## Detailed Overview of the MF-RX250 The **MF-RX250** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. It is commonly referred to as a "multifuse" or "polyfuse." --- ### 1. Key Technical Specifications The following table summarizes the primary electrical characteristics of the MF-RX250: | Parameter | Specification | Description | | :--- | :--- | :--- | | **Ihold** | 2.50 A | Maximum current the device will hold without tripping at 23°C. | | **Itrip** | 5.00 A | Minimum current at which the device will always trip at 23°C. | | **Vmax** | 60 V | Maximum voltage the device can withstand without damage. | | **Imax** | 40 A | Maximum fault current the device can safely handle. | | **Pd (Typ.)** | 2.5 W | Power dissipated from the device when in tripped state. | | **Rmin / R1max** | 0.05 / 0.13 $\Omega$ | Minimum resistance (initial) and maximum resistance (1hr post-trip). | --- ### 2. Functional Principles The MF-RX250 operates based on the thermal properties of its conductive polymer composite: 1. **Normal State:** Under normal operating currents, the polymer is in a crystalline state, and conductive carbon particles form a low-resistance path. 2. **Tripped State:** When an overcurrent or short circuit occurs, the $I^2R$ heating causes the polymer to expand and become amorphous. This breaks the conductive chains, causing the resistance to increase exponentially. 3. **Resetting:** Once the fault is removed and the device cools down, the polymer shrinks, the conductive paths are restored, and the fuse "resets" itself to a low-resistance state. --- ### 3. Physical Attributes & Package * **Form Factor:** Radial Leaded (Through-hole). * **Material:** Cured, flame-retardant epoxy polymer (meeting UL 94V-0 requirements). * **Lead Material:** Tin-plated copper. * **Marking:** Typically marked with the Bourns logo, "RX250," and a date code. --- ### 4. Common Applications Due to its 60V rating and 2.5A holding current, the MF-RX250 is widely used in: * **Power Supplies:** Protection against short circuits in DC power rails. * **Industrial Controls:** Protecting PLCs and sensors from wiring errors. * **Motors:** Protection against stall conditions or overloads in small motors. * **IEEE 1394/USB:** Overcurrent protection for high-speed ports. ---