RESISTORS
What is Resistor?
Types of Resistors
Each resistor has two main characteristics i.e. its resistance (R) in ohms and its power rating in watts (W). The resistors having wide range of resistance (from a fraction of an ohm to many mega ohms) are available.
The power rating may be as lower 1/10 W to as high as several hundred watts. The value of R is selected to obtain a desired current I or voltage drop IR in the circuit. At the same time wattage of the resistor is selected so that it can dissipate the heat losses without overheating itself. Too much heat may burn the resistor.
From the operating conditions point of view, the resistors may be classified as fixed resistors and variable resistors.
1. Fixed Resistors
The resistors which have fixed value of resistance are called fixed resistors. These resistors may be carbon composition resistors for wire wound resistors.
①Carbon-Composition Resistors : Most common in electronic circuits are carbon resistors with a low power rating (2W or less). This type of resistor is made of mixture of carbon or graphite and clay. The two materials are mixed in the properties needed for the desired value of R. The resistor element is enclosed in a plastic case, for insulation and mechanical strength. The leads, made of tinned copper are joined to the two ends of carbon resistance element.
This type of resistors are readily available in values ranging from one ohm to 22 Mπ, having a tolerance range of 5 to 20%. The power rating is generally 1/4, 1/2, 1 or 2 W. The relative size of resistors with higher wattage increases since they are to carry-more current and dissipate higher losses (heat losses). These resistors are quite cheap in cost. A resistor may cost as low as fifty paise even less than this.
The other variety of carbon composition resistors is film-type resistor. A homogeneous film of pure carbon (or some metal) is deposited over a ceramic or other insulating core. Only approximate value of resistance is obtained by this method. To obtain the desired value either the layer is trimmed off or a helical groove of suitable pitch along its length is cut. Thus, very accurate value (with an accuracy of +-1%) of resistance can be obtained. Therefore, these resistors (film-type resistors) are sometimes called precision type resistors.
②Wire Wound Resistors : For its construction, a resistance wire (nichrome, tungsten or manganin) is wrapped around a hollow porcelain cylindrical core. The ends of the wire are atached to metal pieces fixed at the two ends of the core. This assembly is coated with an enamel containing powdered glass and is heated which develops the coating known as vitreous enamel, This hard and smooth coating provides mechanical protection to the resistance wire. It also helps in dissipating heat away from the unit quickly.
The value of resistance depends upon the resistivity of the material used and the dimension and area of (length cross-section) of the resistance wire. These resistors are readily available in values ranging from 1 ohm to 100 kπ and more. The power rating of these resistors varies from 2W to 500W. The value of resistance is generally printed on the body of the resistor.
RESISTOR COLOUR CODING
Since carbon composition resistors are very small in size, it is difficult to print the significant value of resistance on their bodies. Even if the values are printed, the letters will be so small that it would be difficult to read them by naked eyes. Therefore, colour bands are printed on one end of the resistor casing. This system of representing the resistor value is called colour coding. The colour coding is standardized by the Electronic Industries Association (AIE).
The table given below shows the numerical value associated with each colour. To detemine the value of resistance, the colour bands are always read left to right from the end that has the bands nearer to it. The first and second bands represent the first and second significant digits of the resistance value respectively. The third band is the multiplier and represents the number of zeros that follow the second digit. Sometimes, the third band is of golden or silver colour, then these colours represent a multiplying factor of 0.1 and 0.01 respectively. The fourth band represents the resistor's tolerance. It is a measure of the precision with which the resistor was made by the manufacturer. The silver band represents 10% tolerance whereas, the gold band represents 5% tolerance. However, if there is no fourth band, it shows that the resistor has 20% tolerance.
An image is given below to understand the calculation of the value of resistor by color coding.
| Color | Significant Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | 10⁰ = 1 | – |
| Brown | 1 | 10¹ = 10 | – |
| Red | 2 | 10² = 100 | – |
| Orange | 3 | 10³ = 1000 | – | Yellow | 4 | 10⁴ = 10000 | – | Green | 5 | 10⁵ = 100000 | – | Blue | 6 | 10⁶ = 1000000 | – | Violet | 7 | 10⁷ = 10000000 | – | Grey | 8 | 10⁸ = 100000000 | – | White | 9 | 10⁹ = 1000000000 | – | Gold | – | 10⁻¹ = 0.1 | ±5% | Silver | – | 10⁻² = 0.01 | ±10% | No Color | – | – | ±20% |
2. Variable Resistors
Sometimes, in electronic circuits, it becomes necessary to adjust (control) the values of currents and or example, to change the volume (loudness) of sound in transistor radio and television, to adjust the brightness and contrast of the television picture etc. These adjustments can be made with the help of variable resistors. The variable resistors may be carbon-composition resistors or wire-wound resistors.
①Carbon-Composition Resistors : A thin carbon coating on pressed paper or a molded carbon disc constitutes the carbon-composition resistance element. The two ends of the disc are joined to the external soldering lug terminals 1 and 3. The middle terminal is connected to the variable arm that contacts the resistor element by a metal spring wiper. As the shaft of the control is turned, the variable arm moves the wiper to make contact at different points on the resistor element which changes the value of resistance inserted in the circuit.
The carbon-composition variable resistors are available with 1000 π to 5 Mπ approximately having power rating of usually 1/2W to 2W. A carbon control (resistor) is often combined with a power on-off switch.
②Wire Wound Variable Resistors : In this case, a resistance wire is wound over a dough-shaped core of bakelite or ceramic. The two ends of the resistance wire are joined to the external soldering-lug terminals 1 and 3. The middle terminal is connected to the variable arm that contacts the resistor element. As the shaft of the control is turned, the variable arm moves the wiper to make contact at different points on the resistor element which changes the value of resistance.
Adjustable wire-wound resistors are also available in which the resistance may be varied with an adjustable slider moved along an exposed portion of the winding.
The variable resistors can be either linear or non- linear. In the linear type, the former over which wire is wound is of uniform height, that is why the value of resistance varies linearly with the rotation (movement) of moving contact. In case of non- linear resistors, the former has non-uniform height, that is why the value of resistance varies non-linearly (logarithmic variation). These resistors are mostly used as volume control in sound equipment.
In the electronic circuits, a variable resistor can be used as a rheostat or potentiometer. When it is used as a rheostat, the variable resistor is connected in series with the load and only two terminals are inserted in the circuit. The purpose of resistor is to vary the amount of current in the circuit. However, when it is used as a potentiometer (generally called pot), all the three terminals are inserted in the circuit, The purpose of the resistor is to vary the potential (voltage) at the output.
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