The **MF-RX135/72** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. It is commonly used in electronic circuits to protect against overcurrent and short-circuit conditions. --- ### 1. Key Technical Specifications The following table outlines the critical electrical parameters for the MF-RX135/72: | Parameter | Value | Description | | :--- | :--- | :--- | | **$I_{hold}$** | 1.35 A | The maximum current the device can hold without tripping at 23°C. | | **$I_{trip}$** | 2.70 A | The minimum current at which the device is guaranteed to trip. | | **$V_{max}$** | 72 V | The maximum voltage the device can withstand without damage. | | **$I_{max}$** | 40 A | The maximum fault current the device can safely handle. | | **$P_d$ (Typ.)** | 1.7 W | Power dissipated by the device when in a tripped state. | | **$R_{min}$ / $R_{max}$** | 0.12 / 0.30 $\Omega$ | Initial resistance range before tripping. | --- ### 2. Physical and Design Characteristics * **Package Type:** Radial Leaded (Through-hole). * **Material:** Conductive polymer integrated between two metal electrodes. * **Encapsulation:** Cured, flame-retardant epoxy polymer (meeting UL 94V-0 requirements). * **Marking:** Usually marked with "MF-RX", "135", and a date code. --- ### 3. Functional Mechanism The MF-RX135/72 operates as a **non-linear thermistor**: 1. **Normal State:** The polymer is in a crystalline state, and conductive carbon particles form a low-resistance path, allowing current to flow. 2. **Fault State (Overcurrent):** When current exceeds the trip point, $I^2R$ heating causes the polymer to expand and transition into an amorphous state. 3. **Tripped State:** The expansion breaks the carbon chains, increasing resistance by several orders of magnitude, effectively "opening" the circuit (leaving only a small leakage current). 4. **Reset:** Once the fault is removed and the device cools down, the polymer contracts, the carbon chains reconnect, and the fuse resets to a low-resistance state. --- ### 4. Common Applications * **Power Supplies:** Protection for DC power ports and transformers. * **Industrial Controls:** Safeguarding PLCs and sensor interfaces. * **Security Systems:** Protection for alarm panels and cameras. * **Automotive:** Used in non-critical electronic modules and motors. ---