Capacitors

A capacitor is a small device that can be charged up with electrical energy, store it and then release it. Just like a rechargeable battery. But unlike a battery, it does not use a chemical reaction and it can only hold a very small charge. A very large capacitor can only light up an LED for a few seconds. They come in many shapes and sizes and a few are shown below. The bigger the capacitor, the more charge it will hold.

A capacitor is made from two metal plates or metal foils separated by an insulator called a Dielectric material.
The Dielectric materials can be made from Ceramic, Mica, Polypropylene, Polyester, Electrolytic, Tantalum and even air.
The larger capacitors look like tubes, this is because the metal foil plates are rolled up with an insulating dielectric material sandwiched in between.

The value of capacitance is determined by

The size of the plates,
The distance between them,
The type of dilectric material used.

 

The Unit of Capacitance (C)

Capacitance is measured in Farads. (after Michael Faraday 1791 - 1867)
The Farad is too big a unit so values are measured in:-
microfarads (μF), nanofarads (nF) and picofarads (pF).
Largest value is 22000μF Lowest value is 1.0pF

1F =1,000,000μF     1μF = 1000nF     1nF = 1000pF

Circuit Identification.

In circuit diagrams a fixed capacitor is identified with the letter C. i.e. C1 C2 ... C12
Variable capacitors/Trimmers are identified with the letters VC1 VC2 .....

 

Polarised Capacitors.

Electrolytic and Tantalum capacitors are POLARISED and they must be connected the correct way round. (correct polarity).
The casing is marked showing the Negative lead which should be connected to the Negative rail (0 Volt). The circuit symbol shows the + Positive lead.
All others capacitors can be connected either way round.

Use of Capacitors.

Capacitors are used in following ways:-

1. Store a voltage for a period of time,
2. Create a time delay circuit.
3. Shorten or extend pulse lengths,
4. Smooth fluctuating voltages,
5. Filter unwanted frequencies,
6. Allows Alternating Current (ac) to pass to another part of a circuit but blocks Direct Current (dc).

Working Voltage.

The working voltage of the capacitor must not be exceeded. It is good practice to choose a capacitor with a working voltage 50% higher than the circuits normal working voltage. Care should be taken with polarised Electrolytic and Tantalum capacitors as they have low working voltages. For a 9 Volt circuit choose a 16V or higher capacitor. The higher the voltage, the bigger and more expensive they get. Manufacturer's catalogue will give you all the information you need.

Leakage Current.

The dielectric is an insulator and the current should not flow through it. However a perfect insulator does not exist and a small leakage current will flow out eventually discharging the capacitor.

Capacitance Code:-

Most capacitors have a tolerance of 20% and have the following numerical values

10    15    22    33    47    68    82

As many capacitors are small, the values are printed with a three number code. The first two refer to the numerical values and the last gives the numbers of zeros.
Some old capacitors are colour coded in a way similar to resistors.

 

How a Capacitor works.

At the start the capacitor is fully discharged. When the switch is closed, the capacitor is charged up from the energy stored in the battery until the capacitor has the same voltage as the battery. At first it charges up rapidly and then gradually slows.   Open switch. The capacitor remains fully charged. Pushing the RESET button short circuits the capacitor and the energy stored in the capacitor is now discharged, slowly at first. With small capacitors the energy discharge is very fast, almost immediate. With large capacitors, this can take a long time. This is why capacitors are used in timing circuits.

This Animation is incorrect in two ways.

The Reset button is pushed to connect both ends of the capacitor together. This causes the capacitor to discharge.
The animation shows the Reset button being used to trigger the the capacitor to discharge. Once you release the reset button the capacitor will stop discharging.

The capacitor is shown charging and discharging at a steady rate. This is incorrect, see the graph and animation below. (Due to limitations of my JavaScript coding)

Time Delay

If a resistor is put in series with the capacitor, the time it takes to charge and discharge can be slowed down. The resistor is slowing down the current flow into the capacitor. Note again, how the charge rate gradually slows down as the capacitor becomes fully charged or fully discharged.

 

CR Time Constant

The Time Constant T is the time taken to charge the capacitor to 2/3 the supply voltage. (Shown green in the above animation). It also applies when the capacitor discharges. It can be found from the following formula:-

Time Constant    T = C x R

T is in Seconds. (t)     C is in MicroFarads. (μF)     R is in MegaOhms (MΩ)

Some books use the formula T = 1.1 C x R. This is unnesessary. It is impossible to calculate the time accurately as Capacitors have a 20% tolerance and Resistors have a 5% tolerance. Experiment to get the correct time delay.

The following Chart is used to find out the values for the Capacitor and Resistor for a specified time delay.
The blue marks show the available capacitor values and major resistor values.
The values on the graph are reasonably accurate. To get longer time delays, you can use larger values but the result will depend on the ACTUAL value of the components. You will have to experiment to get the correct time delay.

Switch Spark Protection

When a switch opens the current tries to keep going by jumping across the gap creating a spark. (You can see this through the switch cover plate when you turn off a mains light switch).
Adding a Resistor/Capacitor network across the switch absorbs the energy of the spark. This protects the contacts from being worn away by errosion.
Injection Plastic moulds are machined using this principle of Spark Errosion. Very complex shapes can be made to a very high finish.