Types of resistors

When a wire or a piece of a material is used as a component for producing resistance in a circuit, it is called a resistor. Resistors used in electronic equipments are classified as under on the basis of their construction: carbon, wire wound, ballast, PTC, NTC, varistor, LDR, etc.  

Carbon resistors:

Carbon Film Resistor

Carbon is a non metallic and ideal material for making resistor. Resistors made of carbon or graphic are called carbon resistors. They are of following two main types:
1.    carbon composition resistors :
Resistors made by compressing carbon powder while using a suitable resin binder are called composition resistors. They are of following two types:
(a)    Pallet or slug type :

Pallet Resistor

For making this type of resistors, carbon black or graphite powder is moulded into small cylindrical piece while using an appropriate resin binder. During moulding, two leads of tinned copper wire are also attached, one on each side. A defensive coating is made over moulded pieces. After heat treatment the resistor becomes ready for use.

(b)    Filament type carbon composition resistor:
For making this type of resistors, carbon powder is deposited over porcelain or glasses both the ends, one on each side. Finally, the resistor is enclosed with a phenolic (water proof) layer.
Carbon composition resistors are made in the range of 2 ohms to 10 mega ohms with 1/8, 1/4, 1/2, 1 and 2 watts power ratings.

2.    Film carbon resistors:

Carbon Film Resistor

These types of resistors are made by depositing hydrocarbon vapors on a ceramic capillary pipe at 900-1100°c temperature. Resistor s of small values are made by depositing metal oxide (of palladium, nickel, chromium metals) on a ceramic capillary pipe. Resistors of first type are also known as pyrolytic resistors and of second type as metal oxide film resistors. Tinned copper or silver wires are used as leads for these resistors.

This type of resistors is made in range of 1 ohm to20 mega ohms with ½ watts power rating.

Properties of carbon resistors:

(A)    Merits :
(1)    Size is quite small up to 6mm ×1mm.
(2)     Mass is quite less up to 1 gram.
(3)    High value resistors (1 mega ohm and above) can be made easily.
(4)    They can carry high frequency   circuits.
(5)    Their temperature coefficient lies within 0.02 to 0.1.

(B)    Demerits:
(1)    Temperature   and humidity affects their values.
(2)    Their current carrying capacity is restricted, since their wattage is limited up to 2 watts.
(3)    It is complicated to make a carbon resistor of a value below 2 ohms.
(4)    Their stability and reliability are poor.


Wire wound resistors:

Wire Wound Resistor

These types of resistors are made by winding a wire on an insulated former. Generally, porcelain pipe pieces are used as former and Eureka wire (an alloy of 60% nickel and 40% copper) is used as resistance wire. This type of resistor is made in the range of 0.01 to 1 mega ohms with 5 to 50 watts power rating.

They are also able to handle much higher electrical currents than other resistors of the same ohmic value with power ratings in excess of 300 Watts. These high power resistors are moulded or pressed into an aluminum heat sink body with fins attached to increase their overall surface area to promote heat loss and cooling.

   Wire wound resistors are classified as follows on the basis of their resistance control:
(1)    Fixed value resistor:

Fixed Value Resistor

A resistor having a fixed value of resistance is called a fixed value resistor. It has no arrangement for changing its resistance value.

(2)    Tapped resistor :

Tapped Resistor

A resistor having two, three or more values made in a single unit is called a tapped resistor.

(3)    Adjustable resistor:

Adjustable Resistor

A resistor having arrangement for changing its value within a definite range is called an adjustable resistor. It may also have an arrangement for two three values an in a tapped resistor. This type of resistor is used in mains operated value receiver and it is known as ‘mains resistor’, e.g., 1500 ohms/0.1A, 1000 ohms/0.3A.

(4)    Rheostat:

Rheostat Resistor

The resistance value of a rheostat can be varied quite easily. It is used in laboratories.

Properties of wire wound resistors:
(A) Merits:
(1)  They have a lowest tolerance value –up to 5%
(2) They have a high power rating.
(3) It is easy to construct a low valued resistor –as low as 0.01 ohms.
(4) They have a good stability and reliability.

(B)    Demerits :

(1)    Their size is quite big which is unsuitable for small sized equipments.
(2)    It is difficult to construct a high valued resistor.
(3)    It is difficult to construct variable wire wound resistor and working of such resistors is not satisfactory.
(4)    They are unsuitable for high frequency circuits because their effective resistance is increased due to presence of inductive reactance in them.


Potentiometer:

Potentiometer Resistor

 A resistor whose resistance value can be varied easily is called a potentiometer. The resistor used as volume control and tone control in a radio receiver is a potentiometer.
It consists of circular strip which is equal to 3/4th of a ring.  A carbon film is deposited on it. A moving arm which is connected t o a shaft is mounted in such a way that by rotating the shaft any resistance value between zero and maximum can be obtained.

