## MF-RX250/72 Overview The **MF-RX250/72** is a radial leaded **PPTC (Polymeric Positive Temperature Coefficient)** resettable fuse manufactured by Bourns. It is commonly used for overcurrent protection in circuits where traditional fuses are impractical due to the need for manual replacement. --- ### 1. Key Technical Specifications | Parameter | Value | Description | | :--- | :--- | :--- | | **$I_{hold}$** | 2.50 A | Maximum current at which the device will not trip (at 23°C). | | **$I_{trip}$** | 5.00 A | Minimum current at which the device is guaranteed to trip. | | **$V_{max}$** | 72 V | Maximum voltage the device can withstand without damage. | | **$I_{max}$** | 40 A | Maximum fault current the device can safely interrupt. | | **$P_d$** | 2.5 W | Power dissipated when the device is in a tripped state. | | **Temperature Range** | -40°C to +85°C | The operating environment limits. | --- ### 2. How It Works The MF-RX250 acts as a "resettable fuse" using the following mechanism: 1. **Normal State:** The device has a low internal resistance, allowing current to pass through to the load with minimal interference. 2. **Overcurrent Event:** If current exceeds the trip threshold ($I_{trip}$), internal heating (Joule heating) causes the polymer matrix to expand. 3. **Tripped State:** The expansion breaks the conductive paths within the polymer, causing the resistance to increase exponentially. This effectively "opens" the circuit, limiting current to a leakage level. 4. **Reset:** Once the fault is removed and the power is cycled (allowing the device to cool), the polymer shrinks, re-establishing conductive paths and returning to low resistance. --- ### 3. Physical Characteristics * **Form Factor:** Radial leaded (through-hole). * **Material:** Cured, flame-retardant epoxy polymer (meeting UL 94V-0). * **Lead Material:** Tin-plated copper. * **Marking:** Typically marked with the Bourns logo, "RX250", and a date code. --- ### 4. Typical Applications Because of its 72V rating and 2.5A holding current, this part is ideal for: * **Security Systems:** Protecting sensors and control panels. * **Industrial Controls:** Safeguarding PLC I/O ports. * **Power Supplies:** Protection against short circuits in DC power rails. * **Telecommunications:** Protection against power cross and lightning surges. ---