FAQ- Why a cap is useless!

Pimento

New Member
caps=crap

The only cap ive had that made a difference was a rockford 50 ohm one, and it only raised voltage for about a minute or so.
 

Blue3.8

New Member
you guys flame too much.

Mostly I hear of people using a cap to stop their lights from dimming when they drive with their sub(s) on pretty loud. That would make sense because the only charge from the cap you would need was that to make up for the power draw from your amps/subs when they hit. The cap takes some blow from your car's electrical system for that extra half second or so when the amp(s) need that extra power. You only need as much exra charge to power your headlights/whatever for the difference in power consumption there is from dimmed lights to fully functional lights.

Main point I'm trying to make: Less than half a second for cap to charge - half-second extra amp draw = full power everywhere else in your car.

Makes sense to me... I could be totally wrong though
Nice post wexdafid :thumbup:
 

bj350hp

New Member
you guys flame too much.

Mostly I hear of people using a cap to stop their lights from dimming when they drive with their sub(s) on pretty loud. That would make sense because the only charge from the cap you would need was that to make up for the power draw from your amps/subs when they hit. The cap takes some blow from your car's electrical system for that extra half second or so when the amp(s) need that extra power. You only need as much exra charge to power your headlights/whatever for the difference in power consumption there is from dimmed lights to fully functional lights.

Main point I'm trying to make: Less than half a second for cap to charge - half-second extra amp draw = full power everywhere else in your car.

Makes sense to me... I could be totally wrong though
Nice post wexdafid :thumbup:

thats the only reason I can think of to get one......:sleep:
 

JayLS5

New Member
Why a Capacitor is NOT useless (corrections need to be made to the FAQ-Why a CAP is useless!!!! thread)

I am writing these because of all the incorrect post that have been posted to the Why a CAP is useless!!! Thread. The thread appears to have been brought to and end by Urainium with some people agreeing with what he wrote. While the post by Urainium is very insightful with lots of equations and calculations it is also incorrect.

The problem with Urainium’s example is that he has made a very bad error based on an extremely poor assumption that any engineer with his education background should have realized. While the formulas and equations themselves are correct he poorly assumed the resistance of the wire between the cap and the battery being to high. He as also incorrectly assumed the amplifier’s resistance. These misassumptions threw off his RC calculations therefore ruining any supporting evidence.

“Your wire from the battery to the cap has a total resistance of 1 ohm” – Urainium

If the resistance is 1 ohm your amplifier would not even work because there would be too much voltage drop across the wire. An amplifier is capable of drawing anywhere from say 1 amp to 100’s of amps. If the wire were 1 ohm then by I=V/R you could only have a maximum total current draw of 12 amps. 12 amps X 12 volts means we could only have at max a 144 watt amplifier. This is just not going to happen. The actually resistance in the wire between your battery and amplifier is many times smaller than 1 ohm. This is why your capacitor example is flawed and I will post corrected charge and recharge times for you later.

According to Introductory Circuit Analysis Seventh Edition by Boylestad, if we were using a 10 AWG# wire the resistance per 1000 ft. would be .9989 ohms. Let’s say we are using 15ft. of wire from battery to Cap. That comes to .015 Ohms for 15 ft. of wire. Now I did not calculate the wire from the negative of the battery to the cap because as we all know it is grounded as soon as possible directly to the car. So for this example we will use 15ft. of wire.

Let’s not get the capacitor charge/discharge calculations just yet. We have one more major error to fix.

“When we discharge, the R is 4 ohm (for the speaker load).” – Urainium

If we assume that we have 1 amplifier with 1 speaker attached then by I=V/R the most we could apply to this speaker (assuming a 100% efficient amplifier) is 3 amps. Well we all know that our amplifiers put out well more than three amps to our speakers so I will give us an estimated resistance of the amplifier to the battery by using known values and ohms law. Let’s say that we have an amplifier that outputs 500 Watts to the speakers and the amplifier is 70% efficient.

Output Watt / % efficiency = Total Watts consumed by amps and speakers
500 Watts / .70 = 714.28 Watts (lets keep it simple and say 714)

Now lets find the resistance of the amplifier

Amps
Watts / Voltage = Amps
714 Watts / 12 Volts = 59.5 Amps

Ohms
V / I = R
12 Volts / 59.5 Amps = .20 Ohms

Now we can do the RC circuits.

Charging the Cap

Let’s say we are using a 1 Farad cap. 5(time constants) * R * C = Time to charge in seconds. This comes to .075 seconds to fully charge a cap. Now I can tell you there is a problem here in that the calculated current draw at first is well more than what the battery could supply. So this .075s is not the actually amount of time but it is MUCH faster than the time you would get using Urainium’s incorrect model.

