Functionality of Thermal Control Devices

In order to determine how to select the best thermal control device for your application, it is good to know what operating characteristics you desire for a thermal control to exhibit in an application. Just like there a variety of thermal control devices which are available, there are also a variety of ways in how they can be used.

The main uses for thermal control devices are:

  • Temperature Control
  • Over Temperature Protector
  • Temperature Sensor
  • Manual Reset Device
  • Non-Reset Device
  • Self-Hold Device

Temperature Control

Temperature Control functionality is when a thermal control is utilized in an application to control or regulate the temperature of a heated application. The primary function of the thermal control is to maintain a defined operating temperature which a design engineer has determined to provide for optimum performance of the end application. In these applications, the thermal control device will open and close at a preset temperature which will regulate the operating temperature of some type of heater.

This is the most difficult application type for thermal controls since the controls are required upon to carry the full electrical load of the application. However, the devices that provide functionality in these applications are designed, construction, and tested to provide optimum performance under these conditions.

Over Temperature Protector

Over Temperature Protector type controls function like their name implies. The primary function of these devices is to provide high limit temperature protection in applications where the normal operating temperatures have increased due to over work or another type of fault condition. Generally, these devices will carry the full electrical load of the application. Depending on the type of fault condition that exists, many thermal controls utilized as over temperature protectors will function either by the thermal control "seeing" an increase in temperature in the application, or by "sensing" an increase in the electrical load in the application, or through a combination of these two conditions.

Temperature Sensor

Temperature Sensor type controls function again like their name implies. The primary function of these devices is to open or close an electrical circuit that may or may not be the primary electrical circuit of the application. This means that these devices may or may not carry the full electrical load of the application. In general, the function of a temperature sensor is to monitor the temperature of one or more components of an application and either provide a signal function to another device in the circuit or control the function of another device in the circuit. This would include applications like turning on a ready light in an application or activating a cooling fan for other electronic components.

Manual Reset Device

Manual Reset devices are thermal controls which are generally used as over-temperature protectors in an application. Design engineers may want to incorporate an extra degree of safety in an application by using these types of devices. A manual reset device functions by interrupting the electrical load of an application based on increases in temperature. These devices act in a similar manner to a circuit breaker in that once they are activated, they must be manually reset for the application to function again. In some types of consumer applications, a fault condition can occur due to prolonged or misuse of an electrical appliance. In these instances, the overall temperature in the application has increased to a point where the thermal control device has activated and opened the electrical circuit.

Non-Reset Device

Non-Reset types of bimetallic controls are devices which again are generally used as over temperature protectors in applications. These devices will function much like a manual reset type device in that their primary function is to interrupt the electrical load of an application based on increases in the operating temperature. In some application types, design engineers may want to incorporate an extra degree of safety that goes beyond the functionality of a manual reset device. The difference is that once a non-reset type device functions in an application, the end consumer is not able to reset the control and the application will require servicing by an authorized technician in order for the application to function again.

These devices utilize a specially formed bimetallic element which is designed to operate in a temperature range that is determined by the design engineer. The difference is that due to the special form in the bimetal element, the thermal control will not reset unless it is exposed to temperatures around 5° C or less. These devices offer the functionality of a single operation thermal control but unlike a thermal fuse, they can be tested for function prior to use in the application.

Self-Hold Device

Self-Hold devices are an innovative new type of thermal controls which again are generally used as an over temperature protector in applications. These devices offer functionality similar to manual reset and non-reset types in that they offer an extra degree of safety in many different consumer applications but will automatically reset once the consumer disconnects the power source to the application. Again, the primary function of these devices is to interrupt an electrical load based on increases in the operating temperature or an increase in the electrical load in the application. In a fault condition, the bimetallic element activates at a preset temperature which interrupts the flow of power to the application. Once the thermal control device opens the electrical circuit, a heat source that has been incorporated into the design of the self-hold device is activated preventing the thermal control from resetting as long as the application is connected to a power source. Once the power source is removed (turning off the power switch or pulling the plug from the electrical socket), the heat source is deactivated allowing the thermal control to cool which permits the device to reset.