Operating Characteristics and Application Uses
This section of the How-To Guide offers a closer look at the manner in which each of the basic types of thermal control devices operates and the type of applications where they are best used.
Cantilever Bimetallic Thermostats
How They Work:
These devices utilize a bimetallic element to make or break a circuit. The heart of cantilever bimetallic thermostat devices lies in the bimetallic element. This element is made from specially bonded materials that react differently under the influence of a thermal load. The principle behind how bimetals work makes use of the fact that different metals will expand or contract at different rates when heated or cooled. This is commonly referred to as the coefficient of expansion. The materials which are used to produce bimetal react differently when heat is induced into the bimetal element either through exposure to heat generated by an application or caused by the self-heating effect created by passing current through the bimetallic element, or a combination of both. This permits the bimetallic element to "bend" creating the work force required to make or break an electrical circuit.
The technical name for these thermal controls is to call them a cantilever type in order to differentiate them from disc-type bimetal controls. In cantilever bimetallic thermostats, one or both ends of the bimetallic element are restrained to take maximum advantage of the work force created by the bending effect of the bimetal that is created by increases or decreases in the ambient temperature of an application. In most cases, cantilever bimetal thermal controls are divided into two sub-categories, either conductive type controls, or non-conductive type controls or also referred to as a shunted type.
In conductive type controls, the bimetallic element carries the circuit current so the action of the bimetallic element is not only influenced by responding to any changes in temperature but also by the self heating effect caused by the electrical load passing through the bimetal. This dual action provides for circuit breaker characteristics that can enable the device to function on increases in current as well as in temperature. This self heating of the bimetallic element is commonly referred to as "derating." The derating effect can best be described as the difference between the operating temperature of a control under a no-load condition to that when an electrical load is applied. The derating effect increases with increases in the electrical load.
In non-conductive type controls, the bimetallic element does not carry the circuit current of the application, but provides work force against another internal component that does. The component that carries the electrical load is commonly referred to as a "shunt." Although the bimetallic element does not carry the circuit current, there is some derating caused by the electrical load passing through the shunt element.
One advantage of cantilever type controls is that you can customize them to meet the needs of your application. All one has to do, is take advantage of the many different types of bimetals available to easily alter the reactive characteristics. At Portage Electric Products, Inc. (Pepi ®) you are offered a choice of the element used in many of our devices making it easier to meet the specific requirements of your application. Your choice of the bimetal can include choosing a high or low resistance type of element which provides for increased or decreased sensitivity to short circuit conditions in your application. Also, by having a choice as to the type of bimetallic element that best suits the needs of your application, you can tailor the needs of your application which matches the operating specification of the thermal control device. At Pepi ® you have a choice in how it works best for your application.
You also have a choice on the function of the bimetallic element as well. Based on the type of bimetallic element used, you can specify a narrow differential between the opening and closing temperature of the device, a wider differential, or a device that essentially does not reset unless the power source is interrupted, or does not reset at all. Engineers also can choose from many different model variations that are designed in a normally closed configuration where the bimetal element will open the electrical circuit upon increases in temperature or designed in a normally open configuration where the bimetal element will close the electrical circuit upon increases in temperature.
Cantilever type controls which provide for a narrow differential between opening and closing temperatures makes them ideal for temperature control applications where it is necessary to continually make or break the circuit. Cantilever type controls which provide for a wider temperature differential permits lower average operating temperatures in applications where the device is used for over-temperature protection. Variations of these types of controls allow for the device to remain in an open state due to the device having a very low reset temperature that would be much lower than the normal ambient temperatures. This type of device permits for 100 percent testing for function of the device while exhibiting fuse-like operation in the end application. Another variation of this type of control incorporates a heat source either internally or externally to the bimetallic element. This heat source is activated once the bimetallic element opens the circuit current, providing heat to prevent the bimetal from functioning and permitting the device to reclose. The only way the device can be reset, is to disconnect the application from the power source (pull the plug), and the device will reset automatically.
These devices are also generally smaller than other options. With very little room needed to make or break the circuit these cantilever type controls are quite thin, especially when compared to disc-type devices. This small footprint makes it easy for them to fit in tight spaces. Different case configurations and materials also make it possible to fit these flat devices into places other controls simply cannot fit. This includes nestled into the small crevice between AAA cells used in some rechargeable battery packs.
Cantilever devices are easily wired into circuits and do not require any special mounting hardware unless dictated by the application. Pepi ® devices can be ordered with leads attached at the factory making them even easier to install.
Safety and Reliability
Since bimetal devices are resettable they can be tested before and after installation. This not only improves the safety and reliability of the product, but enhances the peace of mind of product designers. This is an especially important feature when comparing these types of devices to thermal fuses which are only effective for a single use and, therefore, cannot be tested. In fact bimetallic devices, with very low reset temperatures, are sometimes used in the same manner as a thermal fuse. Bimetallic thermal controls produced with a low temperature reset point will open an electrical circuit in a fault condition just like a thermal fuse. However, these devices can be tested for function prior to being installed in the end application. This provides design engineer with an extra layer of protection that is not available with single use devices.
