Resistors: Difference between revisions
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Revision as of 03:59, 8 March 2008
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) |