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Resistors and Resistance

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					Resistors & Resistance


Resistors and Resistance

To control the flow of CURRENT in a circuit we use components called
RESISTORS. Resistors restrict the flow of current, their ability to do this
is called RESISTANCE and it is measured in units called OHMS ().

The resistors used in electronics are small CARBON resistors that have
coloured bands on them to tell us how large the resistor is. The details of
this RESISTOR COLOUR CODE are shown below:




                                                             4th Colour band
    1st colour                                               (Usually silver
    band                                                     or gold)
    (RED)
                            2nd Colour   3rd Colour band –
                            band         Indicates the
                            (RED)        number of zero’s
                                         (RED)


The 4th colour band will be ignored in this exercise but it does help to
show the resistor is held the right way around.
From the colour code, the resistance value is 2, 2, 00

Resistance = 2200 
           = or 2.2 k

    Colour          Number
Black                 0
Brown                 1
Red                   2
Orange                3
Yellow                4
Green                 5
Blue                  6
Violet                7
Grey                  8
White                 9

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Resistors & Resistance

The BS 1852 RESISTOR CODE

The values of resistors can also be given by using a code of numbers and
letters. This is the British Standard 1852 Code.

The letters mean the following values:-

R means x 1
K means x 1000 (k = kilo)
M means x 1000,000 (M = mega)

Examples

Value of Resistance      Using the letter code    Correct BS 1852
()                      ()                      marking
470.0                    470.0                    470R
4,700                    4.7k                     4k7
1,800,000                1.8000000M               1M8


IMPORTANT

1k = 1k = 1000
1M = 1M = 1,000,000

Converting  to k - DIVIDE by 1000

6,800 = 6,800 / 1000
       = 6.8k

Converting  to M - DIVIDE by 1,000,000

3,900,000 = 3,900,000 / 1,000,000
           = 3.9M

           Conversion Chart

               1000           1000

                        k            M

              X 1000            X 1000


                                 Page 2 of 5
Resistors & Resistance


Resistor Tolerance
The tolerance of a resistor is the manufacturers guaranteed accuracy
range. The tolerance value is indicated by the 4th band on the resistor.


      Band Colour                         Tolerance
Gold                         +/-5%
Silver                       + / - 10%
NO BAND                      + / - 20 %

Calculating the range of values

+ / - 5% means the resistance could be 5% higher or 5% lower than the
nominal value indicated by the band colour.

Example 1

Nominal value 100 +/ - 5%

Highest possible value, 100 + (5% of 100)
= 100 + 5
=105 

Lowest possible value, 100 - (5% of 100)
= 100 – 5
= 95                        5% of 100
                             (5 divided by 100) multiplied by 100
Range 95 to 105z           (5 100) X 100
                             =5
Example 2

Nominal value 47k +/- 10%

Highest Value 47k + (10% of 47k)
= 47k + 4.7k
=51.7k

Lowest value 47k – (10% of 47k)
= 47 – 4.7k
= 42.3k                     Range 42.3k to 51.7k


                                  Page 3 of 5
Resistors & Resistance


Resistors in Series
Resistors in Series are connected end to end as shown below:

       R1              R2             R3              R4




The total resistance between A and B is given by:-




              Total Resistance = R1 + R2 + R3 + R4



The total resistance is sometimes called the ‘combined resistance’ or
‘equivalent resistance’

Examples

     1) What is the total resistance of 100 and 220 in series?
     Ans: Total Resistance: 100 + 220 = 320

     2) What is the total resistance of 220, 470 and 670 in series?
     Ans: Total Resistance: 220 + 470 + 670 = _____

     3) Find the total resistance of the combinations shown below, show
        ALL workings.


A)                             B)


C)                                             ANS: A) Rt = 47+100 = 147
                                                    B) Rt = 390+470 =860
                                                    C) Rt = 1.5k+2.5k+15k
                                                          = 19k



                                 Page 4 of 5
Resistors & Resistance


Resistors in Parallel
Resistors in parallel are connected one on top of the other
                          R1




                          R2




The total resistance between A and B is given by:

           Total Resistance = R1 x R2 = Product of 2 resistors
                              R1 + R2       Sum of 2 resistors
           For more than 2 resistors in parallel:

           1 / Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4……etc

The total resistance is sometimes called the ‘combined resistance’ or
‘equivalent resistance’

Examples

   1) What is the total resistance of a 4 in parallel with 12?




                                 ANS: R1 x R2 4 x 12 = 48 = 3
                                     R1 + R2 4 + 12 16

   2) What is the total resistance of 12 in parallel with 60
   ANS: R1 x R2 / R1 + R2 =12 x 60 / 12 + 60
   =720 / 72
   =10


                                Page 5 of 5

				
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