Opening/closing behaviour of bimetal thermal protectors may be significantly strengthened by including an additional, flexible disk. In addition to higher electricity bearing capacity, thermal protectors with such apparatus also face less contact abrasion with longer life cycle.
In Figure 2, function and opening/closing behaviour of a sample thermal protector with additional flexible disk (yellow) is illustrated as example. Figure 2-a displays the situation at room temperature; the flexible disk presses with full force. In Figure 2-b, a protector that is close to the opening/closing temperature is shown; the flexible disk still continues pressure at full force without any prevention. In Figure 2-c, the contact (red) of bimetal is opened, the flexible disk is held under by the bimetal and it presses on the bimetal only with a slight force. At the example in the figure, a flexible disk with defined nonlinear force curvature is shown. This construction requires the bimetal disk force to be higher than the counter force of the flexible disk at snap change.

Figure 2: Comparison of the typical behaviours of thermal protectors sensitive against high current with current-heated : current-bearing bimetal disk (a) and thermal protectors without bimetal disk (b)

Figure 4: Force – Distance Curve (characteristic curve):
a simple disk (linear curve)
b additional flexible disk (nonlinear curve)

Continuous running bimetal disk thermal protector

As bimetal disks run, they age depending on the load suffered and the time of use, and thus their functioning parameters change. This aging cannot be prevented completely. Although it is not possible to reduce the thermal load of the bimetal disk due to application, its parametric stability can be enhanced by reducing the mechanical stress. As is known, continuously running bimetal disks (Figure 3) suffer lower mechanical stress compared to non-continuous disks (snap disks).

The following advantages are provided by the continuous bimetal disk systems:

  • High elastic power with small spring oscillation
  • Higher operation capacity
  • Higher contact stability / higher contact pressure
  • Better area use due to round structure shape (miniaturised)
  • Simple geometry: high mechanic strength
  • Low cost production (non-complex sets)
  • Longer life cycle
  • Longer stability period
  • Lower contact resistance
  • More sensitive opening/closing point
  • No early opening/closing
  • Opening/closing sensitivity based on environmental conditions

Figure 3: Automatic reset, temperature-sensitive opening type thermal protectors, additional flexible snap disk (with non-linear curve) and