How to test a bimetal thermal switch?

 Before you start to test a thermal switch, a test oven has to be ready.

1. The test oven should be specially designed for temerature testing. It must have the following functions:

1.1      The internal temperature of oven should be even and stable. The temperature difference between any two points should be less than 1℃;

1.2      The temperature rise speed of oven must be under control. Before the temperature inside of oven reaches the 10℃ lower than bottom limit of rated temperature tolerance value, the oven can run under full load and the fast temperature rise speed is acceptable. Once the temperature reaches the level, the speed has to be controlled at or lower than 1℃ per 2 minutes.

2.  After making sure that the oven has the right function, you can install the thermal switch to the test oven. The body of thermal switch should contact the testing facility tightly and effectively. The circuit connection should be workable. If there is a temperature sensing head or thermocouple inside of the equipment, it should be mounted on the effective position.

3.       Then, close the test circuit and the oven starts to work. If the test oven is equipped with a electronic recorder, the test curve would be recorded. If no, you have to prepare a notepad to write all data down, at least 10 cyclings should be recorded, both OFF and ON temperature.

4.  Analyzing the test data and check if the thermal switch can function properly. If the quality is good, the test curve after initial 2 cycles should be uniform. If no, the curve would have variaties.

5. All test equipment like oven, thermocouple and meters should be calibrated regularly, to ensure the reliable function.

General information about thermal switch

Thermal switch is a temperature control device. It has many models and classifications, which sometimes make engineers confused. As an expert of making thermal swith, we'd like to share our own understanding with people who are interested in this issue.

1.  Bimetal thermal switch: there is a bimetal strip or disc inside of the switch which composed by two kinds of metals. The two metals have different heat expansion rates. When temperature changes, the two metal would expand. Because of the expansion rate is different, the strip would bend to the side with smaller expansion, which would finally drive the bimetal jump and disconnect or connect the circuit.

Button type bimetal thermal switch TS01 serial





2. Capillary thermal switch: Also called capillary thermostat. It is very easy to recognize because it has a long capillary attached to the body. Inside of the capillary, some liquid is filled. According to the different temperature setting, the liquid can be hot medium or cold medium. Some of capillary thermal switches can be used to electric appliances, like electrical stove, oven, refrigerator, AC etc. And some of them would be used to gas appliances, like gas stove, gas oven, gas water heater etc. For different applications, the construction of thermal switch would also be different.

Capillary thermal switch WKF

3. Snap action thermal switch: it is for bimetallic thermal switch. The bimetal inside can action very quickly and cut the circuit off. It does not close immediately, but a time delay. After the temperature decreased to a certain value, it close the circuit again. This thermal switch can be used for temperature protection.



Snap action thermal switch TS05 for battery pack

4. Creep action thermal switch:  it is a type of bimetal thermal switch also. It can open the circuit and close it in a short time. The temperature differential between OFF and ON is very small. It has good performance for temperature balance, but can not be used for thermal protection.



Creep action thermal switch 17AME-H

5. Auto reset thermal switch: it means that the thermal switch can reset automatically and no manual reset button needed.



Auto reset thermal switch for automotive

6. Manual reset thermal switch: it means that the thermal switch can not turn ON after it turns OFF. A manual reset button or power disconnection design would be necessary.



Manual reset thermal switch 17AME+PTC

7. Temperature sensing thermal switch: the main factor that influences the thermal switch OFF and ON is temperature. Current might have some influences, but not the most important issue.



Temperature sensing thermal switch 17AME

8. Current sensing thermal switch: the main factor that affects the thermal switch OFF and ON is current. It is normally used for over current protection purpose. The temperature also can make the thermal switch turn OFF, but normally current sensing actions earlier than temperature.



Current sensing thermal switch 3MPE/6APE serial

9. Fixed temperature thermal switch: The temperature is fixed after it was released to customers. The end user cannot change the temperature setting.



Fixed temperature thermal switch KW serial

10. Adjustable thermal switch: the temperature setting is changeable and the end user can change the temperature by rotating the knob.

Adjustable thermal switch KST serial

Why does not thermal switch action when temperature reaches the rated value?

In my 15 years experience of serving the bimetal thermal switch customers, this question was asked too many times. Different applications or operating engineers would have different situations. But most of  possible reasons which might cause the thermal switch not action properly are as follows:

1. The temperature rise speed is too fast. For example, the temperature rises to the prescribed value in very short time. Then, the bimetal inside of thermal switch has no enough time to reaction. The temperature of bimetal is still much lower than the rated value or outside temperature. So, the thermal switch would not action. The first trip off temperature would much higher than the rated value. It is so called overshot. This problem can be solved by controlling the temperature rising speed, or making thermostat action 3 cycles. Then, the thermal switch would action properly.
2.  There might be misunderstanding for rated temperature. It means a tolerance range,  normally 10℃, instead of the specific value. For example: a thermostat is rated at 100℃, the common action temperature range is 95~105℃. It is not just 100℃ only. So,  the action temperature for some units can be 104 or 105℃. These units are qualified products.
3.  The thermal switch is malfunctioned. The temperature might be out of tolerance, or the two contacts of thermal switch were burned and connected together forever. It would be a serious defection and might cause fire or other safety problems.
4. Other uncertain reasons also are possible, like incorrect mounting method, improper temperature setting, damage during shipping etc. To solve these problems, we need to analyze the samples together with the applied product. Some mock-up tests are necessary.

 

Most of thermal switches provided by Yangzhou Wuyue Electric Co., Ltd have both UL and VDE approval. All products are 100% tested before releasing to our noble customers according to ISO9001 and TS16949 quality system. The defective rate we promise to our customer is 100 ppm.

 

Low temperature thermal switch 17AME-H