If a material with increasing temperature the resistance value increases, this will have a positive temperature coefficient. All materials have metal-like conduction properties with respect to their electrical resistance has a positive temperature coefficient (PTC characteristics). With increasing temperature the resistance increases almost linearly.
Materials such as conducting polymers have in a very narrow temperature band a strong increase in resistance, from which the overcurrent protection modules or overcurrent can be produced. In general, these components are referred to as Polymeric PTC (PPTC).
Here is the PPTC for the English name of Polymer Positive Temperature Coefficients. The PPTC's are also known under the trade names PolySwitch or multifuse.
How it works:
Under normal operating conditions, the generated by the block and the effluent to the environment heat keep the scales at a very low temperature. Increases, the current intensity in the block, while the ambient temperature remains constant, in the module, more heat is generated and the temperature increases accordingly. As long as the current is not too large, o a lot of heat to the environment can still be given that the block is stabilized, albeit at a higher temperature.
If instead of a current increase the ambient temperature increases, the block also stabilized at a higher temperature. This could finally also be obtained by an increase of current and temperature. Increase the current and / or the ambient temperature then continues to rise, the device eventually reaches a point at which the resistance increases steeply. Any further increase of the current intensity or ambient temperature then has the consequence that in the block more heat is generated than can be discharged to the environment. A rise in temperature is the result. At this stage, even a slight temperature change leads to a strong increase in resistance. This is to ensure the normal working range of an activated block. As a result of the increase in resistance increases the current decreases accordingly, which helps to protect the circuit from damages.
To select a PPTC (PolySwitch / multifuse) they need
- The current flowing in the normal operating current in amperes (operating current = Ihold)
- The maximum occurring in the event of a fault current in amperes (fault current = Imax)
- The maximum voltage in volts (circuit voltage = Vmax) Usually 30V to 72V.
- The working temperature range.
Please note that with increasing ambient temperature, the current load is smaller. The rated current Ihold applies at a temperature of 23°C.
e.g. we go by the guy from the MF-RX250 at 23°C a Ihold of 2.5 A and an I trip 5A has, so these values are at 50°C ambient temperature only at Ihold 1.8 A and 3.6 A. I trip Further details can be found in the table eg (Thermal Derating Chart) in the corresponding data sheets.
Select the correct PPTC grades on the basis of the thermal derating chart and its maximum operating temperature, the Ihold should be slightly larger or equal to the current flowing in the normal operating current.
Check that the maximum values of Vmax and Imax of the selected product family of them are larger than the maximum voltage and maximum fault current in their application.