Parts of electric Iron [contd...]
The Heating Element
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 insulating 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.
The Cover Plate
The cover plate is made of thin sheet of iron. It is placed on top of the base plate and it covers all the internal parts of the iron. The handle and connector are only attached to the cover plate.
The handle can be made either with wood or with plastic. The handle is attached to the cover plate with the aid of screws. Studs can also be used for this purpose.
The pilot lamp is housed in the cover plate of the electric iron. One end of the pilot lamp is connected 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.
When it comes to an automatic electric iron, the thermostat is the most important item. It uses a bimetallic strip to operate the switch which is connected in series with the resistance (or) heating element.
The bimetallic strip is a simple element which converts a temperature change into mechanical displacement. A bimetallic strip consists of two different metals bonded together. The two metals should have a different coefficient of expansion. If such a strip is heated, it starts to curve towards the metal having the lower co-efficient of expansion. On cooling, it straightens and comes back to the normal position.
Now we might wonder why such an element is used in iron. What is the purpose of this element in an electric iron?
The bimetallic strip is attached to a contact spring through small pins. The contact point between the strip and contact points remains closed. When the temperature rises significantly, the unusual expansion causes the strip to curve and the contact between strip and contact spring opens. Thus the supply to heating element is temporarily stopped (until the temperature goes down to normal). A special device called the cam is placed is placed near the contact spring, with which we specify the amount of curving of bimetallic strip required to separate the contact.
Thus using bimetallic strip the temperature is kept constant within certain limits.
The thermostat helps in maintaining the temperature within limits. But frequent making and breaking of circuit damages the contact points and it may also result in interference with radio reception. To avoid this, a capacitor of certain range is connected across the two contact points.