The **MF-R900** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. These components are widely used for overcurrent protection in electronic circuits. --- ## Technical Specifications: MF-R900 | Parameter | Specification | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 9.0 Amperes | | **Trip Current ($I_{trip}$)** | 18.0 Amperes | | **Max Voltage ($V_{max}$)** | 30 Volts (DC or AC) | | **Max Current ($I_{max}$)** | 100 Amperes | | **Power Dissipation ($P_d$)** | 4.5 Watts | | **Packaging Type** | Radial Leaded (Through-hole) | --- ## Key Operating Principles ### 1. How it Works Unlike a standard one-time fuse that blows and must be replaced, the MF-R900 is **resettable**. * **Normal State:** The conductive particles inside the polymer form a low-resistance path, allowing current to flow to the load. * **Tripped State:** When a fault (short circuit or overcurrent) occurs, the internal temperature rises. The polymer expands, breaking the conductive chains. The resistance increases dramatically, reducing the current to a safe leakage level. * **Reset State:** Once the power is removed and the fault is cleared, the device cools down, the polymer contracts, and the conductive paths are restored. ### 2. Physical Characteristics * **Material:** Conductive polymer. * **Coating:** Flame retardant epoxy polymer (meets UL 94V-0 requirements). * **Marking:** Usually labeled with "R900" to indicate the 9.0A hold current rating. --- ## Applications The MF-R900 is commonly used in high-current environments where space allows for radial leaded components: 1. **Motor Protection:** Protection against stalls or overheating in small DC motors. 2. **Power Supplies:** Secondary side protection for transformers and switching power supplies. 3. **Battery Packs:** Protection against external short circuits in high-capacity battery systems. 4. **Industrial Controls:** Protecting I/O ports and sensitive controllers. --- ## Performance Considerations ### Temperature Derating The $I_{hold}$ rating of 9.0A is typically measured at **23°C**. As the ambient temperature increases, the amount of current the device can carry without tripping decreases. ### Time-to-Trip The MF-R900 is not instantaneous. At its rated trip current (18A), it may take several seconds to "trip." The higher the overcurrent, the faster the device will react.