Basics / Resistance / Types of Resistor

Types of Resistor

There are different types of resistors. An electrical circuit is incomplete without a resistor.  A resistor is a passive component. The component that cannot amplify or process an electrical signal is a passive component. Resistors have many interesting shapes and sizes.

Before classifying resistors, we should understand two important parameters of a resistor.

Resistance (in Ω)

  • R – value (i.e. Resistance value) which we select to control current in the circuit.
  • The typical range of resistance is available in the market. The range is 0.01 Ω to 1012 Ω.

Power Rating (in Watt)

  • Power rating represents the maximum heat loss that can take place through a resistor without overheating.
  • Typical Range: 0.125 W to 250 W.
Types of Resistor
Types of Resistor

As in the above figure, we can classify resistors broadly into three categories;

  1. Fixed Resistors
  2. Variable Resistors
  3. Special Resistors

Fixed Resistors

As the name suggests, these resistors have fixed resistance and power ratings.

Symbol of Fixed Resistor
Symbol of Fixed Resistor

Further, this type of resistors has three sub-categories.

Carbon Composition Resistors (CCR)

Carbon Composition Resistor
Carbon Composition Resistor

Specification:

  • The body is cylindrical in shape with color codes on it.
  • Power rating ranges from 0.1W to 2W.
  • The range of R-value is up to 22 MΩ.
  • It has a tolerance range. This range is from ±5% to ±20%.

Composition:

  • The material used to make CCR is carbon clay composition. The carbon clay composition is a mixture of graphite and clay.
  • It has two connecting leads made up of copper.

Advantages:

  • Carbon composition resistances are cheap, reliable and small in size. Its diameter is less than 10mm.

Disadvantages:

  • This type of resistance develops noise when current flows from one carbon profile to other carbon profile.
  • The resistance value changes rapidly with temperature.

Wire Wound Resistors (WWR)

Wire Wound Resistor
Wire Wound Resistor

Specifications:

  • A wire wound resistor has a large cylindrical shaped body.
  • Its power rating may be from 5 to 200 W. Therefore, we can use these resistors in high power applications.
  • Its range of resistance value from 1Ω to 1 MΩ.
  • The tolerance ranges from 0.01% to 1%. So, it has less tolerance as compared to a carbon composition resistor.  Less tolerance is always a good indication for accuracy.

Composition:

  • As the name suggests here we wrap wire on an insulating core.
  • We generally use constantan or manganin as the material for the wire. Because they have high resistivity and low-temperature coefficients. So resistance does not change much with the temperature variation.

NOTE:- After wounding the wire, resistors body is coated with ‘Baked Enamel’ (an insulating material).

Wire Wound Resistor

Advantages:

  • Less sensitive to temperature variation.
  • More accurate R-value than CCRs.

Disadvantages:

  • Numbers of turns increases the “inductance value” of the coil so formed and this inductance value associated with the resistor is undesirable (condition gets worse at high frequency because reactance (XL = ωL) increases).

NOTE:- The above problem can be overcome by “bifilar windings” (can’t be explained here due to word limit).

Metal Film Resistors (MFR)

Metal Film Resistor
Metal Film Resistor

Specifications:

  • Again, as the name suggests, a film (of metal) is coated over an insulating substrate/core.
  • The range of R-value: Up to 10000 MΩ
  • Tolerance: ± 1%

Composition:

  • Material for the film: Pure carbon or some metal.
  • Substrate/core: Glass or ceramic.
  • End caps are fitted on both sides of which leads made of ‘Cu’ are welded.

NOTE:- For obtaining accurate/ desired R-value ‘helical grooves are formed by trimming the layer thickness of the film.

Advantages:

  • Much smaller in size than WWR’s.
  • The problem of inductance has overcome.
  • High accuracy and low-temperature coefficient (since metals have a low-temperature coefficient).
  • Very low noise.

Carbon Film Resistors (CFR)

Carbon Film Resistor
Carbon Film Resistor
  • CFR is a type of MFR in which Carbon is used for film material.
  • CFRs are commonly used nowadays because they have much lower tolerances and temperature coefficient than CCRs.

What is the difference between CCR and CFR?

In CCR, the carbon material (i.e. graphite) is mixed with clay to form the core but in CFR. Carbon is only coated over another core material. Unlike CCRs, CFR doesn’t make noise.

Variable Resistors

Resistance can be varied manually. These resistors have 3-terminals; two fixed and one movable.

Symbol of Rheostat
Symbol of Rheostat
Symbol of Potentiometer
Symbol of Potentiometer

Rheostat

We all have seen rheostats in the laboratory. It is a large cylindrical resistor which has the same construction as wire wound resistors (WWR’s) except the fact that in a rheostat a sliding contact is provided at the top (which is the movable terminal).

Rheostat
Rheostat

In a rheostat, only two lead terminals (One fixed and one moving) are connected to the circuit. The third one is not connected. Sliding the movable contact, the resistance between terminals can be varied.

Potentiometer (POT)

With all three terminals connected to the circuit, pots are generally used as Voltage dividers. Potentiometers are available in many shapes but the most common shape is Round shaped General-purpose Pots.

pot or potentiometer
POT or Potentiometer

The videogame joysticks contain potentiometers, the volume control knob of Radio, TV (old) contain Potentiometer.

Special Resistors

These types of resistors are generally made up of PHOTOCONDUCTOR and SEMICONDUCTOR materials.

LDR (Light Dependent Resistors)

  • Material: Cadmium Sulphide, a photoconductor material. When these materials are exposed to light, they show a variation in their Resistance.
  • Initially, under no-illumination, it has high resistance but when it absorbs light then its resistance decreases.

VDR (Voltage Dependent Resistors)

  • Material: SiC(Silicon Carbide) or Zinc oxide.
  • VDRs are made up of those materials which show non-linear behaviour between Resistance and Voltage.

Thermistors

  • Material: semiconductor materials.
  • Thermistors have a negative temperature coefficient (i.e. the resistance of Thermistor decreases with increase in Temperature). Some positive temperature coefficient thermistors also exist.

Leave a Comment