# Basics of resistor

## What is 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. The circuit symbol is as shown in the following figure.

[Resistor circuit symbols]

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.

## Material composition and role of resistors

Resistor is basically composed of following four elements.

### Base substrate:Holds resistive body and terminals

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.)

### Resistive element: Determines basic characteristics and performance of resistor

Electrical resistance

The electrical resistance of the conductor is given by the following formula.

Electric resistance R=ρ・L/S [Ω]

・Cross-sectional area [cm2]: S
・Length [cm]: L;
・Specific resistance of material [Ω・cm]: ρ

Resistance & T.C.R. of major metals

Metals Symbol Electric resistance
(μΩ・cm)
T.C.R.
(10-6/K)
Gold Au 2.4_ 4,000
Silver Ag 1.62 4,100
Platinum Pt 10.6_ 3,900
Nickel Ni 7.24 6,700
Tin Sn 11.4_ 4,500
Bismuth Bi 120___ 4,500
Tantalum Ta 15.0_ 3,500
Iron Fe 9.8_ 6,600
Copper Cu 1.72 4,300
Nichrome Ni-Cr (80:20) 108___ 100
Manganin Cu-Mn-Ni 40___ -5～+10

### Terminal:Electrically and mechanically connects the resistor and the board (circuit pattern).

The electrical connection point of the resistor, where voltage and current are applied. The structure and material are configured so that it does not affect the characteristics of the resistor.

Select a resistor with a terminal structure and shape suitable for the mounting method. Surface mount components (SMD) are now the mainstream in order to improve the mounting density and reduce the mounting cost..

### Example of resistor structure

• Flat Chip Resistor