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RESISTORS

V. Ryan © 2002 - 2015

PDF FILE - CLICK HERE FOR PRINTABLE EXERCISE BASED ON WORK BELOW

Resistors determine the flow of current in an electrical circuit. Where there is high resistance in a circuit the flow of current is small, where the resistance is low the flow of current is large. Resistance, voltage and current are connected in an electrical circuit by Ohmís Law.

When a resistor is introduced to a circuit the flow of current is reduced. The higher the value of the resistor the smaller/lower the flow of current.

   
Resistors are used for regulating current and they resist the current flow and the extent to which they do this is measured in ohms (Ω). Resistors are found in almost every electronic circuit.

The most common type of resistor consists of a small ceramic (clay) tube covered partially by a conducting carbon film. The composition of the carbon determines how much current can pass through.

   

Resistors are too small to have numbers printed on them and so they are marked with a number of coloured bands. Each colour stands for a number. Three colour bands shows the resistors value in ohms and the fourth shows tolerance. Resistors can never be made to a precise value and the tolerance band (the fourth band) tells us, using a percentage, how close the resistor is to its coded value. The resistor on the left is 4700 ohms.

 

PDF FILE - CLICK HERE FOR BASIC QUESTIONS ON RESISTOR VALUES AND OHMS LAW

   
The value of a resistor can be written in a variety of ways. Some examples are given below:


47R means 47 ohms
5R6 means 5.6 ohms
6k8 means 6800 ohms
1M2 means 1 200 000 ohms

   
A common value is 'K' which means one thousand ohms. So if a resistor has a value of 7000 ohms it can also be said to have a value of 7K.
   

 

   

RESISTORS IN SERIES AND IN PARALLEL

   

Resistors can be connected together in two ways to give different overall values. This is especially useful if you do not have a resistor of the correct value and need to make it up from other available ones.
 

   

1. Resistors in SERIES - When resistors are connected in series, their values are added together:

 

R total=R1+R2

 

For example: 1K+1K+3K9=5K9 (total value)

   
2. Resistors in PARALLEL -When resistors are connected in parallel, their total resistance is given as:

 

1/Rtotal = 1/R1 + 1/R2

 

For example: 1/Rtotal = 1/1K + 1/1K = 0.5K or 500 ohms

OR= R1 x R2

      R1 +R2 

 

    = 1 x 1 = 1

               1 +1  = 2  =  0.5k

   

Click here for more resistors in parallel questions

   

VARIABLE RESISTORS

   

Variable resistors have adjustable values. Adjustment is normally made by turning a spindle (e.g. the volume control on a radio) or moving a slider.

SYMBOL

 
   

The rotary variable resistor is the cheapest type of variable resistor. A smaller version of this variable resistor is the pre-set resistor. The pre-set resistor is the type usually used in small electronic projects (you will probably use this type in school based projects).

PRESET RESISTOR

   
 
   

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