my setup is jlaudio 250.1 pushing a jlaudio 10w6. when i turn up the volume and the bass hits my lights dim so i guess i need a cap. my question is does the brand matter for capacitors? theres alot of those swap meet brand caps.. such as legacy, fusion, american pro..do i need a digital one..? are they better? my local audio shop has a lightning audio cap 1 farad for 80 bucks..should i buy it or can i get a better deal ?


[Modified by Agent Daimon, 4:50 AM 4/7/2003]

 

How bad are they dimming? I am running about 3 times the wattage you are and my lights only dim at extremely high volume... Is your amp clipping?

 

I would install one.
Yes there is a difference between brands. You want one with the lowest ESR value.

No you do not want a digital one they do not benifit you at all. It can be argued that they actually use some of the capacitance.

 

blah. caps really do pretty much nothing. ifn you need more power upgrade your alternator for like $180 and never have to worry about dimming or anything. and a farad is a farad brand does not matter.

 

Please explain how you came up with this. Enlighten us.


[Modified by nsxxtreme, 11:43 PM 4/6/2003]

 

i agree with the caps. well to an extent. They do help, but the alternator has to have the extra capacity to refill them. If your alternator is already weak, this extra strain is going to knock it out after time (or has the potential to). Getting your alternator re-wound or getting a rebuilt one would be the best option. But caps do help hold the extra power reserve you'll need sometimes. It's a band-aid in alot of people's eyes, but it does help. Inbetween the big hits, it captures all the energy that could be going to waste, and just "reapplies" it at a better time

 

Capacitors are not band aids. They have a specific function, to lower the ripple voltage going to the amplifier. If your altenator can not handle the load then a capacitor will do no good. Every car is different so you would need to determine this yourself. Your lights will still dim with a higher output altenator. A cap can actually increase the ouput of your altenator. It can also help remove some of the strain on it.

ESR stands for equavalent series resistance.
The capacitor (or capacitor bank) will have a ripple voltage (seen as noise on the supply voltage) proportional to the ESR of the capacitor (or capacitor bank).
V = I x R ... Ripple voltage = Current X ESR
If the current increases from 4A to 20A, within a circuit, the ripple voltage will increase by a factor of FIVE.
This increased noise cannot be tolerated, so the ESR of the capacitors must be reduced.

There was a really good explanation of how a capacitor works and why ESR is important a while back on a different website. Maybe someone here knows about it and can link to it. I am not going to go into this one because his got 10 pages long with people still telling him he didn't know what he was talking about. The guy did an excellent job even going into the physics of how it worked. I believe someone from PG also entered into the conversation.


[Modified by nsxxtreme, 8:47 AM 4/7/2003]

 

I absolutely agree with this one...that's why you should make sure you have enough alternator output.

I don't agree with this one. The only way that your lights dim is if you run out of current from your alternator, and you start pulling juice from your battery...drop in voltage causes the dim in your lights. Size the alternator properly, and you won't have to worry about dimming

You sure about that one? Last time I checked...adding a cap in your trunk does nothing to modify your alternator.

I think caps have their place/applications in some instances, but some people think that they can run a 3000W system on an 75A alternator by adding 3F of caps...it just ain't going to happen!!!

I think the tread you were refering to was on carsound.com - it was headed by Richard Clark. The discussion basically said that those large caps that you see (5F, 15F, 50F...etc) are pretty much worthless, while a smaller cap 1F etc does have a place, but it still isn't a current generating device..it stores energy...it doesn't make any.

 

Your lights will dim when there is an instantaneous demand that the altenator is not able to supply. This is what a capacitor is for. It is not a replacement for a battery. Even if you add a second battery in the truck fully charged, right next to the amplifier your lights will dim. The battery is not able to supply the charge fast enough for the amplifier. This can be reduced by buying a battery that has a lower internal resistance. Ahh ESR again.

Yes by smoothing out the ripple comming from the altenator. Kind of like the large capacitors you see in 120V power supplies.

