“Thermal Sensors can be used in various applications and play a very important role in Electronic equipment, maintaining stable operation of equipment by monitoring temperature and detecting abnormal temperatures. The temperature sensors on the market generally use mechanical contacts (such as reed switches, Reed Switch) and semiconductor switches (such as thermistors, Thermistor). Many temperature sensor manufacturers are also constantly developing new technologies to apply magnetic force to the sensor, giving it more advantages and realizing low-cost, high-precision and fast-response thermal switch solutions.
Author: Kevin Chow
Thermal Sensors can be used in various applications and play a very important role in electronic equipment, maintaining stable operation of equipment by monitoring temperature and detecting abnormal temperatures. The temperature sensors on the market generally use mechanical contacts (such as reed switches, Reed Switch) and semiconductor switches (such as thermistors, Thermistor). Many temperature sensor manufacturers are also constantly developing new technologies to apply magnetic force to the sensor, giving it more advantages and realizing low-cost, high-precision and fast-response thermal switch solutions.
Here is a simple comparison of the 3 more common types of temperature sensors:
TABLE 1: A SIMPLE COMPARISON OF THE THREE THERMAL SENSORS
In general, engineers will usually first consider using a thermistor (NTC or PTC) or a traditional bimetal thermostat. Thermistors monitor temperature through changes in resistance caused by changes in temperature. The original manufacturer usually provides the corresponding table, but circuit control is required. If the physical switch of the bimetal is used to provide the ON or OFF function – that is, using the traditional principle of the expansion rate of the bimetallic element – the operation of the ON/OFF function can be made simple, but the response speed and recovery time are also sometimes considered factors of choice. – Applications such as turning on fans or temporarily turning off the power supply when the system is warm, but also needing to resume normal operation when the system cools down.
There are also manufacturers who use temperature-sensitive ferrite (Thermo Ferrite) as a magnetic switch, which is a ferromagnetic material that has soft magnetic properties even at the Curie temperature (*Note 1). The saturation magnetic flux density decreases as the temperature of the material increases. When the temperature of the material reaches the Curie temperature, it will rapidly become paramagnetic, that is, the ferrite loses its magnetism.
Figures 1 and 2 below show the properties of Thermolite, a temperature-sensitive ferrite developed by KEMET. Figure 1 shows the temperature response in which Thermolite rapidly loses permeability at the Curie point, and Figure 2 shows a comparison diagram when the material’s saturation magnetic flux density is also close to zero when its temperature is close to the Curie point.
*Note 1: French physicist Pierre Curie discovered that at a certain temperature, magnetic materials lose their magnetic properties. Now, this temperature is called the so-called “Curie temperature”.
There are two types of this temperature sensor, namely “Break Type” (Figure 3) and “Make Type” (Figure 4). Its structure is mainly composed of ferrite, reed switch and permanent magnet (if it is “on type”, there will be a gap). The internal reed switch senses temperature with the ferrite and converts the thermal energy into magnetic field energy that separates the reed contacts.
The “disconnect type” is a normally closed sensor (NC), which consists of a ferrite, a reed switch, and a permanent magnet, as shown in Figure 3 below. When the working temperature is lower than the trigger temperature, the switch is closed; when the working temperature reaches or exceeds the trigger temperature, the switch is open.
Figure 3: Construction of a disconnect type sensor (Image credit: Kemet)
Figure 4 : Definition of Disconnect Type Sensor (Image source: KEMET)
We learn more about the role of this ferrite in a disconnected thermal sensor. When the temperature is below the trigger temperature, the ferrite is magnetic and generates a toroidal magnetic field, as shown in Figure 5. In a magnetic field, the reed switch is magnetically induced to N and S polarity. The contacts of the reed switch make contact due to magnetic attraction, and the reed switch becomes conductive. When the temperature of the ferrite reaches the trigger temperature (i.e. the Curie temperature), the ferrite loses its magnetic flux and becomes non-magnetic. The contacts of a reed switch are separated, i.e. open.
Figure 5: Schematic diagram of the operation of the disconnect type sensor (image courtesy of KEMET)
The “on type” is the normally open type, consisting of a ferrite, a reed switch, a gap, and a permanent magnet, as shown in Figure 6. When the operating temperature is lower than the trigger temperature, it is open circuit state operation. But when the working temperature is higher than the trigger temperature, the switch is closed, as shown in Figure 7 below.
Figure 6: Construction of a switch-on type sensor (image courtesy of KEMET)
Figure 7: Definition of a switch-on type sensor (Image source: KEMET)
On-type sensors have a gap space between the ferrite material, as shown in Figure 8. When the temperature is lower than the trigger temperature, the permanent magnet and ferrite generate a toroidal magnetic field and the contacts of the reed switch are separated and become open circuit. When a ferrite reaches its Curie temperature (trigger temperature), the ferrite loses its magnetic flux and becomes non-magnetic. The magnetic reed switches on both sides are induced to N polarity and S polarity respectively by permanent magnets, and the contacts are attracted by the magnetism, so that the reeds are connected.
Figure 8: Schematic diagram of the operation of the switch-on type sensor (Image source: KEMET)
In summary, the main advantages of using ferrite reed switches over traditional bimetal thermostats and thermistors are as follows:
・ High reliability
・ Excellent environmental resistance to dust, explosion, moisture and corrosion
・ Wide temperature setting range
・ High precision
・ Short response time
・ High stability
Choice of temperature sensor
On the Digi-Key website, different types of temperature sensors are covered. Search for the keyword “temperature sensor” and it will be clear at a glance.
After clicking on the “Temperature Sensor – Thermostat – Mechanical” option, pick basic operating parameters such as “Circuit”, “Switch Temperature” and “Reset Temperature”, as well as performance parameters such as “Tolerance” and “Number of Switches” ”, buddy appearance options (dimensions such as Termination Style, Mounting Type, and Package/Enclosure), the entire process is very simple, allowing engineers to choose materials step by step.
If engineers require a temperature sensor with low tolerance and high switching count, KEMET’s TRS5 would be a good choice. For example, the TRS5-90BLRU of the KEMET TRS5 series. It has excellent temperature accuracy of ±2.5°C and lead wire mounting characteristics, making it ideal for temperature sensing and electrical overheat monitoring applications such as home appliances, rice cookers or air conditioners. If size is the preferred requirement, consider Sensata’s 6600 series, such as the 66L120-036, which are bimetallic and 4-pin surface mount types suitable for use in computers, automotive and test equipment.
In addition to traditional bimetal thermostats and temperature sensors with semiconductor switches, engineers can also consider using temperature sensors with magnetic switches, which have the advantages of high reliability, short response time and high stability. Provides another stable and reliable temperature control solution.
The Links: MDC100B-18 2MBI400TB-060 7MBR150VN120-50