### Overview of the MF-R055/90 (Multifuse®) The **MF-R055/90** is a **Polymer Positive Temperature Coefficient (PPTC)** resettable fuse, manufactured by Bourns. These devices are commonly used for overcurrent protection in electronic circuits where a "self-resetting" feature is preferred over traditional one-time fuses. --- ### Key Technical Specifications | Parameter | Value | Description | | :--- | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.55 A | The maximum current the device can carry without tripping. | | **Trip Current ($I_{trip}$)** | 1.10 A | The minimum current at which the device will switch to high resistance. | | **Max Voltage ($V_{max}$)** | 90 V | The maximum voltage the device can withstand without damage. | | **Max Current ($I_{max}$)** | 40 A | The maximum fault current the device can safely handle. | | **Power Dissipation** | 0.9 W | Typical power dissipated when in the tripped state. | | **Form Factor** | Radial Leaded | Through-hole mounting style. | --- ### How it Works The MF-R055/90 operates based on the thermal properties of its conductive polymer compound: 1. **Normal State:** At room temperature, the polymer is in a crystalline state with conductive carbon particles forming paths, allowing current to flow with low resistance. 2. **Fault Condition:** When an overcurrent or short-circuit occurs, the $I^2R$ heating causes the polymer to reach its phase transition temperature. 3. **Tripped State:** The polymer expands and becomes amorphous, breaking the carbon chains. The resistance increases dramatically, limiting the current to a safe leakage level. 4. **Reset:** Once the fault is removed and the device cools down, the polymer contracts, the carbon paths reconnect, and the device returns to a low-resistance state. --- ### Common Applications * **Security Systems:** Protecting sensors and control panels. * **Industrial Controls:** Overcurrent protection for PLC I/O ports. * **Power Supplies:** Protecting against short circuits in transformers or DC outputs. * **Medical Electronics:** Used in equipment requiring high reliability and resettable protection. --- ### Comparison: PPTC vs. Standard Fuse | Feature | PPTC (MF-R055/90) | Standard Fuse | | :--- | :--- | :--- | | **Resettability** | Automatic (after cooling) | Permanent (must be replaced) | | **Resistance** | Moderate | Very Low | | **Reaction Time** | Slower (Thermal) | Fast (Melting) | | **Voltage Drop** | Higher | Minimal |