# Selection of Fuses

## General Selection

• Voltage Rating (Maximum Voltage): Should be greater than the supply voltage with an appropriate margin.
• Rated Current: Should be greater than the operating current under normal conditions (rated current is not the same as the fuse current rating).
• Operating Temperature: Allow for a 25% margin.
• Voltage Drop / Cold Resistance: Generally, lower values are preferable (minimizes power loss).
• Fuse Characteristics: The most critical electrical parameter of a fuse. It indicates that as the current exceeds a certain level, the fuse's temperature gradually rises until it eventually melts, breaking the circuit. Based on their characteristics, fuses can be categorized as fast-acting and time-delay types. Time-delay fuses are commonly used in circuits with significant inrush currents during changes in circuit states, such as inductive or capacitive circuits. They can withstand the impact of surges during power cycling. Fast-acting fuses are often used in resistive circuits to protect sensitive components from current variations.
• Breaking Capacity: Represents the maximum current a fuse can safely interrupt under specified voltage conditions.

## PPTC Selection

Polymeric Positive Temperature Coefficient (PPTC) fuses, also known as self-resetting fuses, have low resistance when operating normally, resulting in minimal voltage drop. When an overcurrent situation occurs, causing an increase in temperature, the resistance of the PPTC fuse rises significantly, reducing the current in the circuit to a safe level. This protects the downstream circuit, and once the overcurrent condition is resolved, the PPTC fuse automatically resets to its low-resistance state, eliminating the need for frequent fuse replacements.

Parameters:

• Maximum Voltage $$V_{max}$$ (Rated Voltage): The maximum voltage that can be withstood at the rated current.
• Maximum Current $$I_{max}$$ (Maximum Current): The maximum current that can be withstood at the rated voltage.
• Hold Current $$I_{hold}$$ (Hold Current): The maximum current at which the fuse remains in its untripped state, without experiencing a significant change in resistance.
• Trip Current $$I_{trip}$$ (Trip Current): The minimum current required to initiate the fuse's tripping action, typically twice the hold current.
• Power Dissipation $$P_d$$ (Typical Power): The power consumption when the fuse is in the tripped state.
• Maximum Trip Time $$T_{trip}$$ (Max Time to Trip): The maximum time it takes for the fuse to trip under specified current conditions.
• Static Resistance $$R_{i_{min/max}}$$ (Resistance Tolerance): Initial resistance value before soldering.
• Resistance After One Hour of Soldering or Tripping $$R_{1_{max}}$$: The maximum resistance value after one hour of soldering or tripping.

## References and Acknowledgments

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