Current Sensitivity in Bimetal Thermal Controls What You Should Know

A major advantage enjoyed by conductive thermal controls is the ability to easily tailor them to meet your particular needs. They are extremely versatile. Start with the bimetallic element at the heart of these devices. There are literally hundreds of different types of bimetal currently available. At Portage Electric Products we have used dozens of these different bimetals to meet the various sensitivity needs of many different applications. This versatility helps conductive bimetallic devices enhance safety and prevent nuisance tripping in a wide range of applications. It even allows you to increase the ability of a thermal control to function like a circuit breaker rather than strictly functioning as a temperature control.

The major difference between bimetals lies in the internal resistance of each given type. Under an electrical load, each type of bimetal will then exhibit a different internal resistive level. This is commonly referred to as the resistivity of the bimetal. The higher the internal resistance, or resistivity of a bimetal, the more heat will be generated by the effects of the current passing through. The more heat generated, the quicker the bimetal will deflect or bend. The self-heating effect of the bimetal provides the reaction needed to function in applications where a thermal control must sense increases in either temperature and or current.

Bimetal resistivity is similar to the way water moves through a small diameter hose as opposed to a large diameter hose. The smaller hose presents greater resistance to the water's movement causing water to spurt out at a greater rate. If you use a larger diameter hose the water will face less resistance and will move through more slowly. The higher the resistivity, the faster the reaction.

The resistivity of any particular bimetal is expressed in ohms per circular mill foot (ohms cm/ft) and determines how quickly a change in temperature will make the bimetal deflect thereby breaking a circuit. The flexivity of a bimetal is the amount it bends at a certain temperature. Resistivity determines the amount of self-heat generation and aids to the speed with which a bimetal will deflect. Flexivity determines how much a bimetal will deflect under the influences of heat generation. This heat can be what is generated through the effects of heat generated by the application or through the self heating effect of current passing through the bimetal.

The other way to alter the sensitivity of a conductive bimetallic device is to vary the composition of the materials used in any other current carrying parts found in the thermal control. For example, the resistivity of the metal used in the manufacture of a support arm used to support a set of electrical contacts or used to in conjunction with a bimetallic element, can also be increased or decreased, thereby altering the internal resistivity of the device.

Improving Safety

Increasing the current sensitivity of a device will enhance the safety characteristics. The quicker a device senses a change in current, the quicker it will shut down the circuit to prevent a catastrophic occurrence. In cases where you want to alter the sensitivity of a device in order to improve safety you don't want a device to react normally. You want it to sense the danger and shut down before a serious problem occurs.

Avoiding Nuisance Tripping

On the other hand, you don't want a thermal control to be so sensitive that it activates prematurely. There has to be a balance between too much resistivity and too little. In every case the needs of the application will determine the proper balance. A vacuum cleaner motor, for example, will have higher power needs than the blower motor in a furnace. The right thermal control might not be the same for these two motors but Pepi products can be found in both applications. Our experienced Sales Engineers will help in determining the right thermal control for your application.