Resistors: –
Resistors are current-controlling electrical components used to control the current by adding resistance to the path of the current (to limit the flow of electrical current).
SI unit of the resistor is ohm or Ω.
Resistors division: –
We divide the resistors mainly into two groups:
- Linear resistors
- Non-linear resistors
Linear resistors:
The resistors whose value of resistance doesn’t vary with the change of temperature. Such resistors follow ohms law, which follows the linear line between current and voltage.
Fixed resistors:
A resistor whose resistance value is fixed. The symbol of such resistors is like
There are a variety of fixed resistors which are available in the market. On a material and characteristics basis we divide the fixed resistors as follows:
- Carbon composition resistors
- Wire wound resistors
- Thin film resistors
- Thick film resistors
Carbon composition resistors
This type of resistor is one of the oldest types of resistors made up of powered carbon and ceramic combination. these resistors are not much used due to less efficiency of resistance value as compared to the others resistors. These types of resistors are not suitable for high-power applications.
Wire wound resistors:
This resistor is made of thin wire over an insulating core. The wire of the resistor is coiled over an insulator. The resistance of this resistor changes from 1ohm-1Mega Ohm as well as power dissipation can change from 5- 250 watts. The range of tolerance may range from 5% to -10 %.
Thin film resistors:
This type of resistor is used in critical applications where precision is very important. It is made up of an alloy of nickel and chrome called nichrome, other metals include gold with platinum or tantalum nitride. It is also called metal film resistors. The thickness of the resistor is 50-250nm with a tolerance range is 0.1%–2%.
Thick film resistors:
Also called a power resistor. These types of resistors are produced by firing the special substrate onto the substrate of a mixture of glass and metal oxides. These types of resistors where we concern with high power. it is also between 0.1%- 2%
The tree of fixed resistors is as follows:
Application of fixed resistors:
Variable resistors:
Resistors whose value can vary by changing the value of resistance. Their value of resistance is change on demand of the user
There are two types of variable resistors
- Potentiometer
- Rheostat
- Trimpots
Potentiometer:
This type of resistor consists of a knob and changes its resistance value with the angular moment. A metallic wiper over a resistive track is used to adjust the resistance value. This type of resistor produced a linear but logarithmic output with a tolerance of 10-20%. The potentiometer is a single rotation resistor that can rotate up to 270 degrees.
Rheostat:
It is a large resistor usually used for high-wattage and high-current applications. It is a wire wound over an insulator like a coil. It has three terminals two are fixed and the third one is variable. As the formula of resistance:
R=ꝬL/A
As we increase the length of the rheostat the value of resistance increases and vice versa.
Trimpots:
Trimpots resistors are the same as the rheostats but they can turn/ rotate multiple times to increase the decrease the resistances. The material used for the resistive track is made of carbon composition or cement. It is used in the circuit for tuning and recalibrations. Its resistance is usually adjusted by using a screwdriver. Trimpots may be single turns or multiple turns (5-25). It is usually used as to tune sets but when it is used as a variable resistor the duty cycle of the circuit should be less than 200 cycles.
Application of variable resistors:
Non-linear resistors:
The resistors whose value changes with the change in temperature. It doesn’t obey ohms law and does not give the linear curve. Here we discuss the three types of non-linear resistors.
Thermistors:
As the word “therm” suggests it is related to the temperature. With the thermistors, we get the temperature sensors. Nickel, strontium, and metal oxide of manganese are used to make these types of resistors. The resistors whose value of resistance decreases with the increase in temperature, so it has a negative temperature coefficient (NTC)but positive temperature coefficient (PTC) resistors are also present.
Varistors (VDR):
This type of resistor deals with voltage. It is usually used to protect our circuits. When the voltage in our line is increased the varistors turn on and decrease the voltage level to the safe side.