Generally, the potentiometers are also made for special purpose in the range of 1 to 100 ohms (e.g., for controlling the e.m.f. acting in multimeter).
Carbon potentiometers are of following two types:

(1)    Linear potentiometer:

Linear Potentiometer

In this type of potentiometer, the variation in resistance value is proportional to the angle rotated by the moving arm.

(2)    Logarithmic potentiometers:

Logarithmic Potentiometer

 

In a radio receiver or an radio amplifier the A.F. signal strength has to be increased ten, hundred or thousand times for increasing the output one , two or three times respectively. Because of this reason only, the unit for the measurement of sound intensity had been selected a logarithmic unit called decibel. 

      Hence, potentiometer used in radio receivers or an audio amplifiers are of logarithmic type. In this type of potentiometer, the variation in resistance value is not proportional to the angle rotated by the moving arm. For equal angle rotation, the order of variation in the value of resistance is 1, 10, 100, 1000 ohms etc.

   Pre-set: it is a potentiometer of a very small size. It is generally mounted on the PCB of equipment. Its ohmic valve can be adjusted by means of a small screw driver during alignment of the equipment sometimes, a pre –set is referred as a ‘POT’ also. These are made in ‘linear’ as well as in ‘logarithmic’ types ranging from 100 ohms to 1 mega ohms. These are extensively erased in T.V. receivers etc. 


Special types of resistors:
 
Besides carbon and wire wound resistors some special types of resistors are also made for specific uses. They are as follows:

(1)    Ballast resistor:

Ballast Resistor

 

It is made by a substance which has a positive temperature coefficient such as nickel iron alloy. It has a property of variation in its resistance value for a change in its temperature. Hence, an increase in the magnitude of current flowing through it causes a corresponding increase in its temperature and which in turn results in an increase in its resistance value. The increase in its resistance value decreases the magnitude of current flowing through it.

In this way, a ballast resistor stabilizes the circuit current within a definite range. These resistors are used for current stabilization in various types of electronic equipments.

(2)    PTC (positive temperature coefficient )resistor :

PTC Resistor

A resistor made of a substance which has a positive temperature coefficient is called a PTC. The resistance value of PTC is directly proportional to its temperature. Hence, an increase in the magnitude of voltage applied or the external temperature causes an increase in its resistance which in turn reduces the magnitude of current flowing through it. A PTC is used for bias stabilization in transistorised circuits. There, it is connected in the emitter circuit. If due to any reason the magnitude of emitter current rises above a definite value it causes an increase in its resistance value.

 In turn, it results in a decrease in the emitter current. Therefore, the effect of increase in the magnitude of current is controlled by it. It is made by a mixture of nickel oxide, manganese oxide and cobalt oxide. It is known as thermistor also.

(3)    NTC (negative temperature coefficient )resistor :

NTC Resistor

A resistor made of a substance which has a negative temperature coefficient is called a NTC. In the beginning (after switching on the circuit), it has a very high resistance and thus the circuit current is quite small. Slowly and slowly its temperature starts to rise which results in an increase in the magnitude of current flowing through it. An increase in the magnitude of current results in an increase in its temperature which again results in an increase in the circuit current.

 In this way, after a few seconds its temperature rises to a sufficiently high value and the magnitude of current reaches at its maximum value. Under such conditions the resistance value of NTC is minimum. The device is used in timer circuits, valve filament circuits etc. if a valve filament is allowed to be heated up at a slow rate or in other words the magnitude of its heating current is controlled in such a manner that it rises slowly then the working life of a valve filament can be increased up to a great extent. It is made by graphite.

(4)    Varistor:

Varistor Resistor

It is also a special type of resistor and it has following two types:
(A)    Symmetrical varistor :

Symmetrical Varistor

It is made of carbide granules and a ceramic binder material. It has a property that its resistance value is controlled by the magnitude of current flowing through it. At high magnitude of current, the resistance is low and vice versa. Since the magnitude of current in a circuit is proportional to supply voltage, therefore, a varistor can be used for controlling voltage fluctuation within a limit of few volts. It is also known as VDR (voltage dependent resistor) and it is used for controlling voltage fluctuations.

(B)    Unsymmetrical varistor :

Unsymmetrical Resistor

It is made of a semi –conductor material. It has a property of controlling the magnitude of current of a circuit by the direction of flow of current. It is also called a varistor diode. It is used as a rectifier also in certain types of circuits.

(5)      LDR(light dependent resistor ):

LDR

It is made of selenium. It has a property that its resistance value decreases on the incidence of light rays on it and thus the magnitude of current flowing through it is increased. In other words it is a photo sensitive resistor. It is used in light operated control circuits.

 You can make a counter for counting the persons passing through a narrow path, a garage door opener which can open the door automatically on the incidence of light rays from a torch or car headlight, an automatic street light control which can switch on street lights in the night and switch off them in the day light automatically.