Yes do you have a link. This was very well written. There was more there than just what you mentioned. You just have to read carefully and understand what is going on.



[Modified by nsxxtreme, 10:30 AM 4/7/2003]

 

Very valid points...Clark and Navone info came out during a time
when hi-powered D class amps were not in existance...and people
sort of ran with the 1 farad per 1k watts idea. As for the larger
farad units I beive that they have there place,but your right....
no amount of caps will work if your alternator can't keep up
with the need.

 

Hence the statement...size your alternator properly, and you don't have this problem....adding more batteries to an already weak charging system is a recipe for disaster. Get a big enough alternator, and you don't have to worry about caps.

A cap does NOTHING TO THE ALTERNATOR...I'm sorry, but the output of an alternator is only affectedy by how it's wound and what rpm it's spinning at.

Unless I'm missing something big here, I don't see what measurable effect placing a cap down stream of an alternator will do to make it produce more current...meaning if I have an alternator, and I measure 100A of output at 2000 rpm, I don't think adding a cap in my trunk will magically make my alternator's output increase (i.e. 120A) all of a sudden.

Seems to me that you would be defying the laws of physics with that one. I've been wrong before, and I'll be wrong again....give me some kind of way that a cap will increase the output of your alternator and I'll change my stance, but until then, I just don't see it.

Caps stabilize your voltage...that's about it. They don't create anything...the only part on your car that creats current is your alternator..plain and simple.

I don't have a link, and I know carsound recently cleared out a ton of old posts, but it may be saved in the archives somewhere...I would just do a search. There was a lot more there going on, and the discussion was very technical. What I posted was the cliff notes version of what I got out of it...again, that discussion was centered around large caps.

With the money you spend on a large cap, you can just get a new alternator and be done with the problem!!

BTW, to the original poster...if you only have a 250/1, you shouldn't have dimming problems.


[Modified by rcurley55, 7:00 PM 4/7/2003]

 

Boy got myself into a discussion I didn't want to because of all the misconceptions about capacitors.

Yes you will.
Current is defined by i=dq/dt voltage is defined by v=dw/dq you see that there is a delta t element in i. Your alternator does not produce lets say 120A all the time. It produces it on demand. It takes time to do this, this is where the capacitors role comes into play. It is able to store a charge and deliver it much faster than an alternator or battery.

This would be hard for me to explain. Try finding something on the net about half wave rectifiers with an RC load and it may help explain it to you. I think I know where the confusion may ly. Capacitor will help maintain a higher peak current. If the voltage drops on the alternator you will end up with less sustainable current. If you have a capacitor that is able to help maintain the higher voltage you will end up with a more sustainable current. i.e the current will not drop. Resulting in an altenator that is able to sustain a higher peak current level. I don't know if that helps.




[Modified by nsxxtreme, 4:37 PM 4/7/2003]

 

No, your lights will dim because even a fully charged brand new battery has a voltage rating of 12.6 (for a good, fully charged 12v battery). your alternator turn out 13.7-14.4 typically. When you pull too much from the alternator you start dipping into the battery which, putting out 12v's, is going to be a noticible drop in brightness. (and yes I know, batteries do not put out power as fast as a cap, but i'm talking even just the strict voltage drop between the two is a noticable factor)

 

ugg If you are dipping into the battery you are exceeding the limits of the alternator. Capacitors are to help when you have instantaneous changes in current as mentioned above. Your amplifier does not pull maximum current all the time.

Yes correct you would need 214 Amps plus enough current to run the car.

I give up everyone go buy expensive alternators and see if your dimming light problem goes away.


[Modified by nsxxtreme, 7:04 PM 4/7/2003]

 

heh i said the same thing above. I was just throwing the other post in for fun. Caps will work, but will put more stress on your alternator killing the diodes.. errr rectifiers, over time due to the extra stress. A larger or rewound alternator would be your best bet, but even then you can run into problems. Largers alts have more of a gap under high loads between pulses, which can lead to bad engine noise

 

Are we talking about dimming at idle, or while moving? Even high output altenators don't produce much current at idle. So you will still get dimming with no cap right?

