The **MF-RX065/72** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. These components are commonly used for overcurrent protection in electronic circuits. --- ### 1. General Specifications The part number can be decoded to understand its primary characteristics. The "065" refers to the hold current, and the "72" refers to the maximum voltage rating. | Parameter | Value | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.65 A | The maximum current the device will pass without tripping at 23°C. | | **Trip Current ($I_{trip}$)** | 1.30 A | The minimum current at which the device will switch to a high-resistance state. | | **Max Voltage ($V_{max}$)** | 72 V | The maximum voltage the device can withstand without damage. | | **Max Current ($I_{max}$)** | 40 A | The maximum fault current the device can safely handle. | | **Typical Power ($P_d$)** | 0.72 W | Power dissipated by the device when in a tripped state. | --- ### 2. Physical and Material Characteristics The MF-RX series is characterized by its "radial lead" package, which is designed for through-hole mounting on Printed Circuit Boards (PCBs). * **Material:** Conductive polymer combined with carbon black particles. * **Form Factor:** Circular or rectangular "teardrop" shape with two metal leads. * **Encapsulation:** Epoxy polymer (Flame retardant to UL 94V-0). --- ### 3. How it Works (Theory of Operation) Unlike a standard one-time fuse, the MF-RX065/72 is **resettable**. 1. **Normal State:** Under normal current levels, the polymer is in a crystalline state, and the carbon particles form conductive paths. Resistance is low. 2. **Trip State:** When an overcurrent occurs, the $I^2R$ heating causes the polymer to expand and turn into an amorphous state. This breaks the carbon paths, causing resistance to spike instantly. 3. **Reset State:** Once the power is removed and the fault is cleared, the device cools down, the polymer shrinks, and the conductive paths are restored. --- ### 4. Typical Applications Because of its 72V rating, this part is ideal for applications where higher-than-logic voltages are present: * **Security Systems:** Protecting sensors and control panels. * **Industrial Controls:** Safeguarding PLC I/O ports. * **Telecommunications:** Protection against power cross-contingencies. * **Power Supplies:** Secondary protection for transformers and DC adapters. --- ### 5. Resistance Range The resistance of the component changes based on its history and temperature: ```text Rmin: 0.31 Ohms (Minimum resistance at 23°C) Rmax: 0.72 Ohms (Maximum initial resistance at 23°C) R1max: 1.15 Ohms (Maximum resistance one hour after tripping) ```