Bimetallic Thermal Control Devices: Applications and Types

Bimetallic thermal control devices are used primarily for temperature control, over-temperature protection, temperature sensing, and thermal fusing. They are also available in an equally wide range of package styles and are used in a wide range of industries, from consumer to industrial and military/aerospace. Some carry the full load current, while others are designed to send a signal-level output.

A temperature control device is used in applications where it’s necessary to regulate the temperature of a heated process. These devices are designed to maintain operation within a specified operating temperature range, usually controlling the operation of a heating element. Temperature controllers typically carry the full load current and are required to be ruggedly constructed.

A temperature sensor may or may not carry the full load current. These devices are placed so they can monitor the temperature at a critical location in the application. For example, they can turn on a light when an oven has reached its operating temperature. In other cases, they can be used to turn on a cooling fan when a higher-than-desired temperature has been reached. Temperature sensors have a reset switching temperature that’s typically 15 to 30 K below the initial switching temperature.

For example, a series of industrial temperature sensors with switching temperatures of 50 to 130 °C are available. The temperature is preset at the factory and can’t be changed in the field. Temperature sensing is implemented with a bimetal disk.

These industrial sensors are designed to protect compressors, pumps, and other machinery. They have a normally closed contact that opens when the temperature exceeds the preset level. When the temperature falls below the reset temperature, the contacts reclose. In addition, these temperature sensors are available with a second normally closed contact or with an internal Pt1000/Pt100 temperature measuring device (Figure 1).

Figure 1. These bimetallic disk temperature sensors are available with a second normally closed contact or with an additional Pt1000/Pt100 temperature measuring element. (Image: *WIKA*)

Bimetallic temperature sensors are also available for use in general electronics, test equipment, and computer applications. Like industrial temperature sensors, these devices are based on a bimetal disk. They send an overtemperature signal to alert operators of abnormal operating conditions.

One of these temperature sensors is offered in a TO-220 package. It was developed primarily to support thermal management in electronic power converters. It’s suited for sensing the temperature of the surrounding air or on surfaces like heat sinks and circuit boards (Figure 2). This device has a switch capability of up to 0.5 A for 48 Vdc and is rated for up to 100,000 cycles at 5 Vdc and 0.010 A.

Figure 2. This TO-220 packaged temperature sensor uses a bimetallic disk and is rated for 100,000 switching cycles. (Image: *Sensata Technologies*)

Over temperature protection

In addition to general thermal controls and sensors, over-temperature protection (OTP) is a key application for bimetallic thermal control devices. Like thermal controls, OPT devices typically carry the full electric load of the application. In some applications, the thermal control device is also used to provide over-temperature protection. There are three types of OTP devices:

Manual reset OTP devices are the thermal equivalent of an electric circuit breaker. Once they have tripped, they need to be manually reset to enable the system to return to operation. They are often found in consumer devices and white goods.

Non-reset OTP devices can provide an extra level of safety and control. They trip if the preset temperature is exceeded, but the user is not able to manually reset the device. Resetting requires the intervention of a service technician. Non-reset OTP devices are used when the root cause of a high-temperature condition needs to be determined and whether the application requires further maintenance or replacement.

Self-hold OTP devices require the user to disconnect the primary power source from the appliance to reset it. They generally require that the user wait for the system to cool sufficiently for the device to reset, enabling the appliance to return to service.

Thermal fuses

As the name implies, thermal fuses are the temperature equivalent of electric fuses that protect from over-current conditions. Thermal fuses, also called thermal links or thermal cut-outs (TCOs), are only good for one use and must be replaced to return the equipment to operation. They are often used as a backup to a primary shut-off device, as required by some safety agencies. They are used in systems where high-temperature conditions are indicative of serious safety concerns, like fire dangers.

Summary

Bimetallic thermal devices are highly adaptable and can provide monitoring, control, and protective functions. They are available in a variety of latching and non-latching configurations. These devices are suitable for harsh industrial environments, consumer appliances, general electronics, power conversion, and other applications.