 

Capacitors should reduce the stress on the alternator by reduceing the instantaneous peak current demands on the altenator. Its benifits should out weight the negative affects. I gave the formula for current that relates to charge. So how much current can a capacitor deliver instantaneously? Extra batteries add a load to the alternator they are not able to supply a charge instantaneously.

I'm done.




[Modified by nsxxtreme, 8:37 PM 4/7/2003]

 

agreed. That's why I have to laugh at those people who add 2nd batteries to their systems without adjusting anything else. The constant pull from both batteries on the alternator... well... whatever.

I also agree the caps help between major power draws, and can help clean up the ripple effect (such as those I was refering to when using a higher output alternator under draw). It helps give a little more 14.4 volt or whatever was put in at charging surge during the bass hit. But for what you're going to spend in $$ on caps it might be worth it to upgrade your alternator first, or in conjunction with.

 

Sorry, but if you are going to make large claims (like adding a cap will increase the output of your alternator) you had better back it up with hard evidence.

Not trying to bust your *****, but I really disagree with the fact that adding a storage device downwind of a mechanical element (like an alternator) will magically increase it's output.....I'm still waiting for some kind of actual, hard evidence.


[Modified by rcurley55, 6:00 AM 4/8/2003]

 

You have mail sorry if the images aren't that great my book is a hardcover and does not open that far.

 

no you don't need a cap, all you need a better battery. try a yellow top optima battery. that's what i have, w/ no problems.

 

a battery won't do crap. The voltage of a new, fully charged, is only 12.6 tops. An alternator will put out 13.8-14.4 typically. It's this 1.2-1.8 or higher volt drop that causes the dimming. A bigger battery only helps when the car is off, keeps your system running longer before the voltage of the battery starts to drop. Not to mention a bigger/more batteries can really start to tax your alternator over time with more demand to charge them.

 

I totally agree with the fact that batteries are only for starting and running the electrical system with the engine is off. It does however offer a little backup when the charging system is asked for too much current.

A few things to add to this discussion:
I do not have the facts to back it up immediately, though I can if anyone disagrees. Here it is: a 12v battery will offer it's current more readily at 12v than it will at 14v.

What's happening when a major current request is made the alternator tries to feed the amplifier, then can't keep up so the voltage drops. As the voltage drops at the amplifier, the current increases (watts/volts = current) causing even more strain on the alternator.

At that moment the battery isn't providing much current because the electrical system voltage still above 12v, though it is dropping. As the amplifier is still requesting power, the system voltage drops further (lights dim) and then the battery begins to offer more current to the system.

If the amplifier is still playing at the same level; and the voltage continues to drop as the current demands increase. The total elapsed time of the paragraphs above is 1 to 2 seconds or longer.

So....
It's imperitive to maintain voltage because if it goes down, it's a deadly spiral. The battery can maintain peak current only near 12v. A second battery is just another load on the alternator.


[Modified by PupaScoopa, 12:28 PM 4/8/2003]

 

I'll work through that doc and let you know what I think...the only thing that really applies on my first go through is that you are trying to represent an amplifier with the resistor, and you have a cap across it in parallel. By increasing capacitance you decrease ripple voltage across the resistor...I still don't see what affect this has on the voltage across the power supply...let me look at it some more...keep in mind I'm a mechanical engineer, not an electrical one.

 

A resistor represents a load on the power supply (whatever it may be). IM me if you have further questions.

This only happens if the amplifier has a regulated power supply. Otherwise the voltage will drop to a sustainable level with reduced power from the amplifier. Most regulated amplifiers will run down to 10V before they shut off.

I'm done again


[Modified by nsxxtreme, 11:04 AM 4/8/2003]