Air Conditioner Thermostat Under Your Control...

Could you imagine your life without air conditioner thermostat, in air conditioned space? Your fingers will become numb from switching on, and off the air conditioning unit – at least.

We need a form of temperature control air conditioner part, that is automatic, once we have set the determined temperature.

There are three main types of environment control within an air conditioned space;

  • the manual control, where control of temperature is achieved by varying necessary parameters of an air conditioner. These parameters could be, opening and closing of ducts, and controlling liquid refrigerant into evaporator.

    It is fully controlled by human. This type of control is very rare now

  • the semi-automatic control. Thermostats are categorised into this control.

    Semi-automatic means, you have to set a temperature that you want, and the equipment will switch on, and off the compressor at correct intervals, to deliver the desired temperature.

  • the automatic control, achieve temperature, humidity, fresh air and recycled are mixture, and draft controls, to deliver the desired comfort air to occupants, based on parameters set on programmable logic controls.

The most distinguishable difference between automatic and semi-automatic control is that, automatic control is set using average comfort level of its occupants. Semi-automatic control is set based on the user, from time to time.

That’s the overview of air conditioner controls. Now,

Let us focus on air conditioner thermostat:

The main role of an air conditioner thermostat, is to regulate the temperature within the air conditioned space, to the set temperature.

There are two types of thermostat,

  • the mechanical thermostat, and

  • the electronic, or digital thermostat

Mechanical air conditioner thermostat:

These devices are normally equipped with,

  • a bimetallic strip, fitted with mercury switch,

  • a temperature adjusting lever,

  • heat or cold anticipator,

  • and switches connected to a printed circuit board

Bimetallic strip

Uses the principle of difference in solid’s expansion. Two metals strips of different material are best to be used, due to the ductile property. It can expand and contract without failing. Thus the name of bimetallic strip.

Brass and stainless steel are the normal material to be used in this application. There are other variations, like copper and iron, but the principle is, to have two metal that expands differently at a given temperature.

In our case, brass expands and contracts quicker than steel, at a given temperature rise or drop.

The strip will be wound into a spiral shaped material, with brass being on the inside part of the spiral. Hence, when the heat within the room is up, the spiral will unwind.

Why spiral? Because it is more sensitive to heat variations within the room.

It is easier to bend a long steel, rather than a short steel, but we don’t want a very bulky air conditioner thermostat. This is why the spiral shape came in. It is compact, yet sensitive to heat variations.

The tip of the strip will be connected to a mercury switch

That will tilt together with the motion of the strip.

Mercury switch is a glass bulb partially filled with mercury, and inserted with a minimum of two wires.

Circuit will be closed if the mercury contacts the wires, and will be opened when the mercury is away from the wires.

Mercury is chosen as it is a liquid metal at room temperature. Thus the motion of the liquid can be controlled by the tilting motion of the strip, and hence the on/off action.

The minimum is two wires, but nowadays, air conditioners for heating and cooling are incorporated in a single unit. Hence we need a minimum of three wires.

Mercury will always be in contact with one wire. Heating circuit will be closed when it tilts to one side, and cooling circuit will be closed when it tilts to the other.

Temperature adjusting lever

Will rotate the position of the strip, to your desired setting. If you want the room to be cooler, then you’ll choose a low setting.

Hence the strip and the switch will be tilted such that, cooling circuit is closed longer. It will take the room temperature you have set at the thermostat, for the strip to contract, and open the circuit.

But, the room temperature will be very low before the thermostat “reads” it and act on it, because the air conditioner thermostat is placed far from the air conditioning duct.

Working parts of mechanical air conditioner thermostat

True, and that’s why we need,

Heat or cold anticipator

Is just a simple resistive wire. These are not vital parts of the thermostat, but its keeps the temperature control more accurate with low temperature overshoots, and your utility bill, low.

This wire will be connected in series with the circuit, for the heating cycle, and in parallel for the cooling cycle.

