The **MF-R90** is a specific model of **PTC (Positive Temperature Coefficient) Resettable Fuse**, manufactured by Bourns. These components are widely used in electronics for overcurrent protection. --- ## 1. Technical Specifications The MF-R90 belongs to the MF-R series (radial-leaded) and is designed to "trip" (increase resistance) when current exceeds its limits. | Parameter | Value / Specification | | :--- | :--- | | **Hold Current ($I_{hold}$)** | 0.90 Amperes (900 mA) | | **Trip Current ($I_{trip}$)** | 1.80 Amperes | | **Max Voltage ($V_{max}$)** | 60V (DC) | | **Max Current ($I_{max}$)** | 40 Amperes | | **Power Dissipation ($P_d$)** | 0.91 Watts | | **Resistance ($R_{min} / R1_{max}$)** | 0.20 $\Omega$ to 0.47 $\Omega$ | | **Termination Style** | Radial Leaded (Through-hole) | --- ## 2. How it Works (Theory) The MF-R90 is made of a conductive polymer. Under normal operating conditions, the carbon chains within the polymer form a low-resistance path, allowing current to flow. 1. **Overcurrent Event:** If the current exceeds $I_{trip}$ (1.8A), the component begins to heat up due to $I^2R$ losses. 2. **Phase Change:** Once it reaches a specific temperature, the polymer expands, breaking the conductive chains. 3. **High Resistance State:** The device enters a high-resistance state, effectively "opening" the circuit and protecting downstream components. 4. **Reset:** Once the fault is removed and the power is cycled/cooled, the polymer shrinks, the chains reconnect, and the fuse resets to a low-resistance state. --- ## 3. Key Applications Because the MF-R90 does not need to be replaced after a fault (unlike standard glass fuses), it is ideal for: * **Power Supplies:** Protecting against output short circuits. * **Industrial Controls:** Protecting sensor loops and PLC inputs. * **Computer Peripherals:** USB ports or IEEE 1394 (FireWire) protection. * **Automotive Electronics:** Protection for small motors and interior lighting circuits. --- ## 4. Typical Connection Diagram The MF-R90 is always placed in **series** with the load you wish to protect. ```text [DC Power Source (+)] ---- [MF-R90 Fuse] ---- [Load/Device] ---- [GND] ``` ---