I need a simple formula to calculate the total coulombs

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bradlee

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My problem is that I need a simple (please) formula to calculate the total coulombs in a capacitor.

I am using a generic cap. that has no manufacturer name or data other than 3f at 16v.
(used in auto stereo power supplies.)

Since this cap is being used in experiments to provide our test circuit with an "exact" amount of input power. We would like to have a good idea as to the amount of coulombs that we are working with. This will help with the next stage of experiments.

Thanks for any help Brad
 
Re: I need a simple formula to calculate the total coulombs

By definition, capacitance equals charge divided by voltage. Therefore, to find charge, multiply capacitance times voltage.

Simple enough? ;)
 
Re: I need a simple formula to calculate the total coulombs

Let me add a note about units of measure. One "Farad" equals "One coulomb per volt." So as an example, consider a capacitor of one micro-Farad, and apply a voltage of one volt. The charge stored will be 1 E-6 (one millionth of a) coulomb.
 
Re: I need a simple formula to calculate the total coulombs

Originally posted by bphgravity: I can't imagine a cap of 3 Farad? That would be aweful big.
Click to expand...
If you built it in "free space" with two parallel plates spaced one millimeter apart, the plates would have to be about 11 miles wide and 11 miles long. :eek:
 
Re: I need a simple formula to calculate the total coulombs

I've had .1 and .5 Farad electrolytics and they're about the size of beer can. I don't remember the voltage though, somewhere around 50 I think.

I can't check your 121 square mile plate math right now Charlie, sounds a little big (a little big :D ), you could always separate those plates a little if you start running out of real estate. :D
 
Re: I need a simple formula to calculate the total coulombs

Your beer cans probably had more than one set of parallel (i.e., concentric) cylinders. But if you wish to save real estate, you would need to get the plates closer to each other, not further apart.

C = kE0 A/d
 
Re: I need a simple formula to calculate the total coulombs

Now they have "super capacitors". I have seen these at up to 1 farad in a package about the size of a thimbal. They are used instead of batteries to provide back up power for small electronic devices (like calculators,etc). They are usually very low voltage though, like 3 volts or so. I'll bet the 3F, 16V one is pretty big.

They are electrolytic capacitors that manage to get the small size by using a very small distance between the plates. The plates are separated by a electrolyte that is conductive. It builds a thin layer of insulating oxide on one plate which becomes the plate spacing. They also etch the plates, so a square in of plate area may have a lot more surface area (lots of hills and valleys).


P.S. I'm not really sure what the difference between these and a standard electrolyte cap is, except you get even more capacitance in a smaller package.
Steve

[ September 29, 2005, 05:41 PM: Message edited by: steve66 ]
 
Re: I need a simple formula to calculate the total coulombs

You're using Coulomb's law to get capacitance Charlie? The electrostatic constant, k, and the permittivity of free space, Eo. I only have an equation that gives me force, F, from that.

I have capacitance equals charge on the plates divided by the voltage between the plates.

C = Q/Vo

C = capacitance in Farads
Q = charge in Coulombs
Vo = potential difference in volts

A charge of one Coulomb at a potential of one volt is one Farad.

You're right Charlie, capacitance is inversely proportional to the distance between the plates. Unlike what I said. :(

If you can get an accurate measurement of the capacitors's capacitance, as opposed to the value it says it is, cause the value printed on it wont be very accurate. You can measure the voltage on it and theoretically get the charge on it by:

CV = Q.

or capacitance times voltage equals charge in Coulombs.

Edit: I had CV = Q wrong.

[ September 30, 2005, 12:23 AM: Message edited by: Physis 2 ]
 
Re: I need a simple formula to calculate the total coulombs

Coulomb
He is the guy that smked cigars, wore a rain coat and dorve a nash. I think he was a cop.
:D
 
Re: I need a simple formula to calculate the total coulombs

Thanks guys for the responces, this is what I came up with from:

One "Farad" equals "One coulomb per volt." So as an example, consider a capacitor of one micro-Farad, and apply a voltage of one volt. The charge stored will be 1 E-6 (one millionth of a) coulomb.......

I hope the following is correct. If its wrong, its not because I didn't try!!!! Please feel free to correct me Im here to learn!!!!

1 capacitor rated at 3f @ 16v

3f divided by 16v =.1875f at 1 volt

Next, to calculate amount of coulombs in 1 volt with this capacitor---(Since there is approx. 96500 coulombs in 1 farad)---
96500c x .1875f=18093.75c to 1v

From this- If I charge the cap to 5v would I have the following amount of coulombs stored?????
18093.75c x 5v = 90468.75c

Any and all help is welcomed

Almost forgot, the cap that we are using is about the size of a garden tractor battery....
 
Re: I need a simple formula to calculate the total coulombs

I'm sorry bradlee, I had the equation wrong. It's

Q = CV

The first thing you need to do is find out what the actual value of the capacitor is. Because there values are usually very inacucrate, especially large ones.

The Charge is the 3 farads times what the voltage on it is.

If it's charged to 5 volts and assuming 3 farads then:

3f X 5v = 15 Coulombs.

I'm Sorry I goofed you up.

For fun, a Coulomb is the charge of 6,250,000,000,000,000,000 electrons.
 
Re: I need a simple formula to calculate the total coulombs

Good point about the tolerance, Number 2. A lot of electrolytic capacitors have a -20%/+80% tolerance.

Steve
 
Re: I need a simple formula to calculate the total coulombs

Originally posted by Physis 2:

For fun, a Coulomb is the charge of 6,250,000,000,000,000,000 electrons.
Click to expand...
What is the force of repulsion between two electrons 1 mm apart?
 
Re: I need a simple formula to calculate the total coulombs

Originally posted by bphgravity:
What is the force of repulsion between two electrons 1 mm apart?
Click to expand...
None if they are spinning in the same direction. But this can?t happen can it?
:D
 
Re: I need a simple formula to calculate the total coulombs

The important fact to remember with this cap is that it is rated at 3 farads at "16 volts". So to properly look at this, would I not have to divide the total farads (3) by 16v to get the actual farads per volt????
 
Re: I need a simple formula to calculate the total coulombs

Bradlee, 16V is the maximum voltage you can safely apply to the cap. It has nothing to do with the formula which applies to all caps,

Q = CV
 
Re: I need a simple formula to calculate the total coulombs

Steve, it's been bugging me for a few days that I didn't mention the additional, way out of the ball park, tolerance of electrolytics, which this big ol' 3 likely is, I'm glad you added that.

Bryan, I really wish I had the time to explore your question and derive the math right now. Alas.

My good friend Rattus is correct. Because I haven't applied this formula in this way before my only confidence in it is that math works, but I think him and I right. You do nothing to adapt the values. Just avoid destroying the capacitor.

I want to elaborate on this but I can't right now. Maybe one of the engineers can pick this up and explain why the value "V" behaves this way.
 
Re: I need a simple formula to calculate the total coulombs

Sam,

The complete derivation of the capacitance formula is very involved. I understood it well enough 50 years ago to pass a test, but I am like the Tin Man and would have to use a lot of WD-40 to explain it now.
 
Re: I need a simple formula to calculate the total coulombs

By Bryan:

What is the force of repulsion between two electrons 1 mm apart?
Click to expand...
Let's see if I can guess without looking it up.

Is it, a Coulomb of charge exerts a force of 1 Newton across a distance of 1 Meter?

1 Newton / 6.25 X 10^18 X 1000

Maybe it's a centimeter? I'll have to look it up.
 
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