Whats a Farad?


Unregistered guest
Farad - The basic unit of capacitance. A capacitor has a value of one farad when it can store one coulomb of charge with one volt across it.

A-Hyuk. of all the things ive tried to learn, i just dont get this one. half of everyone says their nearly-necessary for large wattage systems, and the other half say their worthless. I want to know what a 1 Farad CAP will do for two 1000 watt amps bumping JL 12W7s hard. I originally thought that a 1 farad just sounded weak, and wanted to go with a 2 farad for each amp.
Now im seeing electrolytic and carbon combination capacitors, and just plain carbon caps that reach up to 30 farad. If i can justify the boost a 10 farad cap -carbon or electrolytic or both- will give to my system, ill put one on each amp i have.

i gotta know what the hell a farad is, and how much power it dumps out. if its as weak as people say it is, then how many weak farads do i need for it to do something. I will not allow inconsistencies in my power system...

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 1749
Registered: Dec-03
think of 1 farad as the measure of how much charge a capacitor can store when it's fully energized.
a capacitor stores current then releases it as needed. the problem is that a capacitor releases it's charge rather quickly, then needs to be re-charged by the source of electrical energy, which happens to be the alternator.
if the alternator is too weak to supply the current needed for the audio system, a capacitor will discharge within a matter of about ten to twenty seconds, then will no longer be of any use.
*IF* the alternator is beefy enough to handle the audio system, and still have some reserve left if needed during normal use, and something draws more current than the alternator can supply for a brief moment only, then the capacitor will act as a reserve supply, giving the needed current, then recharging itself afterward.
The problem is that this is rarely the situation.
Most car stock alternators are fairly close to what the car already requires for current to handle things like horn, lights, wipers, electronics and spark for the motor, and so forth. When you start adding on big amplifiers that draw a lot of current, there's no way the alternator will be able to keep up.
The solution is to upgrade the alternator.
Once this is done, there is no ned for a capacitor anyway really, as the alternator sill now be able to supply all the current you need.

Unregistered guest
duh! Hi Glass, i shoulda known youd be the first to answer.. shoulda just addressed the Q: to you.
So... 1 Farad is about 1 Volt? I dont get how much juice one would dump, and if you have the extra juice to keep it charged to its optimum every millisecond or so, then that would power the system right?
I knew the cap was a resevoir, and knew it didnt provide a constant stream of added power. and i figured it dumped to the amps, or the subs, whichever way theyre wired, at the deep notes, or the ones where the bass hits really hard but seems not as loud as it just was the note before. know what i mean? of course you do.. im an idiot..
anyways, with that thinking in mind, i thought some good caps would give an added BOOST, past the voltage the amps were putting out. but not too much to heat anything up too much, or damage anything. another juicy option (all with juicy alternator) would be another amp, one powering the two amps? or 3 amps together, powering the 2 W7s. but why... huge cost, more wiring.. maybe no room. blah.. so i thought about caps.

On a juicy system where enough is there to power everything, even the subs with the gain all the way up, or as safe as possible, would any big Caps do anything to help?

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 1768
Registered: Dec-03
"So... 1 Farad is about 1 Volt? I dont get how much juice one would dump, and if you have the extra juice to keep it charged to its optimum every millisecond or so, then that would power the system right?"

ok this page will help you understand a farad (measure of capacitance) and coulombs and capacitors:
as for recharging, a capacitor charges much slower than it can discharge, so to recharge fully could take up to 2 to 5 minutes.. discharge can take a matter of seconds. A capacitor doesn't create electrical energy the way a battery or alternator does. A capacitor merely stores energy then releases it.

" and i figured it dumped to the amps, or the subs, whichever way theyre wired"

to the amplifiers, yes, as the amps demand more current than the alternator is capable of supplying.
ideally you don't want voltage to drop, otherwise you incur clipping.

"at the deep notes, or the ones where the bass hits really hard but seems not as loud as it just was the note before. know what i mean?"

this could be caused by the port tuning of the box. when voltage drops and peak current is demanded, this is usually when the most power is used, and lower frequencies usually need more power to reproduce at what we as humans consider acceptable levels. (our ears are less sensitive to bass so looking at a C-weighted sound curve, what the human ear perceives to be "flat" it's actually an inverted bell curve. more bass, more treble. We call that pink noise.)

" i thought some good caps would give an added BOOST, past the voltage the amps were putting out."

nah current demand works like this.. when the amplifier tries to reconstruct the input line level signal at the output stage, with more power, it does so by increasing the amount of current it uses, to produce a higher voltage (AC) signal to the speakers. Current demand is on an "as needed" basis. The capacitor won't supply anything more than is asked for by the amplifier.
Ideally, we want the amp to be fed all the current it needs, with more in reserve in case it's required on a moment's notice.
This is why we want the alternator (the main source of electricity when the motor is running) to be capable of putting out at least as much current as the combined amplifiers will draw at peak output (RMS values)
at best, all a capacitor will do, if the rest of the electrical system is beefed up to par, is smooth out spikes in current demand so that the current draw is more steady instead of spikey.
This won't reduce the amount of current needed in any way, but by smoothing out the demand, it will, theoretically, put less stress on the guts of the alternator, extending it's life.

"with the gain all the way up"

never turn the gains all the way up.
remember "gains" more properly called input sensitivity, are there for one reason. That reason is to balance the source signal line voltage to the amplifier's input stage.
Gains aren't a volume control, and turning them up doesn't increase the amplifiers' output power.
Turning gains up too far will only serve to over-drive the amplifier's input stage, which will result in clipping at the finals, sending DC voltage in the form of square waves to the speakers and as a result, damaging the speakers.
Also, this will cause unwanted distortion.
Input sensitivity should be properly set using one of a few methods, but ideally, using an oscilloscope and a meter with a tone generator.

