Resistor Details

The resistor is a passive electrical component to create resistance in the flow of electric current. In almost all electrical networks and electronic circuits they can be found. The resistance is measured in ohms. An ohm is the resistance that occurs when a current of one ampere passes through a resistor with a one volt drop across its terminals. The current is proportional to the voltage across the terminal ends. This ratio is represented by Ohm’s law:

V=I/R

Resistors are used for many purposes. A few examples include delimit electric current, voltage division, heat generation, matching and loading circuits, control gain, and fix time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used to as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for electronics.

Overview of types and materials

Resistors can be divided in construction type as well as resistance material. The following breakdown for the type can be made:

Fixed resistors
Variable resistors, such as the:
Resistance dependent on a physical quantity:
- Thermistors (NTC and PTC) as a result of temperature change
- Photo resistor (LDR) as a result of a changing light level
- Varistor (VDR) as a result of a changing voltage
- Magneto resistor (MDR) as a result of a changing magnetic field
- Strain Gauges as a result of mechanical load

For each of these types a standard symbol exists. Another breakdown based on the material and manufacturing process can be made:

The choice of material technology is a specific to the purpose. Often it is a trade-off between costs, precision and other requirements. For example, carbon composition is a very old technique with a low precision, but is still used for specific applications where high energy pulses occur. Carbon composition resistors have a body of a mixture of fine carbon particles and a non-conductive ceramic. The carbon film technique has a better tolerance. These are made of a non-conductive rod with a thin carbon film layer around it. This layer is treated with a spiral cut to increase and control the resistance value. Metal and metal oxide film are widely used nowadays, and have better properties for stability and tolerance. Furthermore, they are less influenced by temperature variations. They are just as carbon film resistors constructed with a resistive film around a cylindrical body. Metal oxide film is generally more durable. Wirewound resistors are probably the oldest type and can be used for both high precision as well as high power applications. They are constructed by winding a special metal alloy wire, such as nickel chrome, around a non-conductive core. They are durable, accurate and can have very low resistance value. A disadvantage is that they suffer from parasitic reactance at high frequencies. For the highest requirements on precision and stability, metal foil resistors are used. They are constructed by cementing a special alloy cold rolled film onto a ceramic substrate.

Uses of Resistors

Though resistors can cause wastage of electricity, it has a lot of advantages and applications in our daily life.

Resistance is one of the main ingredient in the working of a light bulb. When electricity passes through the filament of the bulb, it burns bright as it turns extremely hot due to its smaller size. Though this mechanism wastes a lot of electricity, we are forced to use it to obtain light. The light used nowadays are highy efficient than the older incandascent lamps.
The similar filament working is the main ingredient in the working of some of our usual household stuffs like electric kettles, electric radiators, electric showers, coffee makers, toasters, and so on.
The application of variable resistance is also helpful to us. Our TV’s, radios, loud speakers and so on work on this principle.

Colour Coding

The value of the resistance is found out by colour coding. The resistors have a band of colours shown in their outer covering. Here are the steps to determine the value of the resistor.

All resistors have three bands of colours, followed by a space and then a fourth band of colour. The fourth band of colour will be brown, red, gold or silver.
To read the colours turn it to the position such as the three consecutive colours come on the left and then the space and the rest of the colours.
The first two colours from the left indicate the first two digits of the value. The third colour represents the digital multiplier. That is, it indicates how much you have to multiply the first two numbers with. Thus if you have a resistance with the first three colours being brown, black and red, the value of resistance is 10*100 = 1000 ohms or 1K.
The last band, after the space indicates the tolerance of the resistor. This indicates the range of accuracy of the resistor. Thus, along with the three colours above, if the fourth colour is gold, it means you have a tolerance between +/-5%. Thus the actual value of the resistance can be between 950 Ohms and 1K.
There can also be resistors with five colours. If so, the first three represents the digits, the fourth will be the multiplier and the fifth will be the percentage of tolerance. This indicates that a more precise value of the resistor used can be obtained from a 5-colour resistor.