Bimetal Thermal Switch : types & applications

What is a Bimetal Thermal Switchs?
Bimetal thermal switchs use A bimetallic strip is a strip that consists of two strips of different metals which expand at different rates as they are heated. They are used to convert a temperature change into mechanical displacement. it creates a round arc, like a rainbow. As the temperature changes, the metals continue to react differently, operating the thermal cut-off switchs.

Bimetal thermal switchs are widely used inside heating pads, blankets, medical pads, electric kettles, and in some other devices where the application demands a cheap temperature controller mechanism.

Inside most home electrical appliances, we can generally find bimetal thermal switchs (temperature Snap Action switches) in two varieties – high voltage (Hi Amp) thermal switchs which directly control the flow of electric current to the load, or low voltage thermal switchs which indirectly control the current flow to the load. Schematic symbols for the common bimetal thermal switchs are shown below.

High voltage (Hi Amp) thermal switchs are rated for the voltage, current and power ratings of the loads they can handle safely. For example, a thermal switchs rated at 250V and 10A can be used to control a 2500W(max) load.

Low voltage (and Low Amp) thermal switchs are comparatively smaller and cheaper, but they are more sensitive to changes in heat than high voltage thermal switchs. That means since they are only intended to handle the control current and not the load current, low voltage thermal switchs are highly responsive and can provide more precise control than their high voltage counterparts.

A bimetal thermal switchs opens or closes an electric connection at a predefined temperature. Here is the inside view of a bimetal thermal switchs lifted from an old coffee maker machine. See how everything is put together!

The online customized thermal switch factory in China provides the following complete bimetallic thermal switch parameters and selection:

The first is a 10A~60A/250V resistive load disc bimetal thermostat switch from the KSD301 /KSD302  series. operating temperatures ranging from -40°C to 280°C. Some of them have normally-open contacts, while others have normally-closed contacts.

KSD301 series bimetal thermostat thermal switch

Normally closed KSD301 thermal switch

Hi-Amp – High voltage KSD302 bimetal thermal switch

Next is the TO-220 PCB-bimetal disc thermal switch from the JUC-31F series (PCB-bimetal thermal switch ). Currently, JUC-31F 95°C H This is the most common PCB mounted thermal switch with normally-open (close on rise) contacts.

KSD-01F/JUC-31F 0°C~130°C PCB mounting thermal switch

KSD-9700 most common and the cheap series thermal switch. consists of dish shape high sensitive bimetallic elemeng,movable contact-head,static contact-piece,soleplate,outer case,ghermal resistant lead and so on. When operation,the bimetallic element is in free state and movable contact-head and static contace-piece are closed and the circuit is on. normally-closed KSD 9700-A 250V 5A 70°C thermal protectors ( KSD-9700.pdf).

High temperature cut-off cheap KSD9700 bimetal thermal switch

Customize the parameters and cables of the 17AM-D+PTC thermal protector, and also process the wire harness to add a thermal fuse. 17AM-D+PTC thermal protector (Texas Instruments KLIXON) is a current-sensitive, temperature-sensing protection device connected in series with the motor coil. The protector changes depending on the coil temperature and motor current.

17AM + PTC overheating and overcurrent bimetal thermal switch

Knowledge of bimetallic strips:
A bimetal switch changes its switching status relative to the temperature. If the temperature is sufficient, the bimetall bends, thereby actuating a snap-action switch. This switch can open or close a circuit and in so doing start or stop a process (e.g. heating, cooling, pumping).

A bimetallic strip or bimetal strip is a strip that consists of two strips of different metals which expand at different rates as they are heated. They are used to convert a temperature change into mechanical displacement. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Bimetallic switches are used as part of the control engineering in the electrical industry, especially in auto-temperature control and temperature -indicating devices and appliances like heating, cooling, and pumping. Bimetallic thermal switches work on the principle that different metals bend at different temperatures. When the set temperature is achieved, one of the metals in the bimetallic switch bends to cut off the circuit and protect the appliance from extreme temperatures.

The TB02 series protector is a thermal switch with small dimensions（Referred to as battery pack thermal protector）, insulating case, high temperature sensitivity and long lifetime, which is widely used in lithium batteries, lighting, transformers, electric heating appliances, and other fields.

smallest size Battery pack thermal switches TB02

Features:
Snap action type bimetallic disc has the characteristics of high reliability and repeatability.
Small dimension is suitable for appliances which compactly installed upon winding or inside winding.
Steel structure bracket makes performance more stable.
Automatic temperature calibration process makes it has a high precision of temperature control.
The bimetallic disc overflow structure solves the technical difficulty of current transient overload and the temperature response lag.

Custom specifications according to customers’ req thermal switch
A key advantage of this concept is that the thermal switch can be installed in any desired local/remote location as it demands nothing but a simple two-core low-voltage cable for the interconnection. The low-voltage control circuit can then be used to drive an appropriate electromagnetic relay module (or a solid state relay module) to safely deliver power to the isolated high-voltage load, a mighty heater wire for example.

Basically, the circuitry switches on and off at different thresholds, so there is no leaping between one state and the other as the capacitors charges. That worked very reliably, no tweaking of the basic setup required. I have used this on a few Various electrical appliances designs now, and they have always proven very reliable.