# Capacitor and amount of current containable

1. Jan 10, 2010

### ramonegumpert

Dear Electrical Engineering experts,

I am able to see the voltage rating of a capacitor but not the amperage info.
I searched the net for a while but could not find the right info on this .

Or maybe my question is invalid? I can understand that there is some difference between a capacitor and a battery. But both can provide electricity. One stores electricity while the other stores electrical charge.

Maybe the amperage info is already available but i do not know where to find it?

sincerely
Ramone

2. Jan 10, 2010

Hello ramonegumpert,

I can think of a few ways in which a capacitor can have a current rating:

Manufacturers will sometimes provide current ratings (in Amps RMS) for capacitors intended for highly stressed applications. These ratings must be respected to avoid premature failure, generally due to overheating due to excessive I2R loss. You may see the term "ripple current rating", because they apply to power supply reservoir capacitors. Note that for non-reversible electrolytic types, the ripple must not lead to voltage reversal.

For reversible capacitors working on AC, the capacitor's working voltage rating also imposes a current limitation.

Finally, if you are considering a capacitor as an alternative to a battery, e.g. a CMOS memory backup capacitor, how much current (I) you can draw for a given time (t) depends on the maximum voltage you can charge it to (V1), the minimum voltage you can discharge to (V2), and of course the time and capacitance. I = (V1-V2)*C/t

Given that a Farad is a pretty large capacitance, you will see that capacitors cannot (as yet) store amounts of energy comparable to what can be achieved using batteries.

3. Jan 11, 2010

### ramonegumpert

Thank you for clarifying my doubt.

I was trying to find out how to use a capacitor as a temporary storage to transfer current from low voltage sources to a rechargeable battery. Maybe, i should directly transfer to the battery instead?

I have seen people post their DIY videos on youtube about how to make capacitors using aluminium sheets and some dielectric material.

I was wondering how efficient can this method be compared to a factory manufactured one?
Would a large capacitor made up of a large piece of aluminium like 50cm in diameter be able to store a lot of electricity more than what a ,say 10 farad supercapacitor can hold?

Have a nice day and thanks for reading my post.

sincerely
Ramone

Last edited: Jan 11, 2010
4. Jan 11, 2010

Hello ramonegumpert,

A home-made capacitor to rival a supercapacitor? Here I begin to wonder if you might just be pulling my leg. In case you are being serious here, the formula for a parallel capacitance is

C = ε0*εr*A/d

C is the capacitance, in farads;
A is the area of overlap of the two plates, in square metres;
εr is the relative permittivity aka dielectric constant of the material between the plates
ε0 is the permittivity of free space (ε0 ≈ 8.854×10−12 Fm−1); and
d is the separation between the plates, in metres.

Suppose you built a capacitor with 50cm diameter round plates, with 1mm thick polythene (εr = 2.3) as an insulator.

Can you work out its capacitance from the formula and the numbers I've given above?

5. Jan 11, 2010

### vk6kro

I found a version of that formula that gives the units in a more familiar way.
Gives the same result as the one above.

https://www.physicsforums.com/attachment.php?attachmentid=22999&d=1263260214

To put the result into familiar terms, it would take 250 million circular plate capacitors as above to make one Farad.

6. Jan 12, 2010

Sad but true, SI or otherwise it's still about 4nF. I think those home-made capacitors may be meant to allow operation at very high voltages at a reasonable cost. Perhaps they would be used as an alternative to Leyden Jars, e.g. with an electrostatic generator like a Wimshurst.

It may be worth saying that such things can be horribly dangerous: although everyday static shocks are often fairly mild, even a modest capacitance charged up to a high voltage can kill.

Take care!

7. Jan 12, 2010

### imsmooth

Unless stated, as a previous member said, the capacitors are not meant for heavy current. Some, can handle high current for short periods, like a pulse capacitor. Other can handle high current for extended periods like power film metallized capacitors.

I recently need high current capacitors which I got from Illinois Capacitors. The rating is 14A rms. However, they are not meant as a storage device.

If you are using high current you will have to consider cooling.

8. Jan 12, 2010

### sophiecentaur

The capacitors used for photographic flash can provide very high peak currents - discharging the device in 1/10,000s. But the mean current is only in the order of much less than 1A - a 6V supply from batteries can only give you a few hundred mA and you charge up to 200V, or so the actual charging current will be significantly less.
A huge problem with capacitors as stores is the fact that the voltage is far from constant (exponential devay) that makes it necessary to use a fancy circuit (switch mode) to provide a constant current and also to use inconveniently high charging voltages.
But there is work going on. . . . . .