capacitor question

[A6D9]

I just got a quick question on what a capacitor actualy does, i think i know but I"m not 100% sure.

if I had an old car audio 1 Farad 12 volt capacitor.  Could i use some kind of 2-3 volts to charge it and find some way to when it is fully charged discharge into a 12 volt batery supply, given that i had correct diodes to prevent the bateries from charging the cap?

just an idea for when a wind mill doens;t have enought wind for the full 12 volts but maybe has enought for 2-3 volts

[veewee77]

A capacitor stores electricity. It will not store more voltage than what your souce is, unless there is some kind of step-up circuit in there. In other words, your 3-2 volt source will only charge the capacitor to 2-3 volts. You'd need a DC/DC converter to get  it to the 12V.  But yes, you can do thins that way but not with just a cap and diodes.

Doug

[dinges]

No, it doesn't make sense. Besides, if things were that easy it would have been mentioned on this board long ago and everyone would be doing it.

http://electronics.howstuffworks.com/capacitor.htm

http://electronics.howstuffworks.com/framed.htm?parent=capacitor.htm&url=http://micro.magnet.fsu
.edu/electromag/java/capacitor/

http://en.wikipedia.org/wiki/Capacitor

http://en.wikipedia.org/wiki/Charge_pump

or google 'capacitor'.

[willib]

* peeks in *

yes it can be done , with oztules boost converter , i know/think he tried it with six volts into a 12V batt, i think the usefullness of such capacitors is very underrated .

they have super low internal resistance which makes the perfect for some of  our applications

[terry5732]

Yes you can, but as stated previously, you need to increase voltage before storing in cap. The simplest way is to pulse a transformer with your DC, rectify the output and store in cap. This will have losses but will work to use otherwise useless power.

To charge battery from cap, you will want somewhat over battery voltage. This pulse charging can be beneficial in lead-acid batteries.

[commanda]

just an idea for when a wind mill doesn't have enough wind for the full 12 volts but maybe has enough for 2-3 volts

Capacitor is irrelevant for this application.

Boost converter is in this recent article.

http://www.fieldlines.com/story/2006/3/17/185646/194

Amanda

[dinges]

And by the time you've increased the voltage to the 1F cap, it has become entirely unnecessary because now you can charge a battery directly. So cap is not needed.

What he needs is a boost converter (which uses a few caps inside).

Peter.

[fred480v]

in electrical theory a proper sized cap will discharge 5-8 % more then it,s line voltage. this is used in many circuts, but as stated will not work for you. that is a very small cap. you are talking about. when power returns to "0" plane caps. discharge this maintains power till it peaks again & cap recharges only to discharge when returns to "0" again it's a never ending cycle

               fred480v

[drdongle]

1 Farad is small?

[willib]

Try this....

i have a gen/alt that i was charging several NiMH batteries in parallel (1.2 Vdc)

i used a 110 F ultra capacitor rated at 2.5 Vdc

if the cap wasn't useful then how come when the gen output exceeded the battery voltage for a period of time , the voltage across the cap and batteries was greater than the rest state of the batteries by themselves ? and it discharged this power into the batteries after a couple minutes, acting like a buffer between the alt and the batteries.

[A6D9]

ok so in theary it would work, just not with a cap...need one of those DC to DC thingy.

I got the answer i wanted.  thanx all.

I have slow winds here most of the time.  but usualy at least once a day for a bit it goes pretty hight, so i figured if i can get some kind of power from the below cut in speed, might be able to use it rather then throw it away.

[fred480v]

I don't understand what you are saying. 1.2v batteries in parallel what are you using that for? gen./alt. don't understand. how much greater was the voltage? if the batteries became more pos. or neg. then the cap it will discharge then recharge till this happens again

                    fred480v

[Ungrounded Lightning Rod]

At a few Hz and tens of amps?  You betcha!

The charge on one farad changes by one volt when it has sourced or sinked one amp for one second.  (That's the definition of "farad".)

That's a lot of capacitance for low-current signal processing at audio rates.  But for low frequency power applications it's puny.  (That's why switching regulators run at high frequencies:  The size of the caps and inductors needed is inversely proportional to frequency, and can thus be small.)

[Ungrounded Lightning Rod]

With a bunch of them and a similar number of diodes (assuming your input is AC) you could build a voltage multiplier.

But at 2-3 volts that would be HORRIBLY inefficient, due to diode drop.  You'd lose maybe 0.6 volts to the diode, and more to vlotage droop from the capacitor, in every step.  That's dumping maybe a third of your power for each volt or two you gain.

Further, if you're thinking about using this on a mill when the wind is low, the FREQUENCY is very low, requiring an ENORMOUS capacitor.  A farad is big for mid-to-high audio rates and signal (or even audio power) current levels.  But it's miniscule for low-freuqency power applications.

But there's another problem with trying to get power from a windmill at low winds:  The power AVAILABLE is proportional to the CUBE of the wind speed.  So at low winds you not only have negligible voltage (from slow rotation) but negligible current (from low force on the shaft).  Boost circuits trade current for voltage, and with little current BEFORE you start trading it away, you're left with dribbles.

And you spent a lot of effort and equipment to get those dribbles.  And maybe changed your system to make it less reliable, or less efficient on TYPICAL winds, when there's lots of power to collect - and thus a lot to be LOST by even a small reduction in efficiency from tuning for lower speeds.

If you need more power your resources are normally much better spent to increase the size of the mill (collecting more power at typical wind speeds) and your storage (to save it for low-wind times) than to try to extract a few drops from the mud when the stream has run dry.

[willib]

"1.2v batteries in parallel what are you using that for?"

I was charging more than one at a time.

"gen./alt. don't understand."

 sorry, .. alternator / generator

"how much greater was the voltage?"

i remember that i had to watch that the alternator didnt exceed the 2.5V rating of the cap.                                                      

"if the batteries became more pos. or neg. then the cap it will discharge then recharge till this happens again"

the cap and batteries exceeded the battery rest voltage during and right after a charging session.

this is  an example of a larger system that could be built  using 12v batteries and a higher voltage  1F ,2F ,or 5 Farad ultra capacitor .

it was  effectivly a buffer storing  power during charging , and discharging into the batteries when the alternator slowed or stopped

[fred480v]

what I was asking was what you where using 1.2 v for, thought you may have a special project going.  1.2v in series, at the end of a parallel circut your voltage dose not add current adds. in series voltage adds

[drdongle]

Hadn't considered those criteria, as usual it's all relative

[A6D9]

lots of great answers thank you all, i now have a better understanding of a capacitor.

so what would be a good resolution if the avg wind speeds are 7 miles/hr.

I"m going to guess I would like to be making let's gestimate 1500 wattes/hr

that is big max.  but any other ideas are sure welcome.  

if I don;t produce enought and get a big enought of a storage bank and have to "top it up" once every few days with a gas powerd geni I"m not apposed to that,  just another excuse to make another mill while i do that to make up for the lack of production...lol