What is the impact of different types of loads on the selection of resettable fuses (PTCs)?

Power Circuit Protection
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Different types of loads do have a certain impact on the selection of PTCs (resettable fuses). This is mainly reflected in the following aspects:

  1. Inductive loads: Inductive loads will generate a large inductive surge current when starting, which may cause the PTC to enter a high resistance state instantly. Therefore, when selecting a PTC, it is necessary to consider that its maximum trip current (Itrip) should be greater than the starting current of the inductive load. Inductive loads will generate high voltage spikes when turning off, which may damage the PTC. It is necessary to connect a suppression diode or RC absorption circuit between the PTC and the load to suppress the high voltage spike.
  2. Capacitive loads: Capacitive loads will generate a large surge current when starting, and the maximum trip current (Itrip) needs to be considered when selecting a PTC. The operating current of a capacitive load may be close to the holding current (Ihold) of the PTC. It is necessary to select a PTC with an Ihold value slightly higher than the working current of the capacitive load to avoid the PTC entering a high resistance state during normal operation.
  3. Common considerations:
  • The action time (Ttrip) of the PTC should be less than the short-circuit time of the load so that the fault current can be cut off in time.
  • The working voltage (Vmax) of the PTC should be greater than the working voltage of the load.
  • The rated current of the PTC should be greater than the normal working current of the load to avoid the PTC entering a high resistance state during normal operation.
  • The working ambient temperature of the PTC should cover the working temperature range of the load.
  • The package form of the PTC should be consistent with the installation space of the load.

In summary, when selecting a PTC, it is necessary to fully consider the type and characteristics of the load and select appropriate parameters to ensure that the PTC can effectively protect the load and will not affect the normal operation of the load.

In addition, what is the principle of the PTC self-resetting mechanism?

PTC uses the positive temperature coefficient effect of its material and the dynamic balance of heat to achieve overcurrent protection. When the current is too large, the PTC quickly enters a high resistance state, limiting the current flow and thus protecting the circuit. This self-resetting mechanism allows the PTC to remain effective in multiple overcurrent events, providing continuous protection for electronic equipment.