The Capacitor Part 1 - Theory of Operation

Today we are going to talk about another key element of electronics, the humble capacitor. They come in many forms like electrolytic, ceramic, or even super capacitors, but that's for a future post. Today we are going to discuss the basic idea of capacitors and how they work.

A capacitor is a component that stores electrical energy in an electric field. Technically it is a temporary storage solution, compared to a battery which is a more permanent solution. In it's basic form, a capacitor is two sheets of conducting material separated by an insulator (also known as a dielectric). It's important that these plates don't touch, hence the insulator.

If you want to understand the way a capacitor works, you need to think about charge. Think about the literal electrons flowing through the wire. When a voltage is put across the capacitor, the electrons flow on to the negative plate. This is why the plates need to be separated by an insulator, because the plates cannot complete the circuit. As electrons are negatively charged, the negative plate becomes much more negative than the other plate. The capacitor becomes polarized, and this big negative charge induces a positive charge onto the other plate.

When the capacitor is then disconnected, the charge is still there, in the form of electrons; and it will stay there until the capacitor is used to power something. This charge can flow out of the capacitor through a load, like a resistor, until the charges on the plate equalize.

Next we are going to look at what capacitance is, and how capacitors are rated.

Resistors and Ohm's Law

If there is one thing that every single electronic engineer needs to know, its Ohms law, it describes the basis of all electronics. Published in 1827 as the book Die galvanische Kette, mathematisch bearbeitet ("The galvanic circuit investigated mathematically") Ohm's Law states

That the current through a conductor between two points is directly proportional to the voltage across the two points.

To put it slightly simpler, if there is current flowing through a conductor, then that current is proportional to the voltage across that conductor. The proportional bit is the resistance. Basically, if you have a resistor, with a voltage across it, the current will be a certain amount (calculated using ohms law). If that voltage was doubled, the current would also double.

The equation representing this law is:

V = I x R

V = voltage across the conductor
I  = current flowing through the conductor
R = the resistance of the conductor

Example

Lets say that we have a constant 5v supply, and need to draw a constant 10mA from that power supply. We need a resistor, and we can use Ohm's law to figure out the value. Start with the Basic equation:

V = I x R

Then rearrange it so resistance is the subject:

R = V / I

Replace V and I with the values shown above (remember that 10 mA is actually 0.010A).

R = 5 / 0.010

R = 500Ω