The discrete electronic resistor is used to control the flow of current by way of inserting resistance in the path.
It is used in many electronic circuits for various reasons as mentioned below:
• To obtain desired voltages(power) at various points in the circuit
• To obtain variable voltage from fixed voltage using variable resistor
• To obtain changing voltage using changing current across resistance in voltage amplifier
Figure-1 depicts fixed resistor and variable resistor symbols. While purchasing or selecting resistor for your need tolerance, power rating and stability (against temperature,physical conditions) specifications are considered. Resistance is measured in Ohm.
Fixed resistor does not change resistance value. Fixed resistors are of various types based on their design and material such as carbon composition, carbon film, metal oxide, wire wound etc. Metal oxide offer highest stability over long time. Wire wound resistors are used in multimeter design due to its low tolerance(i.e. good accuracy) and high stability.
Resistor value using Color code
The first three bands provide the resistor value and fourth band provide the tolerance. The first and second band give the first two numbers and third one gives number of zeros. Tolerance is as per fourth band color. This is mentioned below in the two tables.
If the color codes on the resistor is yellow(4), violet(7), red(two zeros) and silver. The resistor value will be 4700 Ohm with tolerance of +/-10%
|+/-20||No fourth band|
Resistors in Series
As shown in the figure, if two resistors are connected in series, following can be derived.
• Current flow(I) will be same from them.
• Total voltage is summation of voltage across both these resistors. (V=V1+V2) equals (I*R=I*R1+I*R2)
• Total resistance is summation of resistance value of both these resistors. (R=R1+R2)
Resistors in Parallel
As shown in the figure, if two resistors are connected in parallel, following can be derived.
• Votage across both resistors will be same.
• Total current is summation of currents through them. (I=I1+I2)equals (V/R) = (V/R1) + (V/R2)
• Total resistance (R) = R1*R2/(R1+R2)
Depending upon applications resistors in series and resistors in parallel configurations are used in the electronic circuit design.
There are many resistor manufacturers across the world which include Mouser Electronics, vishay, Riedon, murata, precision resistor, ohmite, innovativepp, SRT-restech and so on.
Vishay resistors and vishay capacitors are considered to be very accurate and used for wide variety of applications. User has option to choose resistor based on various technologies(thick film, thin film, carbon film, metal oxide film, wirewound) as well as mounting applications(leaded and surface mount).
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