A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors act to reduce current flow, and, at the same time, act to lower voltage levels within circuits.
In electronic circuits resistors are used to limit current flow, to adjust signal levels, bias active elements, terminate transmission lines among other uses.
The resistor's ability to reduce the current is called resistance and is measured in units of ohms (symbol: Ω).
Resistor Symbol
Typical schematic diagram symbols are as follows;
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(a) resistor, (b) rheostat (variable resistor), and (c) potentiometer
Ohm's law
The behavior of an ideal resistor is dictated by the relationship specified by Ohm's law:
V=I R
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.
The ohm (symbol: Ω) is the SI unit of electrical resistance. An ohm is equivalent to a volt per ampere.
Series and Parallel resistors
The total resistance of resistors connected in series is the sum of their individual resistance values.
Req
= R1 + R2 + . . . . . + Rn
The total resistance of resistors connected in parallel is the reciprocal of the sum of the reciprocals of the individual resistors.
1/Req
= 1/R1 + 1/R2 + . . . . . + 1/Rn
A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other.
Dimensions and material affects
The resistance R in ohms (Ω) of a resistor is equal to the resistivity ρ in ohm-meters (Ωm) times the resistor's length l in meters (m) divided by the resistor's cross sectional area A in square meters (m2):
ρ *
ℓ / A
Resistor Color Codes Table
Resistance calculation of 4 bands resistor
R = (10×1st Band+2nd Band)×multiplier
Resistance calculation of 5 or 6 bands resistor
R = (100×1st Band+10×2nd Band+3rd Band)×multiplier
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