Capacitors in a Basic CircuitCapacitors in a Basic Circuit

Up to this point all the components that we have seen so far (resistors, photoresistors, potentiometers and LEDs)

are all components that control current as it flows through them.

Capacitors are a little different in that they don’t allow electrical current to flow but they actual store energy in the form of static charge.

A capacitors is used as a temporary power source that gains static charge when there is a voltage present across it and then discharged when voltage is removed.

Capacitors consist of two conducting plates separated by an insulator (non-conducting material called a dielectric). The insulative material provides a barrier between the plates that allows a static charge buildup to take place. When a power source is connected across a capacitor with the negative leads connected, negative electrons flow to one plate and positive holes are left on the other plate. A capacitor continues to charge up until one plate has received as many negative electrons as it can hold and the other plate has been depleted of as many electrons it can loose. When the capacitor is fully charged its polarity is opposite of the battery.

When identifying a capacitor two rating values are shown on the casing; Capacitance and Breakdown Voltage.

Capacitance is the amount of charge a capacitor can hold and is measured in farads symbolized by the letter “f”.

The Breakdown voltage rating tells you the maximum voltage level that can be supplied to the capacitor before the dielectric breaks down. This will destroy the capacitor.

A farad of capacitance is a very large unit of measurement and is defined as 1 coulomb of electrons holding 1 volt of pressure in one second.

C = Coulombs / Volts (C = Q/V)So in order to get 12 volts out of a capacitor you

would have to pass 75.36 x 10^18 electrons onto one of its plates (that’ huge).

Remember a coulomb of electrons is (6.28 x 10^18)

Most often you will see microfarads (µf) 1/1,000,000 of a farad, or even picofarads (pf) 1/1,000,000,000.

microfarads are farards multiplied by .000001And picofarads are multiplied by .000000001

The symbols are shown using greek symbols

µf for microfarads ρf for picofarads

Capacitors are classified by their dielectric ability to establish and hold a barrier between the plates and this can be determined by any of the three factors below.

The size of the plates The distance between the plates The type of material used between the plates

Typical materials are mica, ceramic, plastic, and electrolytic (aluminum oxide and tantalum oxide).

If the plates are larger more electrons can be held permitting more capacitance,

If the dielectric between the plates is a more insulative material it would take more capacitance before breaking through

If there is more distance between the plates then there is more resistance to overcome before breakthrough can occur.

Current flows in the circuit for only a short time while plates A and B gain and loose electrons respectively, when the plates are fully charged then current in the circuit stops flowing.

The capacitor at this point is a source of power in its own right just like a battery. The capacitor can be removed from the circuit and used to power a load such as an LED independently for a very short amount of time. Highly charged capacitors, like the one found inside television sets, can deliver serious instantaneous shocks and burns, if improperly handled.

Types of CapacitorsTypes of Capacitors

There are basically two types of capacitors, fixed and variable. As the names would imply, a fixed capacitor has a set amount of capacitance, while a variable capacitor can have its capacity changed over a wide range. We will restrict our observations to fixed capacitor in this class.

Fixed capacitors are then broken down into two more categories, polarized and non-polarized. An electrolytic capacitor has a polarity. This means that one lead is to be hooked only to a negative charge and the other lead is to be hook only to a positive charge. This type of capacitor must be connected into a circuit in the correct direction in order for the capacitor to work. Smaller ceramic capacitors do not have polarity.

They can be connected into a circuit in either direction without affecting the circuit. In a schematic drawing polarized and non-polarized capacitors are drawn differently. see fig. 4-4 below.

Ceramic non-polarized Electrolytic polarized

Small Ceramic Disk Capacitors use rating marking similar to a resistor except numbers are used instead of bands. For instance a .03 uf capacitor would be marked as a 303. The first two numbers are place value and the third number is a reverse multiplier (a divider). 30 x .001 = .03.

There are certainly more resistors and capacitors found in electronic devices than any other components. Choosing them wisely and inserting them in the circuit correctly is most important. Just remember there are two things about each you need to know.

resistors: value in Ohms and wattage rating in watts

capacitors: capacitance in microfarad (µf) and voltage rating in volts

Charging Effects of a CapacitorCharging Effects of a Capacitor

An R-C timing circuit ( a circuit with a resistor and capacitor in series) allows us to measure and vary the charge and discharge time of a Capacitor. The formula to calculate this timing is stated here:

τ = R x C Where τ is the time constant in seconds, R is

resistance in ohms and C is capacitance in farads.

When a capacitor is charged it does not charge at a constant rate.

63.2 % of the total charge is realized within the first time constant (τ).

Only another 23.2 % is charged in the second time constant,

then only 7.9% and so on.

It takes 5 time constants to fully charge a capacitor.

The formula to calculate any desired time duration of the first time constant is shown here.

Total Voltage x first time constant x Resistance x Capacitance = time duration

Example

5 x 63.2% x R x C = 31.6

Capacitors are not generally connected into a circuit in the same manner as the previous components we looked at.

The earlier components we have studied are connected in Series. In a Series circuit all components are connect so that current will enter and then exit each component one at a time. Therefore there is only one pathway for the current to flow through. Due to the fact that capacitors do not allow current to flow through them they must be connect in a circuit that will flow to the capacitor plates at the same time it flows through another portion of the circuit. This is known as a Parallel Circuit.

The circuit in fig 4 - 3 shows a parallel circuit where the current at point A splits up and flows through two different pathways until they again combine at point B where the current once again flows together.

fig 4-3

So in order for a circuit containing a capacitor to continue to have a current flow the capacitor must be inserted in the circuit in a Parallel branch as in the example in fig 4-4. At first the current will flow into the capacitor and the resistor at the same time, then as the Capacitors plates become full of electrons all of the current will be diverted to the resistor branch.

fig 4-4

Capacitor can be group together with other capacitors. If they are connected in parallel the capacitance will increase due to the fact that it would be like increasing the plate size. If they are connected in series the capacitance is decreased, however it takes more breakdown voltage. The algebraic formulas below can show these increases and decreases.

in parallel CT = C1 + C2 + C3 + Cn

in series 1/CT = 1/C1 + 1/C2 + 1/Cn

or CT = 1

1/C1 + 1/C2 + 1/Cn