The **MF-RX300** is a specific model of a **Polymeric Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. It is part of the Multifuse® family and is designed to protect electronic circuits from overcurrent and over-temperature conditions. --- ### 1. Key Technical Specifications | Parameter | Specification | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 3.00 Amps | The maximum current the device will pass without tripping. | | **Trip Current ($I_{trip}$)** | 6.00 Amps | The minimum current that will cause the device to "trip" (open). | | **Max Voltage ($V_{max}$)** | 60 Volts | The maximum voltage the device can withstand without damage. | | **Max Current ($I_{max}$)** | 40 Amps | The maximum fault current the device can safely interrupt. | | **Typical Power ($P_d$)** | 2.8 Watts | Power dissipated when the device is in its tripped state. | | **Mounting Type** | Radial Leaded | Through-hole mounting with two wire leads. | --- ### 2. Physical and Material Characteristics * **Material:** It is made of a conductive plastic (polymer) mixed with carbon black particles. * **Form Factor:** It usually appears as a yellow or tan circular/rectangular disc with two metal leads, often coated in a flame-retardant epoxy. * **Encapsulation:** Cured, flame retardant epoxy polymer (meets UL 94V-0 requirements). --- ### 3. How it Functions (The PTC Effect) The MF-RX300 operates on a non-linear resistance principle: 1. **Normal State:** Under normal current ($I_{hold}$), the polymer is in a crystalline state, and the carbon particles form conductive paths. The resistance is very low. 2. **Fault State:** If the current exceeds $I_{trip}$ (due to a short circuit or overload), $I^2R$ heating causes the polymer to expand. 3. **High Resistance State:** The expansion breaks the carbon chains. The resistance increases exponentially, reducing the current flow to a trickle. 4. **Resetting:** Once the fault is removed and the power is cycled or the device cools down, the polymer shrinks, re-establishing the conductive paths. --- ### 4. Common Applications The MF-RX300 is widely used in industrial and consumer electronics due to its 60V rating: * **Power Supplies:** Protection against secondary side shorts. * **Industrial Controls:** Protecting PLCs and sensor loops. * **Motor Protection:** Guarding against "stalled motor" conditions. * **Security Systems:** Safeguarding alarm panels and sirens. ---