It is more and more necessary to protect
equipments against over temperature effects.
This is particularly true for electrical
engines, transformers, power circuits.
Thermal protections are realised either
with a system of detection and measurement
of temperature with NTC or PTC thermistors
(see these chapters), or with a thermal
switch which opens the circuit to protect,
or for high powers a relay, or closes a
contact on a fan to refresh the equipment.
Thermal switch (frequently named, per mistake,
thermostat) is a bi metallic element which
operates with the metals dilatation : two
different dilatation coefficients (e.g. copper
and steel) are put together either by bonding
or electrolytic deposit, then cutted out in blades
or in discs stamped as a bowl.
When the temperature increases, copper which
expands more than steel, make a deformation of
the blade which bends or the discs which reverses,
this allows to open or close a circuit.
These switches are protected with a housing.
Several shapes are available and they can switched
off intensities from 1 to 25 Amp.
When the temperature decreases, the blade or disc
will recover its initial shape and re-establish
An important differential is necessary to avoid a
too fast re-contacting and allows the equipment to
cool down before a re-starting.
The temperature imiting switch is a thermal switch
which needs a manual operation to re-start.