The working of an electric iron is very simple – it takes current from the mains and heats up a coil inside it. This heat is then transferred to the base plate which is pressed against clothes to remove creases.
Back when I was learning how to iron my clothes, I was rather annoyed by the whole process. For no reason whatsoever, it kept switching on and off of its own accord. As much as I was irritated by this, I was also intrigued by the strange phenomenon. Thankfully, I soon came to know that it was the ‘automatic power cut’ feature that prompted this action in the iron.
You’ve almost certainly observed this automatic power on/off function in electric irons, but do you know how it works? How does the iron know when to cut off the power? More importantly, how does an iron go about actually doing that?
WHAT DOES THE THERMOSTAT DO IN AN ELECTRIC IRON?
The most important component that helps to regulate temperature in an electric iron is the ‘thermostat’. Everyone has heard of thermostats in reference to air conditioners, water coolers, maintaining temperature balance in the home, and a number of other appliances that deal in temperature control.
The basic function of a thermostat can be deduced from the name alone; the word is formed from two Greek words: ‘thermo’ (heat) and ‘statis’ (status quo or constant). As the name implies, a thermostat’s basic function is to keep the heat constant in a given setting.
WORKING OF AN ELECTRIC IRON
The electric iron that we use to press the creases out of our garments also contains a thermostat, which makes sure that the iron doesn’t get too hot if it’s kept switched on or left unattended for an extended period of time. Let’s take a look at exactly how the mechanism works.
An electric iron relies on a basic combination of heat and pressure to remove creases from clothes. When an electric current is passed through a coil (or any other heating element present in the iron), it gets very hot. This heat is then transferred to the base plate (the smooth, flat surface that you place against clothes while ironing) through conduction, which elegantly and precisely irons your clothes.
However, if the iron is continuously drawing electricity from the power supply, the heating element continues getting hotter. This causes a lot of energy wastage (as an iron consumes a lot of electricity even in a few minutes), ruins your clothes, and in the worst cases, causes nasty (and potentially dangerous!) accidents.
Therefore, it’s essential that the iron doesn’t heat up to hazardous temperatures. This is where the thermostat enters the picture.
BIMETTALIC STRIP
The thermostat in an iron uses a bimetallic strip, and as the name implies, a bimetallic strip is made up of two different types of metal – with dissimilar coefficients of expansion – that are bonded together. This means that in the presence of heat, they expand differently. This bimetallic strip is connected to a contact spring through small pins.
At moderate temperatures, the contact point remains in physical contact with the bimetallic strip. However, when the temperature of the iron exceeds a certain limit, the strip begins to bend towards the metal with a lower coefficient of expansion. As a result, the strip ceases to be physically connected to the contact point, the circuit opens and current ceases to flow.
(a) Under normal temperature, (b) When the iron becomes too hot
Given that the circuit remains open for some time, the temperature of the iron drops, the strip acquires its original shape, and the current flows again. This cycle is repeated until you switch off its power supply from the main electricity source. This is the reason why your iron seems to power on and off of its own accord.
Now that we’ve cleared that up, if you thought that your iron was dysfunctional and were thinking of taking it to a electrician, you should feel proud of yourself – you just saved some money!
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