A bunch of small caps in parallel...?

[frikk 10+ year member]

Hey guys, this is my first post (sorry for the length!)

I have a little experience with electronics (I am a computer engineer). I am curious about something. I realize that car audio capacitors are very expensive. I also realize that small capacitors are very inexpensive. I am wondering what the disadvantage of wiring up several small capacitors in parallel is. It seems like it would work to me. For example:

1 F capacitor is supposedly good for a 1000 watt system. Sounds good to me.

Why could I not take 10 100 Milli-Farad capacitors in parallel (which would be the same as 1 F)? Even if they are smaller and have smaller leads, the amperage would be divided up (by current laws) among the capacitors so the amperage would be smaller per each individual cap.

I am looking at buying some of these:

http://www.allelectronics.com/cgi-bin/item/100/25VR/search/100MFD_25V_RADIAL_.html

They are $.20 each, and I think they are 100 Milli Farads. At allelectronics.com, I've noticed that Micro is designated by UF, Pico is designated with PF - so I figure that Milli Fards is designated with MF. I'll have to call the company and see for sure... but even if this price is wrong what is the flaw in my logic? Can I really replace an $80 capacitor with a $10 one built with home made parts?

By the way - I've been talking with electrical engineers where I work and I understand that the car audio caps probably have very specific "sweet spots" where the capacitance is optimal for the bandwidth found in a car audio system. With the small caps I would not be getting optimal performance most likely. I don't care about signal filtering or quality really, since I'm just using it to stabalize a DC line. Even if I am at "worst case" and bad efficiency (my 1F solution only really gets like .5F), this is only helping the system right?

Thoughts? Comments? Tear my idea apart? //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif//content.invisioncic.com/y282845/emoticons/fro.gif.c695f1f814b01c4ad99fe7f8cccadd29.gif

By the way this is purely theoretical - my audio system most likely will not be pushing more than 350 watts rms, so a 1F or even .5F cap will be overkill (and probably unnecessary). This is more or less me looking for a summer project.

Thanks,

Blaine

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[jmanpc 5,000+ posts]

A bunch of small caps would be just as useless as one big one.

 

[LowLowBass 10+ year member]

waste of money waste of time, big 3

 

[LowLowBass 10+ year member]

i'm pretty sure you dont even need a cap.

 

[Grinder1989 5,000+ posts]

350 RMS? You don't even need a cap. I got a 140amp alt and a decent battery. 1500RMS and my lights don't dim at night.

 

[jdawg 5,000+ posts]

350 RMS? You don't even need a cap. I got a 140amp alt and a decent battery. 1500RMS and my lights don't dim at night.

no he really shouldnt at that wattage, but in your case with that much wattage it depends on the amplifiers effeciency, not just wattage numbers alone, your amp is pretty efficient

 

[envengineer 10+ year member]

Hey guys, this is my first post (sorry for the length!)
I have a little experience with electronics (I am a computer engineer). I am curious about something. I realize that car audio capacitors are very expensive. I also realize that small capacitors are very inexpensive. I am wondering what the disadvantage of wiring up several small capacitors in parallel is. It seems like it would work to me. For example:

1 F capacitor is supposedly good for a 1000 watt system. Sounds good to me.

Why could I not take 10 100 Milli-Farad capacitors in parallel (which would be the same as 1 F)? Even if they are smaller and have smaller leads, the amperage would be divided up (by current laws) among the capacitors so the amperage would be smaller per each individual cap.

I am looking at buying some of these:

http://www.allelectronics.com/cgi-bin/item/100/25VR/search/100MFD_25V_RADIAL_.html

They are $.20 each, and I think they are 100 Milli Farads. At allelectronics.com, I've noticed that Micro is designated by UF, Pico is designated with PF - so I figure that Milli Fards is designated with MF. I'll have to call the company and see for sure... but even if this price is wrong what is the flaw in my logic? Can I really replace an $80 capacitor with a $10 one built with home made parts?