I’m not going to go into the rest of the charging and discharging of the capacitor at this point since I have already busted the model that for some reason this board has accepted to be correct. It will take a little longer for the cap to charge from 12 to 14 (about .045 sec) than to discharge from 14 to 12 (about .03 sec if the cap is the sole source, will probably last about .06 seconds when working together with the charging system). But at this point you need to take into account that music is transient and will be changing its current draw continuously so it would be interesting to see a chart of capacitor charging vs its discharging taking into account the ever changing current demand on the system

Why a capacitor is not always useless

I can speak from experience that some lower quality amplifiers will greatly benefit from the addition of a properly installed capacitor. I have also found that higher quality amps really don’t gain any improved sound quality from a capacitor.

By no means is a capacitor worthless and it does not increase the load on the charging system anymore than if it wasn’t there. A capacitor is not going to act as a band-aid for a weak charging system in anyway but it will act to stabilize voltage in any system. Its sole purpose is to oppose changing voltage.

If you think that a capacitor is totally useless then I invite you to take out the large stiffing caps that are inside your amps. Removing these caps will reduce performance and sound quality of your amplifiers. Adding more capacitance may help in some situations but never hurt. Before deciding if a cap will not help you should try one on a trail basis from a friend and see if it helps any. If it doesn’t then you know you don’t need one.

Bachelor of Science in Electrical Engineering from Purdue


I never checked Uranium's math, but judging by your criticisms I can see he was dead wrong.

That wasn't the basis of the argument anyway. The main point was that the authority on the matter said they were nearly useless. Countless times it has been shown to be a band-aid at best for power aids, in the real world.

If you hook up a completely drained capacitor to the car's battery, it will bog the motor. It pulls more power than the alternator puts out, and pulls it so fast that the strain on the alternator causes the engine's RPM to drop. I witnessed this happen. When someone's system is already stressing the alternator, how would a 1F cap help? Once the cap's small power supply is used (i've monitored how small it is), it just adds strain on the electrical system it purports to help.

Nobody is arguing that it is a great voltage stabilizer; in fact, the first post of that thread says that specifically. One can use it for amps with insufficient internal capacitance, but most amps you buy today don't need it and would have no noticable difference in sound quality. For the audiophile, it might make a tiny difference - but for the general audience seeking to buy these things, they do it to keep their headlights from dimming. Most of us know that it wont help much, if any, in that arena.

For the sake of righting previous mathematical errors, i'm going to work this into the original thread.
 

Member02

Audio GOD
I just wanted to say awsome! I'm glad people are at least technically interested in the truth, I read the added posts, been a long time since I had this put up...

I still stand on my side of the issue, but in the process of the following arguements I learned a couple things.
 

knappoleon

New Member
What's worse, is that in order for a 1 Farad cap to discharge, first the alternator output must have maxed out, and the voltage must have dropped around 1.5 volts. But I thought a cap was supposed to prevent that (voltage drop)!!!!!????? Yep, you got the point.
Im afraid you dont understand the amplifier has a switching power supply inside and no battery can keep up with it. Thats where that low ESR comes into play in which a capacitor has.

Hate to do it but your entire post goes into the "MYTH" category.

Also you never put a uncharged cap on a vehicle. Duh. Both the undrained cap and the battery are both LOADS on an alternator. So if you want to waste your money on merely adding a second battery, be my guest and time will tell on your alternator. Not trying to be combative but Ive done installs for years and speak from experience. Just trying to save someone a headache from some misinformation.
 

knappoleon

New Member
In fact...just to keep things simple and not speak over anyone's head...pop open a power suppy, your computer, a radio shack kit, whatever. You'll find capacitors and not just transformers.

Ask yourself why? Whats its purpose in the circuit?
 

VolitioN

New Member
long story short i c where all the disagreements come from of course, but im glad i got mine for cheap....

i previously had the wiring wrong, cap was sucking pwoer from the amp, which caused everything to dim, no power from subs....but once wired correctly, dimming was prevented....if i were to do it again tho i'd just get another battery to run the subs seperately..
 

knappoleon

New Member
Because its not always how fast your car is...

Ever heard of SLAP?

Sound
Looks
and
Performance


It is it's own competition circuit.

Sometimes its just about enjoying your music and being able to pair it with what you love most...your ride and your friends and being able to share that love with everyone else.
 

Jesse

The Voice of Reason
Why have a system anyway, if it doesn't make you go faster you don't need it, even in a V6. Sorry just had to put in my 2 cents.
sounds like a statement from someone who has either never had a system, or who has never owned a fast car before, let me guess you started moding your 6 and got the bug now you think because its slightly faster you are the ****. There more to the world than your slow little car, in the last month ive built a 500hp car and a car that does over 150db and they are both very enjoyable in their own right.
 
You guys are tough, I just know I can get every bit of performance, from wherever I can, oh and Jesse I have an 03 Cobra so I know a little something about speed, and Gacivic you have a Honda and your on a Mustang site so um who's the douche. As far as a system goes the factory MACH 460 is all the system I need. I see what you get when you say something here, you get replys by a bunch of kids. Thanks
 
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