Resistors

In electronic circuits, materials like copper and aluminium are conductors with negligible resistance, while paper and ceramic are insulators with high resistance. Carbon, with varying resistance, is used as a resistor, measured in Ohms (Ω). Resistance opposes electric current, limiting current, dropping voltage, and dividing voltage in circuits. Combined with capacitors, resistors function as filters or create time delays. 

Carbon Composition Resistors (Fixed Resistors) 

Carbon composition resistors are widely used in electronic circuits. They range from 1Ω to about 20MΩ, with power ratings of 1/8W to 2W. These resistors are low-cost and small-sized, commonly found in laboratories. 

Construction 

1. Materials: Made by mixing carbon granules with a binding material. 
2. Forming: Molded into a cylindrical shape. 
3. Leads: Tinned copper wire leads are inserted at both ends. 
4. Sealing: The structure is sealed with a non-conducting coating. 

The cost of a resistor is based on its wattage, not its resistance value. 

Color Coding of Carbon Resistor 

Another feature of carbon resistors is the color code system, indicating value and tolerance. Tolerance shows the maximum deviation from the stated value, expressed as a percentage. For example, a 100 Ω resistor with ±5% tolerance ranges from 95 Ω to 105 Ω. In color code method, the first two rings indicate the first two digits, and the third ring indicates the multiplier. The fourth ring is used to indicate its tolerance 

Wire Wound Resistors (Fixed Resistors) 

Wire wound resistors are used for high current control and require stable, accurate resistance values.  

a)Construction: They are made by wrapping a resistive wire like nichrome or manganin around a ceramic core, with wire ends attached to metal contacts. The structure is coated with vitreous enamel to protect the resistor and dissipate heat.

b) Drawbacks: Large size, limiting higher values.

c) Advantage: High power handling capacity.

d) Range: Resistance: Fraction of ohms to 100 kΩ.

e) Power rating: 5 W to 200 W. 

Carbon Type Potentiometers (Variable Resistors) 

Variable resistors, or rheostats, are known as potentiometers or "pots" in low current circuits. They consist of a wiper moving on a circular carbon resistive track attached to shaft, which can be metal or plastic.  

Made with resistive carbon on a Bakelite piece, potentiometers have three terminals: two fixed (1 and 3) and one variable (2). Connecting terminals 1 and 3 provides maximum resistance, while connecting 1 and 2 or 2 and 3 allows resistance adjustment by rotating the shaft. Resistance varies from zero (wiper at terminal 1) to maximum (wiper fully rotated to the right). 

a) Specialty: Low Cost, Used in Radio, TV as volume control, brightness control etc.

b) Range: 1 KΩ to 5 MΩ, Power rating 1/2W to 2W

Wire Wound Potentiometers (Variable Resistors) 

The basic principle of a wire wound variable resistor, commonly seen in rheostats, involves winding wire on an open ceramic ring covered with vitreous enamel. Non-linear types use a tapered strip for winding. Resistance varies by sliding the contact. 

a) Drawback: High cost and large size.

b) Advantage: High power handling capacity and precise resistance adjustment.

c)Construction: Wire wound on a ceramic ring, coated with vitreous enamel, with linear and non-linear (tapered) options. 

d)Range: Resistance: 100Ω to 1 MΩ.  

                    Power rating: 2W to 100W. 

Other Resistors 

1. Thermistor: A thermistor's resistance changes non-linearly with temperature: NTC decreases resistance with heat; PTC increases. Used for temperature measurement/control. 
2. Light Dependent Resistor (LDR): It is a photosensitive device whose resistance decreases with increase in light intensity. Cadmium sulfide material is used for this purpose. LPR is widely used in many applications like automatic street light control, burglar alarm etc.
3. Varistor (VDR): A voltage-dependent resistor (varistor) has resistance that varies with voltage. Types include Silicon Carbide and Metal Oxide VDRs. Used for surge suppression in industrial applications and TV circuits.