IN Brief:
- The MF-ASMF family puts polymer resettable overcurrent protection into a 0402 surface mount footprint.
- Bourns describes the series as compact, low resistance and fast to react to fault events.
- PPTC devices raise resistance as fault heating increases, then reset after power is removed and the component cools.
Bourns has introduced its MF-ASMF series of low resistance polymer positive temperature coefficient resettable fuses in a 0402 surface mount footprint, targeting compact circuits that need automatic overcurrent protection without replacing a conventional fuse after every trip.
The device joins Bourns’ Multifuse portfolio as a compact protection component intended to react to fault heating while occupying little PCB area. Its 0402 footprint allows protection to sit close to a connector, battery-fed rail or sensitive load where a larger fuse assembly would consume disproportionate board space.
A PPTC element remains at relatively low resistance under normal current. When excessive current produces sufficient self-heating, the polymer composite changes state and its resistance rises sharply, reducing the fault current rather than physically opening the conductor.
Once power is removed and the device cools, its resistance falls again and the circuit can be returned to service without replacing the component. That behaviour suits equipment where temporary shorts, connector faults or user-induced overloads should not necessarily require maintenance.
The trip process is thermal, so current alone does not determine response time. Ambient temperature, copper area, airflow and overload duration all influence how quickly the device heats towards its transition point. Selection therefore has to account for normal load peaks and start-up events as well as the fault current that should cause the device to trip.
Low initial resistance limits the voltage drop and I²R loss created by placing the protection device in series with the load. That becomes particularly important on low-voltage rails where the available operating margin may already be small.
The 0402 package also makes normal operating temperature more sensitive to PCB layout and nearby heat sources. Placing the PPTC close to a regulator, processor or charging IC can raise its thermal starting point and reduce the current it can carry before entering the high resistance state.
Current curves and temperature derating therefore remain central to device selection. A part chosen only by nominal current can trip during benign transients or, at the opposite extreme, allow too much fault energy to pass before its resistance rises.
The tripped state also differs from an ideal open circuit. A PPTC substantially increases resistance but continues to pass a small current, so the protected system has to tolerate the resulting voltage and residual power condition. Applications requiring complete galvanic interruption may still need a conventional fuse or another protection method.
Bourns describes the MF-ASMF family as compact, low profile and fast to react to fault events. Its addition broadens the company’s 0402 Multifuse options where protection has to occupy the same class of PCB area normally associated with much smaller passive components.
The series entered Bourns’ product register on 1 October 2026. Its defining change is physical integration rather than a new protection principle, placing resettable overcurrent protection into a format suited to increasingly dense low-voltage electronics.