By the way - I've been talking with electrical engineers where I work and I understand that the car audio caps probably have very specific "sweet spots" where the capacitance is optimal for the bandwidth found in a car audio system. With the small caps I would not be getting optimal performance most likely. I don't care about signal filtering or quality really, since I'm just using it to stabalize a DC line. Even if I am at "worst case" and bad efficiency (my 1F solution only really gets like .5F), this is only helping the system right?

Thoughts? Comments? Tear my idea apart? //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif//content.invisioncic.com/y282845/emoticons/fro.gif.c695f1f814b01c4ad99fe7f8cccadd29.gif

By the way this is purely theoretical - my audio system most likely will not be pushing more than 350 watts rms, so a 1F or even .5F cap will be overkill (and probably unnecessary). This is more or less me looking for a summer project.

Thanks,

Blaine

MFD = microfarad = 10^-6 farad

You would need 10,000 of the 100 MFD caps to equal one farad of capacitance. Not quite as economical as you had originally thought.

 

[pilyin 10+ year member]

I believe MFD = milli farad (10 ^-3) as UFD = micro farad so it would work, just not nearly as efficiently because the increased amount of resistance. Still a waist of time. Do a big 3 upgrade and don't bother with it.

 

[djman37 5,000+ posts]

all electronics USED to sell a mini 1 Farad cap. we have a few around here somewhere.

 

[BassAce 10+ year member]

You would need 10 100mF capacitors to equal 1 farad. I also think the total esr would drop since they are connected in parallel.

 

[tc300 10+ year member]

caps kinda sound useless(or not worth it) 2me. if u have that much of a current power issue..go to the source.. the alt. a bigger alt.big 3 upgrade and xtra bats sound like the way to go. caps sound like a quick fix

 

[helotaxi 5,000+ posts]

Or a quick non-fix.

 

[Rawr-DQ 10+ year member]

I'm sure caps work on some level to stabilize voltage drops, people say they do nothing, but maybe they just can't tell...

But with that in mind, a battery that can store a lot more electricity can't do ya wrong either...

In most electric motors that run off of two lines of single phase a/c power need a run capacitor to run properly, they used caps 25-60 MFD in residential air conditioners. The capacitor stores enough power to run motors less than 1 horse power, what they do is simulate 3 phase power...they slow down a phase, I forget the equations, but without them the motors can't rotate on start or without enough torque to complete a revolution. three phase electric motors are more effeicent that the latter.

Although, looking at prices these days, I dont think wiring them in parallel verses a 1-5 farad car audio capacitor will do you any good...Perhaps people just haven't used a capacitor large enough to accomendate a cycle in thier speaker, to the point where they drain before the need of power is satisfied, and then seem useless.

Installation error? hmmm.

 

[envengineer 10+ year member]

I believe MFD = milli farad (10 ^-3) as UFD = micro farad so it would work, just not nearly as efficiently because the increased amount of resistance. Still a waist of time. Do a big 3 upgrade and don't bother with it.

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.

 

[helotaxi 5,000+ posts]

I'm sure caps work on some level to stabilize voltage drops, people say they do nothing, but maybe they just can't tell...

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.

But with that in mind, a battery that can store a lot more electricity can't do ya wrong either...

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

In most electric motors that run off of two lines of single phase a/c power need a run capacitor to run properly, they used caps 25-60 MFD in residential air conditioners. The capacitor stores enough power to run motors less than 1 horse power, what they do is simulate 3 phase power...they slow down a phase, I forget the equations, but without them the motors can't rotate on start or without enough torque to complete a revolution. three phase electric motors are more effeicent that the latter.

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

Although, looking at prices these days, I dont think wiring them in parallel verses a 1-5 farad car audio capacitor will do you any good...Perhaps people just haven't used a capacitor large enough to accomendate a cycle in thier speaker, to the point where they drain before the need of power is satisfied, and then seem useless.
Installation error? hmmm.

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.