Frequently Asked Question
The differential or hysteresis is the difference in temperature, measured in degrees Celsius, between the point where the thermostat first operates and the level where the contacts return to their original state.
Yes, in some cases a specific differential range can be supplied through a selection process however a minimum differential of 10°C must be maintained in order to insure a strong snap of the bimetal disc. Limitations do exist in the results obtainable as we can only select thermostats from within the actual range of temperatures which appeared on our test reports. Usually, a lower set temperature will have a smaller differential and a high set point will have a larger differential.
The snap action thermostat is the result of the design of the bimetallic disc which is formed in order to hold its original shape until the specified transition temperature and then switches the position of the contacts quickly. Also a differential or hysteresis is built into the design which eliminates the rapid and repeated on/off chattering or telegraphing characteristic of creeper type thermostats.
Each Cantherm/Microtherm product is tested before leaving our factory. Depending on the particular model switch that you have purchased, they have either been tested for response temperature on a sample batch basis based on AQL 2.5 level I, or tested on a 100% basis. The pill type thermal protectors (F, B, T) are tested 100% and each device is selected and categorized into the appropriate temperature range and tolerance; the A and D series thermal cutouts are batch tested as per the above level of ANSI Z1.4 – 1993 (was MIL-STD-105-E). The methods vary depending on the type but both the pill and open type thermostats are currently tested in an air oven equipped with 800 individually computer monitored positions from which we record the set and if required the reset temperature of each device. This oven is also used to perform the sample testing on the A & D types. The pill type thermostats can also be tested using specially designed machinery which monitors the contact state after passing over each of a series of temperature controlled plates.
Often the perceived operating temperature of a thermostat in the actual application will differ from the published temperature range because of thermal lag. This is the result of the thermal mass of the device and the thermal resistance of the insulation which slows down the reaction of the bimetal disc. The bimetal disc always operates at roughly the same temperature, however the temperature inside the thermostat and outside may be significantly different and time must be allowed for the heat to penetrate and leave the housing. The faster the temperature rises and the larger the total temperature difference from the beginning of the heating cycle to the specified operating temperature the more lag will be experienced.
In order to standardize results and to be able to compare one thermal switch to another, the industry has adopted a standard rate of temperature change which ranges from 0.25°C to 1.0°C change per minute. The larger and the more thermally resistive the package the slower the temperature rise rate must be. After the rise from room temperature a 10 – 15 minute soak at the lower threshold of the published range of operation is required in order to allow temperatures to stabilize. After the soak the specified temperature rise rate must be used and the contacts monitored closely to determine the correct operating temperature.
A creeper thermostat is generally manufactured with a blade type bimetal which has not been pre-formed as is a disc type. Calibration is achieved by either bending the bimetal or adjusting the pressure of the contacts until the desired result is achieved. Essentially a creeper has almost no differential and once the operating temperature has been reached it will switch off and on at or near that temperature. This is ideal for controlling an electric blanket but their usefulness as a thermal protector is marginal since it will maintain the device at the high temperature limit possibly resulting in deterioration of the product. The successive cycling on and off of the power could damage sensitive electronics or pose a hazard to the operator of a piece of equipment.
A PTCR thermal protector has a tiny PTC (positive temperature coefficient) ceramic heater imbedded into its body, it is connected in parallel with the contacts of the switch and as long as the switch is closed it does not heat. When the contacts of the protector open due to an external heat source the PTC heater begins to heat the bimetal disc which prevents it from reaching its reset temperature. As long as electrical power is available, the contacts will remain open in dead air down to -25°C. The contacts will reset a short time after the electrical power is removed from the product. Therefore the PTCR protector acts like a manual resetting device by permanently removing the power from a potentially dangerous appliance without the inconveniences of having to push a button to reset it.
A thermal fuse is a single operation device similar to a current limiting fuse which is sensitive to surrounding temperatures instead of the current passing through it. The small low current type are made using a fusible link material whereas the higher capacity fuses are equipped with an organic or wax pellet which hold the contacts together by compressing a spring. When their temperature threshold is reached the link or the organic pellet melts thus breaking the electrical circuit. Care must be taken during transportation, storage and installation that the device not be subjected to temperatures near the operating temperature. Special attention must be taken during soldering of the leads. It is also critical that the Tc (continuous temperature) not be exceeded as it could cause deterioration of the link or pellet and lead to eventual failure of the device.
A thermal protector is a device which has been designed to protect an apparatus or appliance against an excessively high temperature. They are typically used in electric motors, transformers and heaters to prevent overheating but can be installed in any type of equipment that is sensitive to high temperatures or to prevent damage, fire or personal injury. Because of the nature of the applications thermal protectors provide significant cooling before they will reset and allow the power to circulate again.
A thermal controller is a precision thermostat which is designed to maintain the temperature of a device or enclosure within a pre-determined range. The contact configuration, the differential/hysteresis and the tolerances of both the set and reset temperatures can be specified within published parameters. Contacts are available to open or close on temperature rise or fall thus eliminating the need to depend on the reset temperature of a lesser device to perform your primary task. This is ideal for applications where a contact must operate on temperature fall in order to prevent freezing or similar outcome.
B or beta value is the thermal exponent of the NTC thermistor, which is defined as the ratio of the difference between the napierian logarithms of the zero power resistance at two temperatures to the difference between the reciprocals of the two temperatures.
You can calculate the RT2 using this formula:
B = 3380
T1 = 298.15 K (25°C)
RT1 = 10Kohm
T1 and T2 are expressed in K with T(K) = T(°C) + 273.15
Yes, all Microtherm thermal protectors sold by Cantherm have UL, CSA and VDE approvals along with certifications by other agencies such as BEAB, CB, and others. Since Microtherm is the registered owner of the certifications they will all appear under the name of Microtherm GmbH, a search at the agency in question will list all of the approvals. The file numbers are listed on the individual product brochures or can be obtained by calling the Cantherm sales department.
Simply perform a search for Microtherm GmbH at any of the following agency links: