The **MF-RX012/250-F** is a specific model of Polymer Positive Temperature Coefficient (PPTC) resettable fuse manufactured by **Bourns**. These components are commonly referred to as "resettable fuses" or "polyfuses." --- ## 1. Device Overview This device is designed to protect electrical circuits from overcurrent and overtemperature faults. Unlike a traditional fuse that blows and must be replaced, the PPTC "trips" into a high-resistance state and resets once the fault is removed and the power is cycled. ### Key Specifications | Parameter | Value | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.12 A (120 mA) | | **Trip Current ($I_{trip}$)** | 0.24 A (240 mA) | | **Rated Voltage ($V_{max}$)** | 60V (Continuous) / 250V (Intermittent/Interrupt) | | **Max Current ($I_{max}$)** | 3.0 A | | **Resistance ($R_{min}$ / $R_{max}$)** | 6.0 $\Omega$ / 12.0 $\Omega$ | | **Packaging** | Radial Leaded (Through-hole) | --- ## 2. Functional Characteristics The "MF-RX" series is specifically engineered for high-voltage applications, particularly in telecommunications. ### Resettable Mechanism 1. **Normal State:** The polymer contains conductive carbon black particles forming a path. Resistance is low. 2. **Fault State:** Excessive current causes $I^2R$ heating. The polymer expands, breaking the conductive paths. Resistance spikes, limiting current to a safe leakage level. 3. **Reset State:** Once the fault is cleared and the device cools down, the polymer contracts, the paths reconnect, and the device returns to a low-resistance state. ### Part Number Breakdown * **MF-RX:** Product series (Radial Leaded Resettable Fuses). * **012:** Hold current rating (0.12 Amps). * **/250:** Maximum voltage rating ($V_{rms}$ or $V_{dc}$ interrupt). * **-F:** RoHS compliant / Lead-free. --- ## 3. Applications The MF-RX012/250-F is frequently used in environments where high-voltage surges (like lightning or power cross-overs) might occur: * **Telecommunications:** Customer Premise Equipment (CPE), ADSL/VDSL line cards. * **Industrial Controls:** Protection for sensors and low-power control circuits. * **Meters:** Utility billing meters where protection against 250V mains contact is required. --- ## 4. Performance Considerations * **Ambient Temperature:** PPTC devices are thermal-sensitive. As the ambient temperature rises, the hold current ($I_{hold}$) decreases. * **Time to Trip:** At 5x the hold current, this specific device typically trips in approximately **1.0 to 2.0 seconds**.