A bunch of small caps in parallel...?

[DBfan187 5,000+ posts]

pica farad

 

[ngsm13 5,000+ posts]

Stiffening capacitors are useless.

nG

 

[frikk 10+ year member]

Thanks for all of the information guys, I didn't expect so many responses.

Yes - you are all correct in that I don't "need" this capacitor. However, you may have missed the point that this is a hobby-ist project for the summer. It is more of an experiment, especially since I will be wiring in some Zener Diodes and LEDs to experiment with voltage detection across the capacitance network.

The theory behind the capacitor works - it says that additional voltage would be stored in the system, available for the parallel branch (the amp). You can also look at a capacitor on a DC line as a filter. It would (theoretically) shield the battery from the AC distortion signal drawing into the amp with it distorts. Does this apply to my system? Would I see a difference? Probably not. Would it be fun and cheap to build? Sure thing! I am just testing the waters to see if anyone here had any comments.

Lastly - why does everyone say that Caps are worthless? If your headlights are dimming and a cap solves that issue - doesn't that imply that it works? I can understand if you don't notice a difference, or if it is placebo (Oooh! my sound is so much more crisp! ... I think?), then you would assume that the capacitor does nothing. At a ridiculous price (> $50) then yes, a cap would be worthless in that respect. Even if the electrical theory is correct, I wouldn't pay $50 - $100 for a capacitor which "works in theory" but doesn't give me any added value.

Thanks again for the responses. I'm sure I'll be posting up some other questions in the forums.

Peace,

Blaine

 

[pilyin 10+ year member]

Check again. MFD = ?F older keyboards (typewriters and whatnot) did not have ? so they used m instead.
Millifarad is almost never used. It is relatively useless. Why write "100 millifarad" when you can write "0.1 farad"?

Find me a 0.1F capacitor ( 100 millifarad, or 100,000 ?F) for anywhere near $0.20 and I will admit that I am incorrect.

He's right. At $.20 it's definitely not millifarad. You would need a lot more than you thought.

Never seen that ?F thing though, but that doesn't surprise me. They don't use typewriters anymore and I didn't graduate (from college) that long ago. I know that millifarad is very seldom used, but in engineering I've seen units of measurement stuff that doesn't make sense a lot, so it wouldn't have surprised me in the least.

 

[jdawg 5,000+ posts]

If I remember right this is how they guy who started doing this did it. Of course you couldnt get a 1farad cap back in the day, but he also said that more smaller ones used together have less resistance than larger ones. Thats from my brain which is missing alot of cells btw.

 

[envengineer 10+ year member]

He's right. At $.20 it's definitely not millifarad. You would need a lot more than you thought.
Never seen that ?F thing though, but that doesn't surprise me. They don't use typewriters anymore and I didn't graduate (from college) that long ago. I know that millifarad is very seldom used, but in engineering I've seen units of measurement stuff that doesn't make sense a lot, so it wouldn't have surprised me in the least.

On my mac I was able to insert a greek "mu" (the thing that looks like a u), now on my PC it is showing up as a question mark. I apologize for any confusion, the "mu" symbol is what I had intended to display. Similar to uF.

 

[helotaxi 5,000+ posts]

Yes - you are all correct in that I don't "need" this capacitor. However, you may have missed the point that this is a hobby-ist project for the summer. It is more of an experiment, especially since I will be wiring in some Zener Diodes and LEDs to experiment with voltage detection across the capacitance network.

For an experiment, do whatever you need for the experiment.

The theory behind the capacitor works - it says that additional voltage would be stored in the system, available for the parallel branch (the amp). You can also look at a capacitor on a DC line as a filter. It would (theoretically) shield the battery from the AC distortion signal drawing into the amp with it distorts. Does this apply to my system? Would I see a difference? Probably not. Would it be fun and cheap to build? Sure thing! I am just testing the waters to see if anyone here had any comments.

The original cap installed on the power lines in a car was done by Richard Clark as a noise filter. It wasn't meant as a current reserve at all. As a noise filter it works just fine, but it's usually 100% unnecessary because the amp has filter caps in the power supply for just that reason. Like I said, in theory and in a perfect world, the cap would store usable energy though a very small amount. Reality's a ***** though and there's minimal overlap between the two in this case.