Bimetallic Disc Thermostats
How They Work:
As with cantilever type thermal controls, bimetal disc thermal controls rely on a specially formed bimetallic "disc" to provide the work force necessary to open and close a set of electrical contacts. The bimetal disc thermostat reacts to changes in temperature in an application and will provide the work energy to open and close the contacts. Since the bimetallic element of the thermal control does not carry the circuit current of the application, these devices are not subject to the self-heating effect caused by the electrical load passing through the bimetal element. This gives disc type devices the ability to handle higher current loads.
Like their cantilever counterparts, disc type devices are available with design options that provide for the controls to either open or close an electrical circuit upon increases in temperature. Many manufacturers of bimetallic disc thermostats also offer devices with a manual reset feature. This means that once the bimetallic element is activated in the device, the device must be manually reset by the end user in order to restore the flow of electricity to the application. This is the same principle as resetting a circuit breaker. Some manufacturers of disc type devices also offer devices which incorporate self-hold functionality. These devices incorporate the use of an internal heat source which generates enough heat to prevent the device from resetting until external power to the application is removed.
The ability of disc-type controls to handle loads is due to the fact they operate strictly on changes in temperature and are designed to be isolated from the circuit current. In disc-type devices, there is no internal heating effect of the bimetal element because of the electrical load passing through the bimetal element. For this reason disc type controls are often more suitable than other controls for resistive current loads above six amps. This includes many common applications in products such as coffee makers and hot water heaters.
The exterior housings of disc type thermal controls are commonly made of ceramic or some type of plastic material. The type and temperature rating of the plastic materials vary between the different manufacturers of these devices. Some manufacturers utilize higher temperature plastic materials that make them suitable for operating temperature up to 200°C. The housing of these controls can also be made from various types of ceramic materials making them good choices for very high temperature applications, in some cases well over 300°C.
Most manufacturers of disc type controls can also control the reset temperatures of the device. This can provide the end user with the option to specify a wide or narrow temperature differential between the opening temperature and the closing temperature of the controls. A narrow differential between the opening and closing temperature makes disc type devices suitable for use in temperature control applications. If these devices are produced with a wider temperature differential between the opening and closing temperature, then these devices are ideal for use as an over temperature protector in many higher load applications. Some types are available with a "manual" reset function. This means that once the device has functioned due to a fault condition in the application, then the user must manually press a button connected to the device, to reset the electrical contacts. This feature is normally associated with applications where a true fault has occurred in the application and it alerts the end user as to a problem. Again, some disc type devices are available with self-hold functionality that essentially prevents them from resetting unless the power source to the entire application is interrupted.
Disc type thermal controls are available with a wide variety of mounting options. Many models are designed with a built in mounting brackets which provides ease of installation. Many manufacturers offer a wide variety of mounting brackets that can meet the requirements of your individual application. These mounting options can include fixed flanges, rotating flanges, or threaded screw caps that provides for positive temperature transfer into the devices. Generally disc type devices are equipped with two terminals that are used to connect the devices into the electrical circuit. There is a variety of options available for these terminals as well. These include quick connect style terminals, solder style terminals, screw connect style terminals, or crimp connect style terminals. Also, the position of these terminals can usually be modified to meet the needs of individual applications.
Just like their cantilever type cousins, disc type thermal controls can be used in a wide variety of applications. Since bimetal elements are still at the heart of these devices, they are resettable and therefore, can be 100 percent tested for function by the manufacturer. As with cantilever devices, some disc type controls are available in a normally "open" or a normally "closed" contact configuration.
Just as with our cantilever type controls, at Pepi ® design engineers have a wide variety of options available with our ½" disc type controls. You can choose the operating temperature range, the reset method of the device, the reset temperature, the mounting configuration, and more. We realize that the operating specifications for many applications vary. So you can choose from many options that allows you to specify a thermal control to meet the individual needs of your application.
Adjustable Temperature Type Devices
How They Work:
There are many different types of controls that fall into this category. Normally, these devices enable the end user to manually adjust the temperature of an application such as in an oven or a clothes iron. These devices normally control the temperature in an application, but sometimes also include a secondary set of electrical contacts that are controlled independently provide high temperature limit protection as well. Adjustable temperature type devices are available in three main designs.
Capillary and Bulb and Capillary Type Controls
These larger and more costly devices are used when remote temperature sensing and control capabilities are required. The switching function of the device is not located at the same place where the temperature sensing is taking place. For these devices to function, the capillary tube of the "bulb," or oil filled reservoir is placed where the temperature sensing is to be preformed. As heat increases or decreases at the capillary end or the bulb end of the tube, high temperature oil expands or contracts and moves through a "capillary tube." This tube is attached to a pressure switch that opens and closes a set of electrical contacts to control the temperature of the application. These devices normally utilize an additional bimetal element to compensate for changes in ambient room temperatures.
