Electronics Fundamentals - Introduction to Resistors

 In the previous post we discussed about voltage, and current. In this post we will know more about the relationship between Voltage and Current i.e. Resistors (unit symbol: Ω or ohm) - heart of electronics! The interesting I versus V characteristics that can be useful are:

  1. Resistors: I proportional to V
  2. Capacitors: I proportional to rate of change of V
  3. Diodes: I flows in one direction
  4. Thermistors: Temperature dependent resistors
  5. Photoresistors: Light dependent resistors

Fig: Resistor.


> Wirewound Ceramic Power Resistors: Example Datasheet 1

20W vitreous enamel with leads (view datasheet for more examples and types).

30W vitreous enamel

5W and 20W with mounting studs

20W Resistor with mounting studs

> Wirewound Power Resistors: Example Datasheet 2

1W, 3W, and 5W axial ceramic

5W, 10W, and 25W conduction cooled / Dale-Type

50W conduction cooled (Dale-Type)

> Carbon, Surface-Mount Resistors, and other types:

2W, 1W, 1/2W, 1/4W, and 1/8W Carbon composition

Surface-mount thick film - 2010, 1206, 0805, 0603, and 0402 sizes

surface mount resistor array: 6-, 8-, 10-pin single in-line package arrays, dual in-line package array

ubiquitous RN55D 1/4W, 1% metal-film type

Victoreen high-resistance (glass, 2 GΩ, and ceramic 5 GΩ)


The current through a metallic conductor or a semiconductor is proportional to the voltage across it. The wire conductors used in circuits are usually have a thick gauge so that these voltage drops are negligible.

A Resistor is made out of a conducting material such as carbon, thin metal, or carbon film, or a wire of poor conductivity with contacts at each end.

R = V/I

In the above equation, R is in ohms, V is in volts, and I in amps. This is known as ohms law.

  • Meaninglessly, Resistance is used as a way to convert voltage to a current, or vice versa.
  • Resistors are used everywhere and of different types.
  • Resistors are used with capacitors as a time constant to act as filters.
  • Used to dissipate power and reduce voltages, measure current, or to discharge a capacitor after turning off the power.
  • Used in precision circuits to establish currents and provide accurate voltage ratios in order to set precise gain ratios.
  • In logic circuits - resistors act as bus and line terminators and as pullup and pull-down resistors.
  • In high-voltage circuits they are used to measure voltages to equalize leakage currents among diodes / capacitors connected in series.
  • RF circuits - set bandwidth of resonant circuits.
    • Used as coil forms for inductors.
  • Resistors range - 0.0002 Ω to 1012 Ω, standard power ratings from 1/8 watt through 250 watts
  • Accuracies from 0.005% through 20%.
  • Common materials used are:
    • carbon-composition
    • ceramic-composition moldings
    • metal foil or metal wire wound on a form
    • semiconductor elements similar to FETs
  • Most widely used packages:
    • cylindrical axial-lead type - typified by the generic RN55D 1% 1/4 W metal-film resistor
    • SMT - Surface mount chip resistor - 5% / 2% / 1% tolerances ranging from 1 Ω to 10 MΩ.
      • 1% 96 values per decade
      • 2% or 5% types have 48 and 24 values per decade
  • Easy to use.
  • Defect in resistors depend upon:
    • Temperature
    • Voltage
    • Time
    • Humidity
    • Also depends on Inductance (more defective in higher frequencies)


  • Series: R = R1 + R2

  • Parallel (used to decrease resistance): R = R1.R2 / (R1 + R2)    OR    1 / ( ( 1 / R1  ) + ( 1 / R2  ) )

Fig: Series and Parallel Resistors connections respectively (image source).