Jiggaman_RENAMED

Hey guys,

Just wondering is there any specific brand of capacitors better than another one or will any capacitor perform the necessary job? I was looking into purchasing an American Accessories 36 Volt Capacitor, is this good? I was also looking to purchase 3 of them instead of going with one 3 Farad Capacitor, do you think this is wise or should I just get the single 3 Farad Capacitor from Audiobahn?

By the way I seen that certain capacitors are 36 volt and others are 24 volt, but both of them have an Operating Voltage between 10 - 24 Volts, what is the difference between the 36 and 24 Voltage Capacitor?

Again, should I just stick with a single 3 Farad Digital Capacitor from Audiobahn or should I get three 1 Farad Capacitors from American Accessories? Either way both is 3 Farads, but which would be better? I think that 3 Digital Farads would look much better when installed in the trunk. Tell me what you guys think

Thanks!!

merlyn

what are your running that you need 3 farads for? I'm running 1k watts right now...and dont need more than a 1 farad. I like lightning audio products...but in reality (for the most part) a cap is a cap.

Southbound

I'm not sure either why you would feel the need for a 3Farad cap. There is a point of diminishing returns... But if it makes you feel better, who am I to argue? If they actually make 3Farad caps, I'd just go with that instead of paralleling (3)1Farad caps. If you were concerned about filtering out RF from your power supply. Then the best way to go would be to use (3)1Farad caps instead of ONE 3Farad cap. The reason is because electrolytic capacitors tend to become inductive at high frequencies due to the fact that they are two aluminum "plates" wound together with a paper solution of an electrolyte as a dielectric. Thus, they begin to act more like a coil than a cap at RF frequencies. However, you are only concerned with filtering out alternator hash in the sonic range and for the purpose of holding the power supply up during short surges of current draw. JUST REMEMBER THAT A CAP THIS BIG SHOULD BE TREATED LIKE A GRENADE!!! If it were to ever have the polarity reversed or an excessive voltage applied, it would BLOW like a grenade! There should be a pressure relief valve somewhere on the cap... MAKE SURE NOT TO BLOCK THIS IN YOUR INSTALL!

The difference between the 24volt and 36volt cap would be a better dielectric material which would provide better insulation between the plates. This would provide a better margin of safety before an internal short and explosion could occur. My guess would be that the 36volt cap would be bigger in size. This would be because the insulating electrolyte between the plates would be thicker. Again, this cap would provide you with a better margin of safety before it would have the potential to blow from overvoltage. But with 14volts as the max that the alternator puts out, 24volts should be fine... BUT WHAT WOULD HAPPEN IF THE VOLTAGE REGUALTOR WENT OUT AND THE ALTERNATOR VOLTAGE WENT SKY HIGH? :wow:

If you can take it... read on below and you will learn more about the effect of caps on your car audio system than you ever wanted to know. It was a reply to another thread asking about how to know whether or not the cap was working...

I don't have any experience with super high powered car audio systems. BUT a cap doesn't provide power, it simply stores it. What it provides you is a little bit of storage so that the power supply doesn't sag momentarily. If you put a prolonged current draw on ANY power
supply it will sag at some point.

In the case of adding a Capacitor across your battery, you are only adding a few milliseconds
of storage before the capacitor will drain down to the level of the lowest SOURCE of power in
your system. In a car electrical system, The alternator does supply the highest level of
voltage. It does this in order to keep the battery charged. The battery by itself DOES
SOURCE power. It is typically somewhere around 12.6volts. The alternator/rectifier and
voltage regulator typically put out about 13.8volts. You can measure this with a voltmeter
across your battery. It will be around 12.6 volts before you start your car and run up to
around 13.8 volts with the engine/alternator running. The alternator has a limit as to how
many amps of current it can supply before it falls down to the level of the battery, which in
turn has a limit of how much current it can supply rated in amp/hrs.

A simple Cap tied across all of this power can only help to hold up the voltage MOMENTARILY
when an excessive current draw on the system would otherwise sag the alternator and
battery voltage.

Don't expect miracles from a cap. It is useful to filter out alternator whine more than
anything else. There is a simple time constant involved with the charge and discharge time of
a capacitor. One time canstant is equal to the capacitance in farads times the resistance in
ohms. One time constant will charge a cap to approximately 63.2% of its value, two time
constants will charge it to approximately 86.5%, and three time constants equal 95%. It is an exponential charge and has to do with the natural logarithm of nature. It is theoritically
impossible to completly charge the cap to 100%. 99.999999999999..... is what actually
happens. MORE IMPORTANTLY. The discharge of a cap is just the inverse of this. One time
constant will discharge the cap down to 36.8% of its original fully charged value. Two time
constants will drain it down to 13.5% of the original value and three time constants will have
it down to 5% of its value, and so on.

For explanation purposes, Let's say for example you have a current draw on a 14volt system
which is pulling 50 amps. Ohms law tells you that the resistance of that circuit is 14 (volts)
divided by 50 (amps) equals .28 (ohms). That means that the power supply voltage source is
looking at a .28 ohm load! With even a giant 1 farad cap, the time constant of 1 times .28
says that the voltage would be drained down to 36.8% of its original 14volt value, or 5.15
volts in only .28 seconds (280 milliseconds). Obviously the power amp would shut down way
before 5 volts and the battery would be what was continuing to supply power long before
that point anyway. The battery would be taking over as soon as the rest of the supply fell to
around 12volts.

