Whether it is electrical equipment or electronic equipment, or even electric motor, transformer electromagnetic conversion equipment, they are about to fail at the beginning of the phenomenon will often appear abnormal temperature. For example, motor, transformer in the occurrence of overload or short circuit fault, the winding temperature will rise sharply; In the electronic circuit, resistance, two/triode type components before breakdown will also occur abnormal temperature rise phenomenon; Contactor/relay coils, etc. Burned out due to overvoltage or interturn short circuit or prolonged use in closed environment. It can be seen that temperature protection for the above equipment has become a very necessary preventive protection means, so today we will talk about some knowledge in this respect.

First, let's look at an irreversible temperature protection device -- the temperature fuse. Different from the use of lead-antimony alloy/silver-copper alloy with higher melting point temperature, which focuses on the current protection of the circuit current value of the current fuse, temperature insurance is made of alloy material which is more sensitive to temperature. Temperature insurance will not only act when the current value rises to a certain value, but also fuse when the temperature of the protected object rises to a certain value, thus playing a preventive protection function. At present, the temperature insurance in the hair dryer, electric cooker and other household appliances; Hand-held power tools, small and medium-sized capacity transformers and other devices are often seen.

At present, there are two types of temperature insurance: cylindrical porcelain tube (mostly found in cookers such as electric rice cookers and electric pressure cookers) and plastic sealed sheet temperature insurance (mostly embedded in primary winding of transformer and small power motor). At the present stage, the two most commonly used temperature insurance operation temperature values are 92℃ and 98℃. Need emphatically pointed

Again, in view of the temperature insurance is a one-time protection device, damaged only to replace the adverse factors, in order to maintain and easy to use and the birth of a recyclable use of temperature protection device - temperature switch. Temperature switch is the use of bimetallic sheet as a temperature sensing element protection device, when the load is working normally, the bimetallic sheet remains in the on-off/on-off state; When the temperature of the protected object rises to the action value of the bimetallic piece, the bimetallic piece is heated to produce internal stress and quickly moves. When the temperature of the protected object falls back to normal, the bimetallic piece returns to its original condition. From the point of view of its protection mechanism, the temperature switch is almost the same with the traditional overload protection device -- thermal relay.

(Photo by KSD)

The rated current value of the temperature switch (the current that flows between the bimetallic plates) generally has three grades: 1A, 2A and 5A. The common packaging forms of temperature switch are plastic sealing sheet, metal shell strip, etc., which are widely used in electric cooking utensils such as electric frying pan, electric baking pan, electric oven, etc. In the soft starter, frequency converter, inverter welding machine and other industrial control equipment, the temperature switch is mostly fixed on the aluminum heat sink behind the high-power components (thyristor, IGBT); In addition, the temperature switch will be implanted in the windings of motors and large-capacity transformers to act as over-temperature warning and temperature protection components.

Thermistor, thermocouple, the two kinds of temperature detection components, they need to be equipped with the corresponding electronic circuit, in order to play the corresponding temperature measurement, overtemperature protection and other functions. Thermocouple is a voltage output type temperature sensor (MV output), its output end has polarity points, its temperature range is wide, the detection accuracy is general, used in the temperature detection of industrial heating equipment. Thermistor is a resistance value output type temperature sensor (see more 10KΩ and 100K two resistance level), the temperature range is narrow, but the accuracy is higher, more used for general temperature measurement, in induction cooker, light wave furnace, electronic thermometer and other devices often used.

In terms of the above four kinds of temperature protection and measurement devices, temperature insurance and temperature switch are the most common in electrical equipment. They can be directly involved in circuit control because of their certain current-carrying capacity. Thermistors and thermocouples are more commonly used in electronic equipment for temperature measurement/protection. They are only used as temperature sensing elements and cannot be directly connected with the main circuit or control loop.