Basic Electronics

Resistors

Bukit View Secondary SchoolDesign & TechnologyMr Siraj S.H / Mr Ong Soon Gee

Resistors determine the flow of current in an electrical circuit.

Where the resistance is low the flow of current is large.

Where there is high resistance in a circuit the flow of current is small.

Ohm’s LawResistance is defined as the ratio of the potential difference (voltage) across a conductor, to the current flowing through it.

( I ) Current is what flows on a wire or conductor like water flowing down a river. Current flows from negative to positive on the surface of a conductor. Current is measured in (A) amperes or amps.

( V ) Voltage is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit, and is measured in (V) volts.

( R ) Resistance determines how much current will flow through a component. Resistors are used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms.

Symbol Name SI Unit

R Resistance Ω

V Voltage V

I Current A

R= V/I

• Resistors are used to regulate current in a circuit.

• 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 standard resistor color code table:

Color 1st digit 2nd digit 3rd digit* Multiplier Tolerance

Black 0 0 0 ×100  

Brown 1 1 1 ×101 ±1% (F)

Red 2 2 2 ×102 ±2% (G)

Orange

3 3 3 ×103  

Yellow 4 4 4 ×104  

Green 5 5 5 ×105 ±0.5% (D)

Blue 6 6 6 ×106 ±0.25%(C)

Violet 7 7 7 ×107 ±0.1% (B)

Gray 8 8 8 ×108 ±0.05% (A)

White 9 9 9 ×109  

Gold       ×0.1 ±5% (J)

Silver       ×0.01 ±10% (K)

None         ±20% (M)

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Resistors can be connected together in two ways; series and parallel

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