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Resistors Knowledge
Release date:
04 Jan,2022
1.「Ohm's Law」- Basics of resistor
To understand a resistor, the important thing is to understand Ohm's Law, first of all.
Ohm's law: the potential difference between two points in an electrical circuit is proportional to the current flowing between them.
If the voltage is E (V: volt) and the current is I (A: ampere), the following relationship holds.
Ohm’s law : E=I・R, I=E/R
"R" represents the coefficient of proportionality and is called resistance (electrical resistance). It is the quantity symbol and Ω (ohm) is the unit symbol.
When the electrical resistance is high, it is difficult for the current to flow, and when the electrical resistance is low, it is easy for the current to flow.
Using this phenomenon, resistor, as an electronic component, provide constant current flow through a circuit or current adjustment as needed. Resistors are also used to reduce or divide voltage.
As seen from above, resistors along with capacitors and inductors (coil) are the major passive components, which are essential basic components for electronic circuits
2. Resistors’ Base substrate
Alumina-based insulators are the most commonly used base substrate. Materials suitable for the application are selected as the base substrate of the resistor. (Based on thermal conductivity, thermal expansion coefficient, mechanical strength, etc.)
Determines basic characteristics and performance of resistor:
the temperature coefficient of resistance (TCR).
Electric resistance R=ρ・L/S [Ω]
・Cross-sectional area [cm2]: S
・Length [cm]: L;
・Specific resistance of material [Ω・cm]: ρ
3. Classification of resistors
Fixed resistors are divided into leaded type (with lead wire) and surface mount type (without lead wire). Surface mount type includes rectangular type and cylinder type. In addition, division goes further depending on the sealing material: resin mold type and ceramic case type.
As for the material of resistive element, followings are available: carbon film, metal film consist mainly of nichrome, metal oxide film, metal glaze composites consist of metal oxide and glass; ones with metal plate, metal wire or metal foil; solid type consist of metal oxide ceramics.
According to its application, there are those with highly accurate resistance tolerance and temperature characteristics, those withstand high voltage and surge, those with large fluctuations in resistance value depending on temperature, and those that also have a fuse function.
It is important to select an optimal combination out of available options according to the purpose of the application when selecting a resistor.
4. Terms and Definitions
●Rated Power
．The maximum value of power which can be continuously loaded to a resistor at a rated ambient temperature.
●Rated Voltage
．The maximum value of D.C. voltage or A.C. voltage(effective value)capable of being applied continuously to a resistor at the rated ambient temperature.
Rated voltage shall be calculated from the following formula.
Rated voltage(V)= √(Rated power(W)×Nominal Resistance value(Ω) )
When the nominal resistance value exceeds the critical resistance value, the Rated voltage is limited to the maximum working voltage.
●Critical Resistance Value
．The maximum nominal resistance value at which the rated power can be
loaded without exceeding the maximum working voltage.
●Maximum Working Voltage
．The maximum value of D.C. voltage or A.C. voltage(effective value) capable of being applied continuously to a resistor.
●Maximum Overload Voltage
．The maximum value of D.C. voltage or A.C. voltage(effective value)capable of being applied to a resistor for five second .Typically loaded voltage shall be 2.5 times larger than rated voltage. However loaded voltage shall not exceed maximum overload voltage.
●Dielectric Withstanding Voltage
．A.C. voltage(effective value) that can be applied to a designated spot between the electrode and the outer coating for a minute in dielectric withstanding voltage test.
●Rated Ambient Temperature
．The maximum ambient temperature at which resistor is capable of being used continuously with the rated load applied. Rated ambient temperature refers to temperature around a resistor inside of equipment, not to outside air temperature.
●Derating curve
．A curve that expresses the relation between ambient temperature and the maximum power, the later generally expressed in percentage.
●Temperature Coefficient of Resistance(T.C.R)
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T.C.R. shall be calculated from the following formulas. |
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T.C.R(ppm/℃) = |
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R: Measured resistance(Ω) at T R0: Measured resistance(Ω) at T0 T: Measured test temperature (℃) T0: Measured base temperature (℃)
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04 Jan,2022
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