The **MF-RX020/72** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. These components are commonly referred to as "multifuses" or "polyfuses." --- ### 1. Component Overview The MF-RX series is designed to provide overcurrent protection for electrical and electronic circuits. Unlike standard fuses that blow and must be replaced, these devices "trip" into a high-resistance state during a fault and reset once the power is removed and the fault is cleared. ### 2. Key Technical Specifications | Parameter | Value | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.20 A | The maximum current the device will pass without tripping. | | **Trip Current ($I_{trip}$)** | 0.40 A | The minimum current at which the device is guaranteed to trip. | | **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 interrupt. | | **Power Dissipation** | 0.41 W | Typical power dissipated when in the tripped state. | | **Form Factor** | Through-Hole | Radial leaded package (Circular/Disc shape). | --- ### 3. Functional Mechanism The device operates based on the thermal properties of its conductive polymer matrix: 1. **Normal Operation:** At rated currents, the carbon granules within the polymer form a conductive path, offering low resistance. 2. **Fault Condition:** When an overcurrent occurs, $I^2R$ heating causes the polymer to expand. 3. **Tripped State:** The expansion breaks the carbon chains, increasing resistance significantly and reducing current flow to a "trickle." 4. **Reset:** Once the power is disconnected and the device cools down, the polymer contracts, the carbon chains reconnect, and the device returns to a low-resistance state. ### 4. Part Number Breakdown: `MF-RX020/72` * **MF:** Bourns Multifuse Product Line. * **RX:** Series designation (Radial Leaded, Through-Hole). * **020:** Hold Current rating (0.20 Amps). * **/72:** Maximum Voltage rating (72 Volts). --- ### 5. Common Applications * **Security Systems:** Protecting sensors and control panels. * **Industrial Controls:** Overcurrent protection for low-voltage motors and actuators. * **Power Supplies:** Short-circuit protection for DC output ports. * **Telecommunications:** Protection against power cross and lightning surges.