, because I'm doing a completely new sound system install, I figured I'd put in a cap as part of the install. I've read mixed opinions on how people feel about using a cap; some are for it, others aren't. I personally think it's a good idea to use a cap. I don't see any disadvantages. I have 2 amps, one is a(1000W peak) 50W RMS/channel (x4), the other is a 140W RMS (for my subwoofer). I'm really not looking for anything beefy, just a pleasant (upgraded) sound system. I like the idea of having a reserved capacitance for any extra power demands at peak times.

Again, your help, Stocker and radu, was much appreciated! I was really curious about that 24VDC spec.

radu_rd2

The problem with a cap is that it is inconvenient - you have to charge it with a resistor before you install it, and you have to do is every time you disconnect your battery (otherwise you would draw a big rush of current when you connect it). Pretty annoying if you ask me. If it had some circuit that would do this automatically, then yeah I would see no disadvanages (other than money paid of course).

The idea sounds nice in theory, but the problem is that at such a low voltage (12V), even with this huge capacitance, it doesn't store enough energy to make a big difference. (e.g. a fully charged 2 farad cap can give you 500 watts for only 0.7 seconds: http://www.google.com/search?hs=1CO&q=...%2F+(500+watts) )

I'm running a 4x105W RMS 4 channel amp and a 500W RMS sub amp and everything works perfectly, with no cap.

Stocker

sound system opinions are like hondas and bellybuttons but here's a piece of mine:

I'll spare you the explanation, but if you can hear yourself think, you are burning fewer than 5 Watts across those speakers.

If you turn it up to levels that make your headlights dim, you might consider a capacitor as a band-aid. It might make your lights stop dimming. It also takes your charging system longer to get back up to the proper voltage. Let me try to 'splain. Your bass hits and the sub amp sucks the cap down to a low state of charge. Then the charging system has to take a few milliseconds (?) or so to get it charged back up. OR Your bass hits, and the lights dim, and maybe the amplifiers start to clip. Those things happen because the charging system output voltage was drooping. When the bass hit stops, the load goes away, and the lights come back up and the amps stop clipping, basically instantly. Can you hear the difference? Only if you should be wearing earplugs (and muffs), and even then, you won't hear it for more than a couple/few years lmao.gif

If you don't turn it up that loud, it's cheaper and easier, and has about the same effect on the sound, to go without a cap.

DTN

Usually you are only using it for .01 second it saves your battery durring alternator dips between the surges. It's always a good idea to have a capacitor. They make your battery last longer.

fury

Yeah, that's true. It is inconvenient, but bearable. I mean, you charge it initially and then everytime you disconnect the battery, but then again, how many times do you actually disconnect the battery, right? So this one doesn't wouldn't discourage me from buying a cap.



Now, THIS POINT, would stop me from buying a cap. You are running close to 3 times the power I'm running on my system, and you have NO problems?!?! That's good to know. I wasn't sure what to expect in terms of power demand on my electrical system, so I thought of being safe than sorry, especially that my car is completely gutted now.




I like to turn it up loud, but I'm gonna try going without the cap for now, given Radu's postive (under-load) experience.

radu_rd2

You don't want to know lol.

Yeah, no problems here. Both amps in the trunk, fed from the battery through a 4AWG knukonceptz fleks cable. Also added a couple of 8AWG grounds between battery and chassis.

Oh, and I forgot the BIG argument against caps. Amps already have a fair number of caps inside their power supply. They have them where it makes sense to have them - after the step-up in voltage. The energy that a capacitor of a given capacitance can store grows with the square of the voltage. So 10 times more voltage, 100 times more energy.

So DTN, for those 0.01 second power dips you mention, the caps in the amps are more than enough.