capacitors

[rokebaum]

What is a capacitor, anyway? I am looking for a device or component (the smaller the better) that can store/build up an electrical charge with a dynamo(?), kind of like a crank flashlight and then release a very high voltage electrical charge that could last at least a couple second. Or could a small dynamo create enough juice to turn something red-hot like a cigarette lighter?

 

[john monks]

A capacitor is something like a battery. It is comprised of two plate separated by an insulator. It holds a charge by piling up electrons on one place and having electrons depleted from the other plate.
You might consider using a super capacitor. They commonly in a one farad size at about 5.5 volt rating.
A small dynamo will probably burn up before heating up a cigarette lighter. But they are fine for flashlights.

 

[rokebaum]

Thanks for the reply. Your expanation of a capacitor made a lot more sense than the one on wikipedia.
So, heres a question. Could a dynamo supply the energy to the capacitor, the capacitor store that energy, flip an in-line switch between the capacitor and some type of element (like the end of a soldering iron or something smaller) and allow that electricity to run through the element which would create a significant amount of heat?
Or: Could I eliminate the capacitor and use a dynamo to charge a small rechargeable battery and could a small battery be powerful enough to make a small element glow red hot, even briefly. Then use the dynamo to recharge said battery for further element use?

 

[john monks]

Thanks for the compliment.
Yes a dynamo can be use to charge a capacitor but at least in my example not enough to power a soldering iron.
A dynamo can charge a nicad battery that can be used to power a small soldering iron for a very short period of time.
A 5.5 volt, 1 farad capacitor will store 15.125 joules of energy or 15.125 watt seconds of energy. So a soldering iron will not get very hot.
A small soldering iron will draw about 25 watts. A human being can typically put out about 150 watts continuously. And there is a loss in a dynamotor. Maybe it is only 75% efficient. But anyway the person will get quite a workout.

 

[rokebaum]

Well, thank you. You have been very helpful. I plan to buy some parts tomorrow and start experimenting. I imagine I will have more questions as I go. By the way, what is a farad? Is it like a watt and what is a watt compared to a volt? Sorry to keep bugging you but this is helpful.

 

[(*steve*)]

A Farad is the unit of capacitance that will store 1 coulomb of charge when it has a potential of 1V across it. Alternatively, if current is flowing into it at 1A, the voltage rises by 1V per second.

1 Farad is ENORMOUS. Large capacitors are measured in thousands of microfarads, small ones in fractions of a billionth of a Farad.

A watt is a measure of the rate at which you are expending energy. 1 Watt is 1 joule per second. If a resistor has 1A flowing through it, and 1V across it (which implies it is a 1 Ohm resistor) then 1 joule of energy is being expended each second. Thus, it is expending energy at a rate of 1 Watt.

This rate is called power (more power means more energy per unit time), and is often designated P.

Similarly, I is current (in amps); V is voltage (in Volts); and R is resistance (in Ohms).

There are a number of equations that relate them to each other.

V = I * R

P = V * I

From these, you can rearrange and substitute to find any of these in terms of any other two.

Look at Ohms Law on wikipedia for more information.

Note that the definition of the capacitor has a 1 F capacitor charging at 1A for 1 second and having 1V across it. This DOES NOT mean it is dissipating 1W because a capacitor stores energy, it doesn't waste it like a resistor. (in practice it wastes a little).

To properly understand capacitors you need to understand calculus (an possibly differential equations).

as an example, the relationship between capacitance, voltage, and current is given by this: i = C * dv/dt Note that time is involved!