Resistors

Resistance and Resistivity

Resistance (R) is the measure of how much an object - a resistor - opposes the flow of electric current. The resistance of an object is measured in Ohms (<math>\Omega</math>)

Resistivity (<math>\Rho</math>) is a measure of how strongly a material opposes the flow of electric current. Resistivity is measured in ohm metres (<math>\Omega</math>m). Related wiki page: table of resistivities

The relationship between resistance and resistivity is:

<math>R = {l \cdot \rho \over A} \,</math>

where

"l" is the length of the object

"A" is the cross sectional area of the object (assuming constant), and

"ρ" is the resistivity of the material

Resistors

A resistor is a passive electrical device that has been manufactured specifically to add resistance to a circuit, to effect a voltage drop.

The mathematical equation that describes the relationship between resistanece, voltage drop and current through a resistor is known as Ohms Law

<math>I = \frac{V}{R}</math>

where

"I" is the current in amperes,

"V" is the potential difference between the ends of the resistor in volts, and

"R" is the resistance of the resistor, measured in ohms

Resistor colour bands

The resistance of a resistor is coded into a series of coloured bands along its length:

• band A is first significant figure of component value
• band B is the second significant figure
• band C is the decimal multiplier
• band D if present, indicates tolerance of value in percent (no color means 20%)

Color	Band A	Band B	Band C (multiplier)	Band D (tolerance)	Temp. Coefficient
Black	0	0	×100
Brown	1	1	×101	±1% (F)	100 ppm
Red	2	2	×102	±2% (G)	50 ppm
Orange	3	3	×103		15 ppm
Yellow	4	4	×104		25 ppm
Green	5	5	×105	±0.5% (D)
Blue	6	6	×106	±0.25% (C)
Violet	7	7	×107	±0.1% (B)
Gray	8	8	×108	±0.05% (A)
White	9	9	×109
Gold			×10-1	±5% (J)
Silver			×10-2	±10% (K)
None				±20% (M)