Types of Resistors

There are many types of resistors are available in the market according to their working principle, type of material used to manufacture, making procedure, and their applications. The classification list is as shown below.

  • 1. Linear Resistors.
    • 1.1. Fixed Resistors.
      • 1.1.1. Carbon Composition Resistors.
      • 1.1.2. Wire wound Resistors.
      • 1.1.3. Thin Film Resistors
        • 1.1.3.1. Carbon Film Resistors.
        • 1.1.3.2. Metal Film Resistors.
      • 1.1.4. Thick Film Resistors
        • 1.1.4.1. Metal Oxide Resistors.
        • 1.1.4.2. Cermet Oxide Resistors.
        • 1.1.4.3. Fusible Resistors.
      • 1.1.5. SMT Resistors.
    • 1.2. Variable Resistors
      • 1.2.1. Potentiometers.
      • 1.2.2. Rheostats.
      • 1.2.3. Trimmers.
      • 1.2.4. Digital Potentiometers.
  • 2. Non-Linear Resistors.
    • 2.1. Thermistor.
    • 2.2. Light Dependent Resistor(LDR).
    • 2.3. Varistor.

1. Linear Resistors

A linear resistor is the type of resistor whose resistance remains constant with increase in the potential difference or voltage applied to it. The V-I characteristics of such resistor is a straight line in other words these types of resistor obeys Ohm's Law very strictly.

These type of resistors are divided into two types. They are:

1.1. Fixed Resistors

The Fixed Resistors are one of the most widely used components in electronic circuits. Fixed Resistors are whose value is fixed at the time of manufacturing and it's value cannot be changed during it's usage.These type of resistors are further divided into 5 types.

1.1.1. Carbon Composition Resistors

Buy Carbon Resistors

The most common type of Composition Resistor is the Carbon Resistor, also known as Carbon Composition. The carbon composition resistors are made from a solid cylindrical resistive element with embedded wire leads or metal end caps. The cylindrical resistive element of the carbon composition resistor is made from the mixture of carbon or graphite powder and ceramic (made of clay). The carbon powder acts as the good conductor of electric current. The solid cylindrical resistive element is covered with plastic to protect the resistor from outside heat. The leads made of copper are joined at two ends of the resistive element.

Resistance of the carbon composition resistor is depends on three factors: amount of carbon added, length of solid cylindrical rod, and cross sectional area of the solid cylindrical rod.

Advantages:

  • Ability to withstand high energy pulses.
  • The entire carbon composition body conducts the energy. So It has a higher energy capability.
  • Carbon resistors can be made with a higher resistance.

Disadvantages:

  • Low stability of the resistance value.
  • The carbon composition contains materials with different heat expansion properties.The mechanical contact between the conducting particles will change, and this leads to a change in resistance value.
  • The operating temperature range is between around -40 to 150 ° C. However, the resistor derate above 70 ° C.
  • Insulation resistance is poor with approximately 109 Ω.

1.1.2.Wire Wound Resistors

A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material. The wire material has a high resistivity, and is usually made of an alloy such as Nickel-chromium (Nichrome) or a copper-nickel-manganese alloy called Manganin. Common core materials include ceramic, plastic and glass. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core.

Resistance of the wire wound resistor is depends on three factors: resistivity of the metal wire, length of the metal wire and cross sectional area of the metal wire.

Advantages:

  • This resistor is used in high power circuits.
  • It will not affect by the noise.
  • It is thermally constant.

Disadvantages:

  • These resistors are used for only low frequencies because it works as an inductor at high frequencies.
  • It is expensive as compared with carbon resistor.
  • It is larger in size.

1.1.3. Thin Film Resistors

The thin film resistors are made of from high grid ceramic rod and a resistive material. A very thin(less than 1 micron) conducting material layer overlaid on insulating rod, plate or tube which is made from high quality ceramic material or glass. There are two further types of thin film resistors. They are Carbon Film Resistors and Metal Film Resistors.

Carbon film resistors are a fixed form type resistor. They are constructed out of a ceramic carrier with a thin pure carbon film around it, that functions as resistive material.the carbon film resistor has a higher negative temperature coefficient than carbon composition. The resistive temperature coefficient lies between 2.5×10-4 Ω/°C and -8×10-4 Ω/°C. Also this type of resistor is protected against chemical influences with a silicone coating. This type of resistor is widely used in electronics. Therefore it is important to note that the small resistors have a capacity of approximately 0.5 pF. Self induction is around 0.01 μH for uncut resistors and up to several μH for spiral cut resistors. These resistors are available in values between 1Ω – 10,000 MΩ and have a power rating of 1/16, 1/8, 1/4, 1/2, 1 or 2 watt.

