Resistors

The resistor is a passive electrical component whose function is to introduce resistance to the flow of electric current in an electrical circuit to limit the current. The magnitude of the opposition to the flow of current is called the resistance of the resistor. A larger resistance value indicates a greater opposition to current flow. The resistance is measured in ohms (Ω), and its equation is as follows.

R=V/I

The voltage (V), current (I), and resistance (R) are related by Ohm’s law. i.e. V = IR. The higher the resistance R, the lower is the current I for a given voltage V across it. It is a linear device.

Resistors dissipate electrical energy given by P=I² R Watts or Joules/sec.

resistor-symbol.jpgresistor-symbol.jpgResistor

Resistors are made using different materials such as carbon film, metal film, etc. However, we will concentrate on the most common varieties and their attributes.

Resistors’ values vary from milliohms to mega ohms and the tolerance of typical resistors varies from 1% to 5%. However, for precision resistor tolerance varies below 1% from 0.1% to 0.001% and hence they are more expensive and are used in analog circuits where precise/reference voltage is needed. Commonly used Resistor are available with maximum power rating of 1/8(0.125W), 1/4W (0.25W), 1/2W (0.5W), 1W, 5W. Based on the values and power ratings, SMD resistors are made in different sizes codes 1210, 1206, 0805, 0603, 0402, 0201. This also includes R-packs resistor network used for pull up /pull down for circuits interfaces.

Different types of resistors by size and form

  • Through-hole resistors
  • Surface-mount resistors SMD/SMT

Different types of resistors by application

  • Common resistor: used in current limiter, setting biases, voltage dividers, pull up, filtering, termination resistors, load resistors, etc.
  • Precision resistor for voltage feedback circuits, voltage references.
  • Current sense resistors
  • Power resistors

Resistor selection parameters

While selecting any resistor in the circuit, the designer needs to consider the following parameters based on the application and real-estate available on the printed circuit board.

  • Resistance value(R),
  • Power (Wattages) dissipated across it,
  • Tolerance (+/- %)
  • Size based on available space on PCB.

Resistor manufacturers: AVX, Rohm, Kemet, Vishay, Samsung, Panasonic TDK, Murata, etc.

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