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.
A clothing iron works based on the combination of heat and pressure to remove wrinkles. Most domestic clothes irons work in the temperature range of 120oC to 180oC. The working premise of an iron is simple. It takes in the current from the mains (power supply). This current heat up the coil inside the iron. As the coil is heated, it transfers the heat through conduction to the base plate. We press this base plate against the clothes to remove any creases.
Back when I was learning how to iron my clothes, I was rather annoyed by the whole process. Apparently, for no reason whatsoever, the iron 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 understand that it was the ‘automatic power cut’ feature that prompted this action in the iron.
You’ve almost certainly observed this automatic 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 the iron go about actually doing that?
What does the thermostat do in an electric iron?
It is the ‘thermostat’ inside the iron that silently tracks the temperature, and with the help of other electronics, it is able to turn the power on and off. The thermostat is arguably the most important component in the iron, as it helps to regulate the temperature. It is not only in clothing irons where thermostats are used. You’ll also find them in air conditioners, water coolers, automatic temperature-controlled rooms, and several other appliances that require strict temperature regulation. In fact, roughly half of the electricity demand in the US comes through thermostatically-controlled loads.
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.
There is one caveat. Many people often confuse a thermostat with a thermometer, or use the words interchangeably. Well, they aren’t really the same thing. The thermometer is a device that measures the temperature, whereas a thermostat tries to maintain or regulate temperature.
Working of an electric iron
The electric irons that we use to press the creases out of our garments contain a thermostat, which ensures that the iron doesn’t get too hot if it’s kept switched on and left unattended for a long 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 an iron doesn’t heat up to hazardous temperatures. This is where the thermostat enters the picture.
The original thermostat conceived in the seventeenth century consisted of a float in a mercury thermometer tied to a damper cover. Whenever the ambient temperature around the thermometer surpassed a certain limit, mercury would rise, displacing the float such that it would close the damper. This basic premise led to the modern thermostats we use today.
The thermostat in an iron generally uses a bimetallic strip. 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.
Given that the circuit remains open for some time, the temperature of the iron then drops, the strip acquires its original shape, and the current resumes flowing again. So, in a way, a bimetallic strip works like a bridge to connect or disconnect the circuit to regulate the heating.
This cyclic on and off of the iron keeps repeating until you switch off its power supply from the main electricity source. This is why your iron seems to power on and off intermittently as you iron your clothes.
Although a thermostat helps in regulating the temperature within safe limits, frequent making and breaking of the circuit to regulate temperature causes the contact points to gradually wear out. This may result in electromagnetic interference, causing trouble with radio reception. To avert this, a capacitor is connected across two contact points. The role of the capacitor is to smooth out the electromagnetic interference. To learn more about capacitors, click here.
Now that we’ve cleared up this domestic mystery, if you thought that your iron was dysfunctional and were thinking of taking it to an electrician, don’t worry about it. You just saved yourself some money!