Page 289 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

5 Pilot Lamp: The pilot lamp is housed on the cover plate of the electric iron.One end of the pilot lamp is con-
nected to supply, while the other end is connected to the heating element. A shunt resistance is provided across
the pilot lamp which assists in providing a voltage drop. The shunt is designed to provide a voltage drop of 2-5
volts.
6 Heating Element:Most heating elements are made with a nickel-chromium wire, having both tensile strength
and high resistance to current flow. The resistance and voltage can be measured with a multimeter to verify if
the element is functioning properly. Heating elements are available in many sizes and shapes.

In an electric iron, the heating element is present between the sole plate and pressure plate. It is pressed hard
between the two plates. The heating element consists of nichrome wire wound around a sheet of mica. The two
ends of the nichrome wire are connected to the contact strips. The contact strips are connected to the terminals
of the iron. There are two reasons for which mica is chosen in the heating material. Mica is a very good insulat-
ing material. Besides that mica can also withstand very high temperatures. The entire assembly of mica sheet,
nichrome wire and contact strips are riveted together resulting in a mechanically sound and robust construction.
There is an asbestos sheet, which separates and thermally insulates the top plate from the heating element.
7 Thermostat:Thermostat is an important component of an electric iron that regulates its temperature. The word
thermostat is very common due to its usefulness in many home appliances that deals with temperature control.
The word thermostat is a combination of two words ‘Thermo’ (heat) and ‘statis’(status quo or constant).The
main function of thermostat is to keep heat constant in a given setting.
The main function of thermostat in an electric iron is to make sure that the iron doesn’t get too hot if it is left unat-
tended to for a period of time. The mechanism work as follows.
When electric current is passed through a coil in an electric iron, the coil becomes very hot. Through conduction
the heat is transferred to the flat base plate of the electric iron which is used to iron our clothes .

8 Bimetallic Strip:The thermostat in iron makes use of bimetallic strip. This bimetallic strip is made up of two
different types of metals (Brass and iron) with different coefficient of expansion bonded together. Therefore, in
the presence of heat, the bimetallic strip expands differently. The metallic strip is connected to a contact spring
through small pins.
The bimetallic strip remains in physical contact with the contact point at moderate temperature. However, at a
temperature of the iron above a certain limit, the strip tends to bend towards the metal with lower coefficient of
expansion. At this point, the strip stops to be physically connected to the contact point and current stops flowing
because of opening of the circuit.

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CITS : E & H - Electronics Mechanic - Lesson 160 - 179