I could post the universal time constant formula, but it's too hard without the proper text
font here. SO let's just say that the cap is only important as long as it holds up the charge
between 12 and 14 volts. Once the cap drain falls below 12 volts, you are going to see your
lights dim, etc. and it will be the battery doing all the work at that point anyway, NOT the
cap. I've done the math, and for the cap voltage to drop down to say 82% of its full value,
which would be down to 11.5 volts from 14 volts.... IT WOULD ONLY TAKE 2/10THS OF A
TIME CONSTANT! .2 OF .28 seconds is only .056 seconds or 56 milliseconds! That's enough
time to only sustain one cycle of full voltage at 17.86 hertz!

BOTTOM LINE... A 1 FARAD CAP WILL ONLY HOLD UP YOUR SUPPLY VOLTAGE FOR ONE CYCLE
OF AN 18HZ NOTE, OR 56 MILLISECONDS with a current draw of 50 amps.

Just the facts... If you're experiencing your lights dimming due to an excessive current draw
from your stereo amp. Don't look for caps as a miracle solution. You could add ten of these
monsters, and you'd still only have enough reserve to keep you up for about a half of a second (560 milliseconds). IF you've got a sustained bass note longer than that, you'd still be in trouble. And you'd also have to have a period of little or no current draw to allow the cap to charge back up in time for the next heavy note to hit... If you are in a constant bass situation that is continuously pulling more current than the alternator can supply, then the cap would only help you out for the first 56 milliseconds, and then it would never be allowed to charge back up until the heavy current draw went away and the alternator again started increasing the voltage to again charge the cap.

You've either got some copper losses in the form of resistance in the wiring or wiring crimps
somewhere that are acting like big resistors during this period which is dropping the voltage
to your amp/speakers. If your system is pulling 50 amps, it would only take a resistance of
.04 ohms to drop 2 volts off of your voltage power source. However, this wouldn't account
for the lights dimming unless the wiring resistance problem is between the alternator and
battery or somewhere else in series with the lights, before going off to feed the stereo. You
might want to re-think the point where ever you tapped off the power from to feed your
amp... If there is no wiring resistance problem, then you are most certainly loading the
alternator more than it can handle.

Like I said, I haven't worked with these high powered car audio systems. I personally don't
need that kind of ground moving bass. But to each his own... I am, however, an engineer
who used to design professional audio amplifiers for stadium use, etc. I'm just trying to help,
even though I have no intention of modifying my system to the extent that it needs a bigger
alternator or battery or gigantic sized caps and double aught wiring... just to keep the lights
from dimming. I guess I must be getting old... fgf Yet, I hope some of this has helped to
clarify some physics to some of you...

preludeshfan

WOW finally someone on the board that knows electronics at my level. i havn't had to use any of that info since i was in EET. anyway you are 100% correct. but from personal experience i know a good farad of storage usually helps quite a bit in helping to reduce the light flicker. my guess is probly the milliseconds of reserve power a cap provides allows the voltage regulator time enough to react to the instanious surge on the charging system. i think that it takes a few milliseconds to energize the stator winding in the alternator before the additional current from the alternator can be realised. so possibly that is why it may help to some degree. who knows
1 thing i have found about car audio and you may also have noticed is the amount of hype in the industry to get people to buy unnecessary things that are supposed to help the sound of a system. i think it is a crime to sell some one RCA cable that has 14 ga wire in it to carry a 100mv-4 volt signal with an input impeadance of 1k ohm you don't need anywhere that gauge of cable to carry a signal that consists of a few microamps. and all the shield in the world won't protect the cable from getting noise. i buy cheap RCA's on purpose to thumb my nose at the industry. i have been using $6.00 cables for years and none of the installs i do ever have any engine noise. it is all in the way you install it.

Southbound

Thanks for the refresher course...

Like I said, I've never actually designed or worked with any circuits that actually required caps of this size. The largest power supply caps that I've worked with are around the 100,000 uF range.

It's hard for me to imagine a need for this kind of capacitance, so I was only stating facts from theory that I have needed to know. I have always had to be concerned with the fact that the larger an electrolytic cap, the more inductive it becomes. I am used to putting ceramic disc or mica caps across them to negate this property when filtering out RF frequencies. YOU ARE CORRECT to point out that the DCR also increases... Again, I've never actually worked with this large of cap or that kind of instantaneous current demand, so that fact slipped my mind.

SO YES, you would be better off to parallel the caps to keep the internal resistance down. I guess I drifted and fell back to looking at things from a lower current point of view, and was thinking about the physical space and wiring required to connect these monsters up.

Glad to see some other knowledgable people on board as well. THANKS for the correction! Also, preludeshfan, the fact that these caps do help with light flicker is a bit of a mystery to explain from a strictly theoretical standpoint. There are a lot of things going on all at once in this complex HIGH POWER scenario. I'll buy your alternator stator theory... sounds good to me
Southbound

nameeri

Southbound, that was a sweet dissertation on caps. "Oh the fond memories of Electrical Circuit Analysis 1"....