An air conditioner thermostat will normally have both anticipators installed.

Heat anticipator is and adjustable resistor. You’ll have to adjust the lever for heat anticipator to the required room temperature.

The larger the setting, the longer the wire length “seen” by the circuit. Longer time will be required before the wire heats up the coil.

However, the wire will heat up much quicker than the room. This heat will be transferred to the coil and unwind it, at the desired “room temperature”.

Cold anticipator is not adjustable, and it is wired parallel to the cooling circuit.

Current will flow through the resistive wire when the cooling circuit is open. This will heat up the wire and coil. Hence the coil will unwind before the room temperature rises too much.

As a result, the cooling circuit will be initiated much quicker than the set room temperature.

Simple illustration of cold anticipator and heat anticipator in mechanical air conditioner thermostat


The analogy of anticipators is similar to capacitors in ac-dc converter.

It “smoothens” the temperature variation in an air conditioned space.

Switches and printed circuit board

A printed circuit board is initially a plastic board with a layer of electrical conductive material on top.

Designated circuit paths for various on and off signals will be marked, and bleached. The marked portion will remain, and the unmarked areas will be washed away.

Switches will be soldered to the board at specified places, to select,

  • air conditioner on or off mode,

  • heating, cooling mode or auto mode,

  • fan on, or fan auto mode

But the real variations of the switches vary from one manufacturer, to another.

However, mechanical air conditioner thermostat needs a bulky system for more complex controls, and

Digital air conditioner thermostat comes into picture:

A simple digital system will consist of,

A programmable digital thermostat. Courtesy: Honeywell
Courtesy: Honeywell

  • a thermistor,

  • a liquid crystal display (LCD) to display various settings and monitored parameters by the thermostat,

  • and a printed circuit board with integrated circuits (ICs)


Is a temperature sensitive semi-conductor.

Semi-conductor is one of man’s greatest discoveries. It is neither a conductor, nor an electrical insulator. It will conditionally conduct electricity.

One of the conditions is temperature. Thermistor will vary its resistance, based on the temperature sensed.

A positive temperature coefficient thermistor will increase its resistance as the temperature increases.

A negative temperature coefficient thermistor will decrease its resistance as the temperature increases.

Manufacturer can choose either type, but the circuit will have to be different.

Printed circuit board with ICs

It is similar with to the printed circuit board for the mechanical system, but it is more complex. In addition to that, we’ll have ICs installed.

An IC is a circuit of more than one transistor, and normally in hundreds for a simple control system.

These sets of transistors will send on and off signal based on programmed inputs.

The input at the push button, will be translated into electrical current, and stored in flip-flop circuit. A flip-flop circuit is a memory circuit. It can store, and erase memory (in form of electrical current) based on sequence of received current.

Therefore, more complex input and output signals can be performed in a digital air conditioner thermostat, for a given size.

Oh yes, these ICs are quite small. Example of an IC is your computer’s processor. The size is about 10 mm by 10 mm, but the amount of transistors packed in it, is in the magnitude of hundreds of millions.

Variations in air conditioner thermostat design:

The variations are,

  • inclusion of mechanical timer or digital clock for programmable thermostat.

    The mechanical timer includes mini sliding clips to set for on and off times. The programmable function is quite limited.

    Digital ones may provide you with weekly programmes and more variations. The power of ICs.

  • inclusion of outdoor sensors, to give more accurate temperature control, especially for central air conditioners

  • inclusion of touch screen for better visibility and easier navigation for complex settings. This one is quite subjective. It depends on your preference

Popularity of digital thermostat is starting to displace mechanical air conditioner thermostat, due to the amount of inputs that can be stored.

However, the simplicity, reliability, and at least 10% cheaper price of a mechanical device makes it still used for air conditioners.

The choice of an air conditioner thermostat is yours, after weighing the cost and benefit analysis.

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