Help me understand capacitors

[fwb_1234 10+ year member]

//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif Currently going to school for electrical engineering.

Me too. I got all excited and then I was like, dang it. It's been done.

 

[Team Maxxsonics 10+ year member]

I can also add that almost every capacitor we have tested comes no where close to the suppliers claims.

We tested several top selling brands and found 1 farad caps that were actually .25-.35 farads inside!

Claimed 5 farads that tested at 2 farads, 10 fards at 3.5 farads!

We did this because we had interest in selling caps that were raited correctly only to discover that some companies were selling them to dealers at or below the same cost we were buying at.

That's when we teste them and determined we could not be competitive because we would actually sell a 1 farad caps that were 1 farad.

Now, there are a few compaies that sell correct value caps but I was should to find out how many cheated by 35% to 60%.

If one companies cap is much more expensive then the others, buy the more expensive one.

There is a real reason for the price difference.

So, I of course will not tell you the name of the bad caps but be careful and buy from the companies that are expensive or you have simply wasted your money completely.

 

[fwb_1234 10+ year member]

I can also add that almost every capacitor we have tested comes no where close to the suppliers claims.
We tested several top selling brands and found 1 farad caps that were actually .25-.35 farads inside!

Claimed 5 farads that tested at 2 farads, 10 fards at 3.5 farads!

We did this because we had interest in selling caps that were raited correctly only to discover that some companies were selling them to dealers at or below the same cost we were buying at.

That's when we teste them and determined we could not be competitive because we would actually sell a 1 farad caps that were 1 farad.

Now, there are a few compaies that sell correct value caps but I was should to find out how many cheated by 35% to 60%.

If one companies cap is much more expensive then the others, buy the more expensive one.

There is a real reason for the price difference.

So, I of course will not tell you the name of the bad caps but be careful and buy from the companies that are expensive or you have simply wasted your money completely.

Why not tell us the names? That would be very helpful.

 

[ultimate157 5,000+ posts]

I would much rather invest my money into a 2nd or 3rd battery than a true 5 farad cap that would cost just as much.

 

[bigbang 10+ year member]

It would apear that from the graph the lowest voltage level occurs before the battery kicks in to supply its voltage. Having more batteries would not defeat the fact that there is a span of time before they can kick in after the alternators output has been surpased to increase the voltage level.

In the case of any stereo system in which the alternators output will be exceeded regularly durring dynamic passages wouldent it make sense to have a capacitor for the sole purpose of supplying a very brief supply of current until the slower chemical-reaction based batteries deliver or until the current demand drops back below the alternator output level?

 

[savagebee 10+ year member]

the only arguement for caps that Ive ever heard is for the aforementioned reason

to "level out" the spikes so to speak

 

[Drowsy 10+ year member]

So all in all, they are not worth it, correct?

 

[helotaxi 5,000+ posts]

It would apear that from the graph the lowest voltage level occurs before the battery kicks in to supply its voltage. Having more batteries would not defeat the fact that there is a span of time before they can kick in after the alternators output has been surpased to increase the voltage level.
In the case of any stereo system in which the alternators output will be exceeded regularly durring dynamic passages wouldent it make sense to have a capacitor for the sole purpose of supplying a very brief supply of current until the slower chemical-reaction based batteries deliver or until the current demand drops back below the alternator output level?

A battery will begin to supply current as soon as the alt voltage begins to sag. The battery has a float charge on it that is actually larger than the usable charge in most caps. Better batteries will have a higher voltage as well. More batteries means more float charge.

 

[ssj2xxgotenxx 10+ year member]

//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif Currently going to school for electrical engineering.

I always have respect for engineers.

 

[ultimate157 5,000+ posts]

I always have respect for engineers.

Thanks. I'm hoping to go for my masters before I'm through.

 

[WhoSayWho? 5,000+ posts]

So all in all, they are not worth it, correct?

That's my understanding -- and really all I need to know.//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[jmanpc 5,000+ posts]

Thanks for the continued explanation.

 

[maldecido33 10+ year member]

50A current draw from a 14v source is equal to ~0.28 ohms of resistance.
The RC time constant is a fixed time interval equal to resistance times capacitance. It takes ~5 time intervals to fully discharge a capacitor.

280x10^-3 ohms x 1 F = .28 seconds. Five times this is ~1.4 seconds. This is from fully charged to fully discharged.

Since the discharge curve of a capacitor resembles exponential decay, it takes a very short time for the fully charged capacitor (sitting @ 14v) to reach 11v. This would happen within 1 time constant, or less than .28 seconds. (Edit: By looking at a discharge curve, 80% charge is remaining at just over 1/5th the RC time constant... in this example that is approximately 0.056 seconds)

It happens VERY quickly when using a capacitor as a voltage source.

You have to remember this is using the capacitor as the only source. Complimenting a 14vdc source will decrease the discharge rate.

I had an awesome reply ready to type up, but you nailed it. Nice post.

 

[maldecido33 10+ year member]

Caps are basically a small battery. They charge up, and once they're charged they stay charged until your system needs the extra boost.
In theory they're good... but in reality they're bad. In essence they're just a weak second battery. So why not spend another $100-50 dollars and buy another battery?! //content.invisioncic.com/y282845/emoticons/wow.gif.23d729408e9177caa2a0ed6a2ba6588e.gif

From my understanding, they add more of a strain on your electrical system and most caps are only 1 farad [i believe an average battery is about ~10,000 farad... probably more]

Wrong, they are not a small battery.

They are sort of like having a little retarded middle man.

 

[maldecido33 10+ year member]

Me too. I got all excited and then I was like, dang it. It's been done.

//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif Currently going to school for electrical engineering.

Thanks. I'm hoping to go for my masters before I'm through.

Same here; At Texas Tech (my university) they offer a 5 year program in which less significant senior electives are replaced with graduate level electives in the area of EE you want to pursue. It's a really good deal, and even without it, you really would be foolish not to purse a MS. PhD's are really only if you'd like to teach; alot of companies won't hire them because they're not willing to pay.