New member
Username: Blindell

Post Number: 2
Registered: May-04
Physics nerd stuff:

A Farad is a measure of capacitance (how much umph you can hold). Total electrical energy is a function of both input voltage and capacitance.

Electrical Energy (Joules) = .5CV^2
C = Capacitance (in farads)
V = voltage (figure 14ish)

So a 1F cap would grant you 98 Joules. Now, what the heck is a joule? A joule is a baisic unit for energy, Voltage is measured in Joules per Coulumb (a unit of a large number of electrons) and Amps are measured in Coulumbs per Second. If we multiply Volts * Amps, we get:


Coulumb cancels out, and we get Joules per second, commonly known as a Watt. So if you have 1000watts (J/second) of powah running to your amp, how the heck is 98 joules going to do anything?! Simple: When bass hits, your monoblock amp which may have been using only a couple hundred watts all of a sudden wants 1000W *NOW*. This 1000W isnt sustained for more than a quarter of a second or so (250J, or 1000Joules per second for .25 seconds --> 250joules) in most good punches of bass, so you can see how having 98J in reserve, ready to supliment your alternator and battery could be useful. Why would your battery/alt not just supply this extra power? They do, they just sometimes arent as quick about it as a capacitor. Caps are pretty much as instantaneous as you can get, power output wise. One good application of a cap is in the Flash function of all cameras. Energy is stored up over a few seconds, and is all released in an instant, making a really bright flash.

An interesting thing about caps is their charging times. If connected to a constant source (like an alternator or a battery) your going to charge the majority of your cap in the first couple of seconds. As charge increases in a cap, so does resistance, and it heads twards infinity as your current capacitence heads for the rated capacitance (1F, 2F, etc). In this reguard, with a 1F cap, you will never hit 1F, but in the first few seconds you will hit .65F, then a few seconds later .8F, then .9F, all the way until .99999...99F. The reason it takes so dern long to charge caps when you first install them is because you have to use a resistor to keep from pulling a few hundred amps in the first second. When a cap has 0 charge, it functions like a short circuit, so it would technically pull infinate amps for the first fraction of a second (no good for the rest of your car). The plus to this is that once your cap drains itself (it will never drain to 0 unless short circuited, but it will get pretty low), it can charge itself back up to a decent level in a split second, ready for the next pounding of bass.

Bronze Member
Username: Pat_l

Tucson, AZ USA

Post Number: 84
Registered: Apr-04
A farad is a waste of Money. Do not buy!

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 2836
Registered: Dec-03
Now Blake, calculate the discharge rate of a 20V, 1Fd capacitor on a 14.4V system, so people know how quickly the cap really discharges.. *grin*


Bronze Member
Username: Blindell

Post Number: 12
Registered: May-04
how quickly? depends on how much current you need and for how long... how fast will a 1F drain pretty low on a 1000W system? damn near instantly =)

Thats their purpose.. if you want to store up charge, do it the right way with a battery.. caps are just good for averaging out voltage when the bass goes boom boom.

98J == 98Watts for 1 second
~ 100watts for 1 second
~ 200watts for .5 seconds
~ 1000watts for .1 seconds

etc.. your not going to get a whole heck of alot of umph in a cap, but youll get enough to supliment your alternator/battery system in case of massive instantaneous current draw.

And the 20V rating on a cap just means that if you hook up more than 20V to it, the charge will jump the dielectric and baisicly short circuit itself (the whole thing functions as a big wire). If you put 14.4in, your going to get no more and no less than 14.4 out.

By the way, when a cap is at 0 charge, it has an ohm rating of about 0 (well, the internal resistance, which is usualy like .03millaohms or something obscenely tiny) so it can charge in an instant.. but as you start building up charge, its resistance increases (heads to infinity as charge heads to maximum capacitence)

What I cant stand is those silly people that go "OHH MAN IMMA GET A BIG CAP AND MY SYSTEM IS GUNNA PUT OUT 20MORE DBS THAN YOUR SYSTEM!!!"
no, in fact if the other persons electrical system is able to supply a constant voltage (not dip when the bass hits), then a cap is 100% worthless. Its when you start dropping voltage for that split second when a cap comes in handy.

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 2862
Registered: Dec-03
yeah, like I've pointed out countless times..
a cap won't make up for a lacking alternator..
Andreas liked to thihk his 5Fd of caps helped power his system when his alternator was insufficient.
Refused to believe when I told him the discharge rate for a 5Kwatt system was all of a few seconds.

Bronze Member
Username: Blindell

Post Number: 18
Registered: May-04
E = .5CV^2
E = (.5)(5)(14.4)^2
E = 518.4 Joules

.1 seconds of 5kwatt (and you wont even get that, youll fry 000gauge before then!)
5000watts = 347amps (coulombs/second)
347coulombs/.1 second == 3470 amps for .1 seconds.

good friggin luck !

But then, if you somehow find yourself in a position where you want to drain that much off your cap.. youve got bigger problems. But ide like to see an alternator produce a 100% constant voltage to any high powered amp without the aid of a cap. And its just rumors, but i can see how it would decrease the stress on the alternator, not making it spike output every few beats, but I have no equations to back that up.

Capacitors have their purpose, and they do it pretty dang well.. but trying to use a cap in place of an alternator is like the two year old trying to fit the square through the circle opening in his play toy.
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