Page 15 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 3 Fig 4

Carbon film resistors are available from 1 ohm to few
Meg ohm and up to 2W and can work from 85°C to 155°C.
Specification of resistors: Resistors are specified normally with the four important parameters
1 Type of resistor
2 Nominal value of the resistors in ohm (or) kilo ohm (or) mega ohm.
3 Tolerance limit for the resistance value in percentage.
4 Loading capacity of the components in wattage
Example
100 ± 10% , 1W, where as nominal value of resistance is 100W.
The actual value of resistance may be between 90 W to 110 W, and the loading capacity is maximum 1 watt.
The resistors can also be classified with respect to their function as
1 Fixed resistors
2 Variable resistors
Fixed resistors : The fixed resistors is one in which the nominal value of resistance is fixed. These resistors are
provided with pair of leads. (Fig 2 to 4)
Variable resistors (Fig 5) : Variable resistors are those whose values can be changed. Variable resistors includes
those components in which the resistance value can be set at the different levels with the help of sliding contacts.
These are known as potentiometer resistors or simply as a potentiometers.
It is provided with 3 terminals as shown in Fig 5 and 6. They are available with carbon tracks (Fig 6) and wire
wound (Fig 6) types. Trimmer potentiometers (or) resistor which can be adjusted with the help of a small screw
driver. (Fig 7).
Resistance depends upon temperature, voltage, light: Special resistors are also produced whose resistance
varies with temperature, voltage, and light.
Light dependent resistor (LDR): The LDRs are also known as photo- conductors. In LDRs the resistance falls
with increase in intensity of illumination. The phenomena is explained as the light energy frees some electron
in the materials of the resistors, which are then available as extra conducting electrons. The LDR shall have
exposed surface to sense the light. These are used for light barriers in operating relays. These are also used for
measuring the intensity of light.
Marking codes for resistors
Commercially, the value of resistance and tolerance value are marked over the resistors by colour codes (or) letter
and digital codes.

Resistance and tolerance value of colour coded resistors.
The colour codes for indicating the values to two significant figure and tolerances are given in Table 1 as per
IS:8186.

The two significant figures and tolerances colour coded resistors have 4 bands of colours coated on the body as
in Fig 8.
The first band shall be the one nearest to one end of the component resistor. The second, third and four colourbands
are shown in Fig 8.

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CITS : Power - Electrician & Wireman - Lesson 60-69