Introduction
From very small surface-mount chip resistors to large wire wound power resistors, many different types of resistors are available for electronics applications. The primary function of a resistor in an electrical or electronic circuit is to resist, control, or set the flow of electric current by using a conductive material with a specific resistance value.
Resistors can be connected in series or parallel to form resistor networks. These networks are commonly used as voltage dividers, current limiters, or voltage droppers in electronic circuits. Resistors are classified as passive devices because they do not generate power or provide amplification. Instead, they reduce voltage or current levels and dissipate electrical energy in the form of heat.
Basic Operation of a Resistor
For current to flow through a resistor, there must be a potential difference across its two terminals. This potential difference accounts for the energy dissipated as heat. In DC circuits, the voltage drop across a resistor is measured as current flows through it. Most resistors obey Ohm’s Law, making them linear components.
By selecting appropriate resistance values, resistors can be used to convert voltage into current or current into voltage, which is extremely useful in circuit design.
Resistor Characteristics
Resistors are available in a wide range of shapes, sizes, and ratings to suit specific applications such as high stability, high voltage, or high current operation.
Important resistor characteristics include:
- Resistance value
- Power rating
- Temperature coefficient
- Voltage coefficient
- Noise level
- Frequency response
- Physical size and reliability
Resistance Values
Fixed resistors are available in values ranging from less than 1 ohm to several tens of megaohms. These resistors have only one fixed resistance value. Variable resistors, such as potentiometers, allow an infinite number of resistance values between zero and their maximum rated value.
Standard Resistor Symbols
In electrical and electronic circuit diagrams, resistors are commonly represented by either a zig-zag line or a rectangular box, with their resistance value expressed in ohms (Ω).
Classification of Fixed Resistors
Modern fixed-value resistors can be broadly classified into the following four categories:
- Carbon Composition Resistors – Made from carbon dust or graphite paste, low wattage values
- Film or Cermet Resistors – Made from conductive metal oxide paste, very low wattage values
- Wire wound Resistors – Metallic bodies for heat sink mounting, very high wattage ratings
- Semiconductor Resistors – High-frequency and precision thin-film surface mount technology
Composition Type Resistors
Carbon Composition Resistor
Carbon composition resistors are low to medium power resistors with low inductance, making them suitable for high-frequency applications.
However, they tend to become noisy and unstable when exposed to heat. Due to their manufacturing process, these resistors have relatively large tolerances. Carbon composition resistors are typically prefixed with the letters CR (for example, CR10k) and are available in:
- E6 series – 20% tolerance
- E12 series – 10% tolerance
- E24 series – 5% tolerance
Their power ratings range from 0.25 W (1/4 watt) to 5 W. Because of their low cost, they are widely used in general-purpose applications.
Film Type Resistors
Film Resistors
Film resistors offer better performance than carbon composition resistors, especially in high-frequency and radio-frequency applications. Metal film resistors provide excellent temperature stability and low noise characteristics. Metal oxide film resistors can handle higher surge currents and operate at higher temperatures.
Thick film resistors, also known as cermet resistors, are made by depositing a thick conductive paste of ceramic and metal onto an alumina substrate. Cermet resistors are commonly used in surface-mount devices, multi-resistor networks, and high-frequency circuits.
These resistors offer:
- High voltage ratings
- Low noise
- Good temperature stability
- Low surge current capability
Metal Film Resistors
Metal film resistors are precision, low-power components. They are typically prefixed with a notation such as MFR (for example, MFR100k).
They are available with power ratings ranging from 0.05 W (1/20 watt) to 0.5 W and are manufactured in:
- E24 series – 5% and 2% tolerance
- E96 series – 1% tolerance
- E192 series – 0.5%, 0.25%, and 0.1% tolerance
Wire wound Resistors
Wire wound resistors are constructed by winding a metal wire around an insulating core and are designed for high-power applications.
These resistors typically have power ratings ranging from 1 W to 300 W or more and are prefixed with W or WH (for example, WH10).
They are available in:
- WH aluminum-clad packages with tolerances of 1%, 2%, 5%, and 10%
- W vitreous-enameled packages with tolerances of 1%, 2%, and 5%
Wire wound resistors are ideal for high-current and high-power circuits where accurate resistance and heat dissipation are required.