Temperature switch

A circuit that opens and closes a conductive path based on temperature.

A temperature switch is one detecting the temperature of some substance. Temperature switches often use bimetallic strips as the temperature-sensing element, the motion of which actuates one or more switch contacts.

Temperature Switch has a vapour filled flexible sensing element actuates a micro Switch by means of a piston when temperature reaches the setpoint. The set point is adjusted by means of a compressible spring or a adjustable screw installed in the switch.

A bi-stable electromechanical device that actuates/ deactuates one or more electrical switching element(s) at a predetermined discrete temperature (set point) upon rising or falling temperature.

Special features of the temperature switch

• Fixed switching temperature

• Automatic reset

• No additional voltage supply
• Switching voltages up to AC 48 V / 3 A

Temperature Switch: Construction

Symbol

Working

Temperature switch is having a sensing part immersed in the process whose 

temperature is required to be controlled. The sensing part can either be a sensing bulb filled with a fluid-liquid, gas or a bimetallic strip that uses the differential expansion 

of two dissimilar metals. Snap action contacts that act to switch on electrical power to the device controlling process temperature.

Liquid filled temperature switches comprises a sensing bulb and a bellows element. The bulb is immersed in the process whose temperature is being controlled. The bellows element senses fluid pressure (liquid or gas) as temperature increases within

the process.As shown in figure, the fluid sensing bulb reacts to temperature variations and increases the pressure in the bellows element when temperature rises. A rise in the sensing bulb temperature compresses the bellows and moves the main spindle upward until spring force and bellows pressure are in equilibrium. This movement of the 

spindle is transferred to the switch and cause on or off action depending on set point of the temperature switch.

Specifications of temperature Switches

Adjustable Range

The span of temperature between upper and lower limits within which the temperature switch can be adjusted to actuate/deactuate. It is expressed for increasing temperature.

Set Point

That discrete temperature at which the temperature switch is adjusted to actuate/deactuate on rising or falling temperature. It must fall within the adjustable range and be called out as increasing or decreasing temperature.

Dead Band

The difference in temperature between the increasing set point and decreasing set point. It is expressed as “typical,” which is an average with the increasing set point at mid-adjustable range with the standard K switch element. It is normally fixed (not adjustable).

Hermetically Sealed

A welded steel capsule with glass-to-metal, factory sealed electrical leads that isolates the electrical switching element(s) from the environment.

Over-range

Over-range temperature is that temperature to which the sensing bulb can be continuously exposed without causing permanent change of set point or distortion sufficient to cause leakage or significant degradation of the fill fluid. Temperatures greater than over-range could cause permanent damage and render the device inoperative.

Maximum Process Pressure

The maximum process pressure to which the temperature sensing bulb should be exposed without being protected by a thermowell.

Repeatability

The ability of a temperature switch to successively operate at a set point that is approached from a starting point in the same direction and returns to the starting point over consecutive cycles to establish a temperature profile. The closeness of the measured set point values is normally expressed as percentage of full scale (maximum adjustable range temperature.)

SPDT Switching Element

Single-Pole, Double-Throw (SPDT) has three connections: C-Common, NO-Normally Open and NC-Normally Closed, which allows the switch to be electrically connected to the circuit in either NO or NC state.

DPDT Switching Element

DPDT is two synchronized SPDT switching elements which actuate together at increasing set point and deactuate together at decreasing set point. Discrete SPDT switching elements allow two independent circuits to be switched; i.e., one AC and one DC. 

Temperature Switch Principle

An alternative design uses a metal bulb filled with a fluid that expands with temperature, causing the switch mechanism to actuate based on the pressure this fluid exerts against a diaphragm or bellows. This latter temperature switch design is really a pressure switch, whose pressure is a direct function of process temperature by virtue of the physics of the entrapped fluid inside the sensing bulb.

The “normal” status of a switch is the resting condition of no stimulation. A temperature switch will be in its “normal” status when it senses minimum temperature (i.e. cold, in some cases a condition colder than ambient – check below note). For a temperature switch, “normal” status is any sensed temperature below the trip threshold of the switch.

How to Select a Temperature Switch

Step 1:

Determine if direct or remote (and capillary length) sensing is required. Select temperature Sensing Bulb Designator from specifications.

Step 2:

Determine the adjustable range required. Select Temperature Switch range as per requirement.

Step 3:

Select Housing for type of service.

Step 4:

Select Switching Element for housing and electrical service.

Step 5:

Select Diaphragm System.

Step 6:

Select Accessories as required for service.

Step 7:

Determine if Thermowell is required or not.

Applications of the temperature switch

• Temperature monitoring and direct switching of electrical Loads.
• Control and regulation of industrial processes.
• Universally suitable for machine building, plant, vessel, apparatus construction Food industry, Chemical industry and Petrochemical industry.
• For measuring points with limited space.