Image source: Kemet
Author | Randall Scasny
Thermal switches help mitigate damage to electronic products and prevent deterioration of operating conditions. This article highlights three methods for controlling heat buildup using thermal switches.
Overheating is a major problem for electronic devices, especially in high-power applications such as industrial automation or robotics. Overheating not only shortens the lifespan of electronic devices but can also quickly lead to component damage, fires, explosions, and even injuries to operators.
Thermal switches mitigate thermal damage in electronic products. They detect abnormal temperatures and shut down the system before any damage occurs. Unlike thermal fuses (which are consumables and must be replaced after melting), thermal switches reset upon cooling. But before we can demonstrate how thermal switches address real-world challenges in electronics, engineers must understand how they work.
A thermal switch is an electromechanical temperature control device that disconnects an electrical connection at a specific temperature. Thermal switches are typically reusable, preventing situations requiring user intervention.
Thermal switches can be implemented using various technologies, including bimetallic switches, thermal reed switches, mercury switches, rod and tube switches with different temperature coefficients, and pneumatic switches. Below are three methods for controlling heat buildup using different types of thermal switches.
01
Thermistors and microcontrollers
A thermistor is a resistor whose resistance changes with temperature. Thermistors are sensitive to temperature fluctuations and can accurately measure temperature. They are used in applications such as circuit protection (surge protection), oil temperature measurement, and automotive coolant applications.
When designing products, engineers should consider two parameters of thermistors: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). The resistance of an NTC thermistor decreases as temperature increases; conversely, the resistance of a PTC thermistor increases as temperature increases.
The change in resistance of a thermistor can be converted into an electrical signal using a Wheatstone bridge or voltage divider circuit. The voltage divider converts the thermistor's resistance into an electrical signal. When the temperature changes, the electrical signal changes accordingly, and its value can be read by the analog input of the microcontroller and displayed on the LCD screen.
02
bimetallic switch
A bimetallic switch consists of two metal strips with different thermal properties, typically a combination of brass and steel or copper and steel. These metal strips are bonded together along their length, either by riveting or by fusion. The structure can be in the shape of flat strips or concentric coils.
As temperature rises, the two metals expand at different rates, causing the strip to bend. This characteristic of bimetallic switches can be used in switching circuits or thermometers, where a pointer indicates the temperature on a dial. Bimetallic switches are commonly used in thermostats, thermometers, clock mechanisms, and heat engines.
The coefficient of thermal expansion of steel is 12 parts per million (ppm/℃). Copper has a coefficient of 16.6 ppm/℃, and brass has a coefficient of 18.7 ppm/℃. When a strip of metal composed of steel and copper, or steel and brass, is heated, the strip will bend towards the steel because steel has a lower coefficient of thermal expansion.
03
Reed switch
A reed switch is a type of magnetic reaction switch consisting of two small iron-nickel (Fe-Ni) alloy plates. As shown in Figure 1, the reed switch can be encapsulated in a glass tube filled with inert gas, thereby reducing the risk of metal fatigue and extending its lifespan. Enclosed reed switches are dustproof, moisture-proof, explosion-proof, and corrosion-resistant.
▲Figure 1: Reed switch structure.
Reed switches open and close in the presence of a magnetic field. For reed switches used as thermal switches, they must be used in conjunction with materials that generate a magnetic field upon temperature change.
The Curie point, discovered by French physicist Pierre Curie, is the temperature at which certain magnets lose their magnetism. When used in thermal reed switches, engineers found that adding a specific amount of dopant could effectively control the Curie point of the material.
Engineers can choose between two types of thermal reed switches: break-type and make-type. Both consist of a thermal ferrite core, a reed switch, and a permanent magnet. The magnetic properties of the thermal ferrite core change with temperature, which in turn opens or closes the reed switch.
Figure 2 shows an open-type switch, which is normally closed. The switch remains closed (ON) if the operating temperature does not exceed the trigger temperature. It opens (OFF) when the temperature rises to equal or higher than the trigger temperature.
▲Figure 2: Disconnect type thermal reed switch.
Figure 3 shows an ON-type switch. When the operating temperature is below the trigger temperature, the ON-type switch is open (OFF). When the operating temperature is above the trigger temperature, the switch is closed (ON).
▲Figure 3: Thermorite switch with on/off capability.
The diagram also illustrates the operating mechanism of the contact-type switch. At temperatures below the Curie point, the thermal ferrite generates a toroidal magnetic field. This magnetic field induces N and S poles in the contacts within the reed switch, generating a magnetic attraction that causes the contact tips to touch, thus placing the switch in a closed or open state.
When the temperature of the heated ferrite reaches the Curie point, it loses its magnetic flux and becomes nonmagnetic. Since there is no longer a magnetic attraction, the contact tip breaks, putting the switch in the off state.
The difference between on and off type switches lies in the addition of a gap spacer between the two sections of heated ferrite. If the temperature does not exceed the trigger temperature, a magnetic field is generated, separating the contacts in the reed switch. When the temperature reaches the trigger point, it becomes non-magnetic. A permanent magnet induces N and S poles in the contacts of the reed switch, generating a magnetic attraction that brings the contact tips together, thus closing the switch (ON).
04
Benefits of thermal switches for manufacturers
Overheating poses a serious threat to the lifespan of electronic components and, if left unchecked, can injure operators. Thermal switches play a crucial role in temperature monitoring; they can shut down the system when its operating temperature exceeds safe levels.
When designing electronic products, engineers should carefully consider operating requirements to determine a suitable thermal switch solution that is simpler, more cost-effective, and most importantly, more reliable.
Key concepts:
■ A thermal switch is an electromechanical temperature control device that can disconnect electrical connections at a specified temperature.
■ Thermal switches offer higher accuracy and overall robustness, as well as better response time and relatively low hysteresis.
■ In preventative maintenance, users can use a variety of tools, among which thermal switches are one.
Think about it:
What benefits do thermal switches bring to manufacturing companies?