Stack Type Devices
This type of adjustable control is used when the control of the temperature is preformed at the same location where the heat is being generated. These devices are normally used in applications requiring higher current loads such as clothes irons and baseboard heaters. These devices consist of multiple "leaf" elements that permit manual temperature adjustment, ambient compensation, and a bimetallic element to provide the work force necessary to open or close a set of electrical contacts to make and break the electrical circuit.
Adjustable Electronic Type Devices
These are probably the most accurate devices available for adjustable temperature applications. However, you normally pay more for this accuracy and the size of these devices is quite large since they require additional electronic circuitry to make them work. Typically, the temperature is measured by one type of electronic circuitry and the actual switching of the load is performed by an electronic relay circuit.
Solid State Devices
How They Work:
There are many different types of controls that fall into the classification of solid state devices. Basically, these devices are activated by increases in temperature and or current, but have no moving parts. One important aspect to consider is that since there are no moving parts to make or break an electrical circuit, then in some cases, the device is never really achieves a true open circuit status. In many instances, the device becomes "non-conductive" under the effects of changes in temperature and will only remain in an “open” state as long as the device remains under load.
The most common types of solid state devices fall into two categories. These are (1) Polymer devices or (2) Ceramic devices. There are also devices called thermistors types of solid state devices are cheaper than other thermal controls, however they do not have the ability to carry current.
Polymer Type Devices
Polymer type devices are basically conductive strips of a plastic like material. They rely on heat generated from increases in temperature or current to generate thermal energy to provide a self heating effect. This results in a change of the polymer matrix increasing in resistance which then causes the device "trip" the electrical load. This device remains at an elevated temperature so it remains in a "tripped" state or until the power is removed. Normally, these can only be used in lower voltage applications and were not designed for continuous-duty applications. Although there have been improvements in their thermal sensing abilities, these devices are not as thermally sensitive as bimetallic controls. Polymer devices rely on the expansion capability of the polymer matrix and are susceptible to problems due to improper installation. Also, once these devices are exposed and function under certain fault conditions their operating characteristics may change.
Ceramic Type Devices
It is difficult to describe all the variations of ceramic type controls. These controls range from devices which only measure or sense temperature increases or decreases in limited area of an application to that of providing a self limiting heat source.
There are many variations of these temperature measuring devices. These are generally categorized as PTC’s (Positive Temperature Coefficient) or NTC’s (Negative Temperature Coefficient). These are again used for temperature sensing applications and require additional circuitry to actually open and close an electrical circuit. These devices on generally used on low voltage DC circuits and are very accurate in their measurement capabilities.
Although some solid state controls can sense temperature, additional logic is required to do something about it. For example, a solid state device in a laptop computer can sense heat, but needs additional circuitry to tell the cooling fan to do something about it.
Other types of ceramic devices actually are used to provide a heat source for an application as well as provide for a temperature limiting function. Although the first generation of these devices was used for low wattage applications there have been advancements to permit their use on higher wattage applications. These can be used in continuous duty applications and include glue guns, curling irons, and small portable space heaters.
These single operation devices work primarily as a temperature sensitive fuse. Due to their compact size and low cost they can be used as a primary protection device in applications where there is little thermal inertia experienced during normal operation conditions. Thermal fuses are also valuable as a cost effective added safety back-up to a primary operating control.
The construction of the most common form of thermal fuses basically uses a contact spring encapsulated into a wax pellet. The pellet is formulated to melt at a predetermined maximum temperature. As the wax melts the spring stretches until it breaks the circuit in the process.
Other types of thermal fuses incorporate the use of a specially formulated solder that effectively melts when exposed to given maximum temperature. These devices are non-resettable and react primarily to changes in temperature. There is some self-heating effect for these devices under higher current loads which may cause the operating characteristics to change over time.
Thermal fuses are also referred to as thermal links, thermal cut-outs (TCO's), or one shots.
In many applications, these devices are commonly used to back up primary devices as required by safety agencies. Thermal fuses are effective for totally shutting down a circuit when a catastrophic failure occurs. If your application demands repeatable operation at or near the maximum operating temperature for the materials you have selected, you may want to consider using a resettable type device. Basically if you choose to use a thermal fuse as a primary operating control, then you must consider that your application will become a disposable item if the thermal fuse functions. Since thermal fuses can not be reused after functioning, it is not possible to test them before or after installation.
Even though there are limitations to how these devices perform, there are very versatile in how they can be custom tailored to meet the individual needs of a customer's application. You can choose the operating temperature range, the length of the lead wires, add terminals, add insulating sleeving, or even custom bend the standard lead wires to make your own termination points.