Lastly - why does everyone say that Caps are worthless? If your headlights are dimming and a cap solves that issue - doesn't that imply that it works? I can understand if you don't notice a difference, or if it is placebo (Oooh! my sound is so much more crisp! ... I think?), then you would assume that the capacitor does nothing. At a ridiculous price (> $50) then yes, a cap would be worthless in that respect. Even if the electrical theory is correct, I wouldn't pay $50 - $100 for a capacitor which "works in theory" but doesn't give me any added value.

Masking a symptom is not fixing a problem. The dimming headlights are not a problem, they are merely a symptom of that problem. The problem is voltage drops due to insufficient current at the peak voltage. The cap will marginally ease the transition between the higher and lowr voltage and the lights fade rather than flickering and the eyes can't pick up the change, but the voltage still drops. Look at it this way, you're in pain from a broken bone. The symptom is the pain the problem the broken bone. You are given some morphine for the pain. Analagizing this to the cap and dimming headlights, morphine is as much a fix for the broken bone as the cap is for the voltage drops. I could just as easily turn off the headlights as add a cap. I don't see the symptom so the problem is fixed. See the issue?

 

[Rawr-DQ 10+ year member]

The cap charges to the voltage of the alternator but their ESR keeps them from discharging with any worthwhile amount of current at a usable voltage. The don't do anything becuase they can't.


Several orders of magnitude more capacity and the ability to actually contribute it at usable voltage.

A motor starter cap and a car audio cap have nothing in common in their application.

In a zero resistance situation the theoretical usable capacity of a cap is 0.5 amp seconds per Farad. A 20F could theoretically give you 10A for a second or more than that for fractions of a second. A typical transient for a large-ish sub amp is going to be on the order of 80A+. The cap could theoretically deliver that for 1/8 second. Problem is the ESR of the cap is going to reduce the power available comensurate with the current that it attempts to deliver. The higher the current from the cap, the greater the loss in voltage. This voltage drop can be enough to completely negate the storage capacity of the cap. Consider that the region in which the cap could be adding anything is the

Getting nothing from a cap in the way of current contribution is in fact installation error. The error lies in the fact that it was installed in the first place expecting any positive results.

I found this article, http://forum.realmofexcursion.com/showthread.php?t=17970

Skimmed over it, and basically, without adding a ton of capacitors, a capacitor does not store enough energy to make a difference. Batteries using chemicals store way more chemical energy than a capacitor storing electrons.

application wise a run cap may not be similar to a car stereo system cap, but they do the same thing, store electrons untill whatever they're hooked to calls for the power.

Eh, buy 200 caps, wire it in parallel, perhaps that'll be enough to make a difference.

 

[helotaxi 5,000+ posts]

I found this article, http://forum.realmofexcursion.com/showthread.php?t=17970
Skimmed over it, and basically, without adding a ton of capacitors, a capacitor does not store enough energy to make a difference. Batteries using chemicals store way more chemical energy than a capacitor storing electrons.

If you crunch the numbers the theorectically usable storage capacity of a car audio capacitor is right at 0.5 amp*seconds of current per Farad of capacitance. Not a lot at all. Subtract losses from resistance and you're looking at right at zero usable capacity. A battery on the other hand measures its capacity in the amp*hour range and resistance is already accounted for. It's capacity is all usable.

application wise a run cap may not be similar to a car stereo system cap, but they do the same thing, store electrons untill whatever they're hooked to calls for the power.

In theory they do the same thing, but the application makes a huge difference. The run cap is a high voltage low current device. The low current is key because the current through the resistance is what causes the loss. A car cap is a low voltage (working voltage range is around 2 V) high current device and poorly suited for that purpose.

 

[envengineer 10+ year member]

There was a good discussion over at DIYMA recently. It's long, but it is a good read. More good info than most of the "caps ****" or "caps saved my system" bullshit threads that are started around here.

http://www.diymobileaudio.com/forum/showthread.php?t=14000