Carbon film resistors are a significant improvement on carbon composition. However, in comparison to metal film and metal oxide film, the commercially available range steadily decreases. Metal and oxide film are not more expensive to produce, and have overall better properties.

Advantages:

  • Low noise than carbon composition resistor
  • Wide operating range
  • Low temperature co-efficient of resistance
  • Low cost

1.1.4. Thick Film Resistors

Thick film resistors are produced by applying a resistive film or paste, a mixture of glass and conductive materials, to a substrate. Thick film technology allows high resistance values to be printed on a cylindrical or flat substrate either covered entirely or in various patterns.

There are three main types in thick film resistors like Metal oxide film resistors, Cermet film resistors, and Fusible resistors.

Metal oxide film resistors

A Metal oxide film resistor is formed by oxidizing a thick film of Tin chloride on a heated glass rod, which is a substrate. They have high temperature stability and can be used at high voltages. These resistors have low operating noise. Metal oxide film resistors differ with metal film ones only regarding the type of film coated. Metal oxide is a metallic compound like tin with oxygen to form tin oxide, which is coated as a film on the resistor. The resistivity of this resistor depends upon the amount of antimony oxide added to the tin oxide.

Cermet film resistors

A cermet is a composite material composed of ceramic (cer) and metal (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide, boride, or carbide. Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on the physical structure of the material, cermets can also be metal matrix composites, but cermets are usually less than 20% metal by volume.

Fusible resistors

The Fusible resistors are similar to wire wound resistors. But these resistors along with providing resistance, act as a fuse.In this resistor, the current flows through a spring loaded connection, which is placed closely to the body of the resistor. The blob that is attached to the spring wire of the resistor takes the heat generated by the resistor due to the current flow. If this heat is increased, the attachment to the blob gets melted up and opens the connection. The value of these resistors is usually of less than 10 ohms.

1.1.5. SMT Resistors

Surface-mount technology is a method in which the electrical components are mounted directly onto the surface of a printed circuit board. An electrical component mounted in this manner is referred to as a surface-mount device(SMD).

They have metallised areas at either end of the main ceramic body, and in this way they can be set onto a printed circuit board that has pads onto which the two ends are set to provide the connection.The resistor is made by taking an alumina or ceramic substrate. The end connection electrode bases are then placed onto this and then this is fired to ensure they are robustly held in place.

Then a thin film of resistive material is deposited - this is typically metal oxide or a metal film. Once the resistive element has been completed it is covered with successive layers of a protective coat which are all allowed to try between the applications. These layers of the protective coat not only prevent mechanical damage, but also prevent ingress of moisture and other contaminants.

1.2. Variable Resistors

Variable resistor is one the resistor which its resistance value can be change by rotating wiper anytime. The electrical symbol of variable resistor is given as below.

This type resistors are divided into four types. They are potentiometer, rheostat, trimmer, and digital potentiometer.

A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.

A trimpot or trimmer potentiometer is a small potentiometer which is used for adjustment, tuning and calibration in circuits. When they are used as a variable resistance they are called preset resistors.

A digital potentiometer (also known as digital resistor) has the same function as a normal potentiometer but instead of mechanical action it uses digital signals and switches. This is done by making use of a 'resistor ladder', a string of small resistors in series. At every step of the ladder, an electronic switch is present. Only one switch is closed at the same time and in this way the closed switch determines the 'wiper' position and the resistance ratio. The amount of steps in the ladder determines the resolution of the digital pot. Digital resistors can be controlled by using simple up/down signals or by serial protocols such as I2C or SPI.

2. Nonlinear resistors

Nonlinear resistors are those resistors, where the current flowing through it does not change according to Ohm's Law but, changes with change in temperature or applied voltage. Few types of Nonlinear Resistors are given below.

Thermistor

A thermistor is a resistance thermometer, or a resistor whose resistance is dependent on temperature. The term is a combination of "thermal" and "resistor". It is made of metallic oxides, pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass.

Thermistors are divided into fundamental types:

  • NTC thermistors, resistance decreases as temperature rises. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter.
  • PTC thermistors, resistance increases as temperature rises. PTC thermistors are commonly installed in series with a circuit, and used to protect against overcurrent conditions, as resettable fuses.

Light Dependent Resistor(LDR)

A Light Dependent Resistor(LDR) or photoresistor is a nonlinear resistor that changes resistance with respect to receiving luminosity on the component's sensitive surface. The resistance of a LDR decreases with increase in incident light intensity.

Varistor

A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor, it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode.

Resistors in Parallel
Resistance value marking methods

User Comments

No Posts found !

Login to Post a Comment.