For low voltage power inputs, e.g. when using an external AC adapter or say a vehicle low DC voltage supply, electrical threats include lightning surges, ESD and external equipment switching spikes.
A fuse or resettable PTC can be used for short circuit and overload current conditions.
For surge and ESD protection, an MOV or TVS diode can be used.
The magnitude of the surge transient needs to be understood. If more severe surges are expected then C-III or UltraMOV varistors or higher-rated diodes (e.g. 600W, 1,500W) need to be selected. If the only threat is expected to be ESD, then multilayer chip varistors can be selected.
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