Basic Components

Resistors are available in values from fractions of ohms up to many millions of ohms (Megohms) although only certain 'preferred' values are readily available. To identify the resistance value of resistors a 'colour code' system is often used

Variable Resistors

Variable resistors or potentiometers are used where the flow of current in a circuit needs to be occasionally varied such as for:-

Volume Control

Frequency Control

Output Voltage Control

They come in a range of sizes.


Capacitors are used to store electric charge which can be released into a circuit when required. The unit of capacitance is the Farad which is a very large unit. Most commonly used capacitors are rated in microfarads - µF (millionths). Large value capacitors are often used as smoothing capacitors in A.C. power supplies. Other uses for capacitors are in timing circuits and filter circuits. There are two main types of capacitor - Polarised and non polarised.


Switches are used to make or break an electrical circuit. They can also be used to divert the current path into one of two or more directions (ways or throws), and can also switch more than one circuit at once.

For example, a double pole double throw switch (dpdt) can be used as either an "on/off" or "changeover" switch controlling two circuits at the same time.


A relay is a device which uses a small control current to switch a much larger load current. They are used in switching circuits where the output of that circuit has insufficient power to drive the output device. They have disadvantages in that they can only operate at low switching speeds and are relatively expensive.

Resistor Formulae:

Resistor in Series (one after the other)
The total value is found by simply adding all the separate values together.

Resistor in Parallel (side by side)


1/Rtotal = 1/R1+1/R2+1/R3+1/R4

1/Rtotal = 1/100+1/200+1/100+1/200 200
Find the common Dominator i.e. (400) 
1/Rtotal = 4/400+2/400+4/400+2/400 
Add value together 100
1/Rtotal = 12/400
Rtotal/1 = 400/12
Rtotal = 400/12 = 33.3 ohms 200

Power Formulae

Watts = Volts x Current
                      Volts = Watts / Current
                      Current = Watts / Volts 
                      Watts = Current˛ x 
                     V = Voltage
                      I = Current
                      W = Watts 
                      R = Resistance

                     W = V x I
                     V = W / I
                      I = W / V
                      W = I˛x R

Electronic Components and Symbols

1. What is the function of a double pole switch?
    a) To disconnect a circuit
    b) To protect the other components in a circuit
    c) To divert the current from one path to another
    d) To resist the flow of current
2. What resistor value would be indicated by the colour bands red, red, orange?
    a) 22000W
    b) 200 W
    c) 220 W
    d) 22 W
3. What resistor value would be indicated by the colour brown, black, green?
    a) 1000 W
    b) 10000W
    c) 100000W
    d) 1000000W
4. If a 1000W resistor has a gold tolerance band, what are the highest and lowest values it could have?
    a) 1100W and 900W
    b) 1050W and 950 W
    c) 1500W and 500 W
    d) 1025W and 975W
5. What is the main function of a capacitor?
    a) To connect a circuit
    b) To store charge
    c) To determine current flow
    d) To resist the flow of current