Different Types of Fuses & Circuit Protection Devices

Diodes: Basic Circuit Protection

Diodes are the most basic form of circuit protection. In their purest form, diodes are unidirectional devices that have low resistance to current flow in one direction and very high resistance to current flow in the other direction. This structure means they can limit the direction of current flow based on the component's orientation. Protection circuits use diodes to prevent current from flowing in an improper direction, which could cause damage to sensitive components.

There are a large variety of different diode technologies at work outside of protection circuits, such as:

• Transient voltage suppressors (TVS)
• ESD suppressors
• Schottky
• Vacuum tubes

Depending on their specific technology, some diodes can operate in voltage ranges from 0.1V to 100kV with varying current capacities. You can also learn more about the different types of diodes and when to use them.

Overvoltage Protection: 3 Types of Circuit Protection Devices

As the name implies, overvoltage circuit protection protects the circuit if the source voltage exceeds the intended amount. You can combat transient voltage spikes with several devices:

• TVS diodes: Transient Voltage Suppressors (TVS), also called transils or thyrectors, are devices used in circuits to protect surge-prone devices from sudden overvoltage conditions, or transients. External influences such as lightning, ESD, or motor arcing commonly cause these voltage spikes. Where these hazards exist, TVS diodes may be necessary to avoid damage to sensitive circuits. TVS diodes, which are sometimes also called zener diodes, can have protection over 1V to 15kV with a response time of one picosecond.
• ESD suppressors: ESD diodes protect devices from electrostatic discharge, which can be detrimental to sensitive IC equipment. When an ESD strike occurs, ESD diodes create a low impedance path to divert current away from the IC and to ground. The term "ESD suppressor" describes the overvoltage protection (likely a TVS diode) device being utilized during an ESD event.
• Varistors: Varistors also protect circuits from transient voltage spikes. However, varistors have varying resistance that changes with respect to voltage in a non-linear fashion. The most common form of varistors are called metal oxide varistors (MOVs). You can also find varistors made from silicon carbide (SiC), which are less desirable than MOVs due to their standby current consumption. Varistors are more cost-effective than TVS diodes, but they are prone to degradation over time and require a higher clamping voltage.
• Thyristors: Thyristors, or TPSD (thyristor surge protection devices), are solid-state components that allow current flow in the event of voltage on the gate of the device. Due to their robustness, you can often find thyristors in high voltage circuits. They connect across the supply line in shunt with the load that needs protection and, at normal operating voltages, maintain very high resistance across its terminals. If the line voltage exceeds the triggering threshold of the TSPD, it switches to the ON state and acts as a near short circuit, diverting the line current away from the protected load. Once fired, the TPSD will not switch off until the current falls below a 'holding' current value. This functionality makes TPSDs desirable for long transient spikes.

Different Type of Circuit Breakers and Fuses

• Circuit breakers: Generally thought of as being in higher voltage systems, circuit breakers are designed to automatically actuate to protect a circuit from overloading or shorting. However, these devices function well in low-voltage, low-current applications as well. These automatic switches trigger to interrupt current flow of systems, and users can reset them automatically or manually.
• Fuses: Fuses are common in applications ranging from highly complex manufacturing systems to simple automotive lighting. Fuses operate on a relatively simple concept, relying on high-current situations to be effective. To create a fuse, you need a metal wire with very low resistance. When subjected to a high-current signal, that wire melts and disconnects the circuit from operation, thus protecting components down the line.Melting a fuse requires replacing the modular fuse with a new one to continue operating the circuit.
• Resettable Fuses: There are, however, resettable fuses that don't require replacement after a single high current scenario. These fuses operate on a more complex design principle. Polymer PTC (positive temperature coefficient) fuses operate similarly to varistors, but instead of voltage transients, they respond to current transients. When PTC fuses encounter higher-than-normal currents, they increase in temperature, thus reducing their conductivity. This sharp increase in resistance is sustained until the high-current source is removed, and the fuse cools and can return to normal low-resistance operation.

Thermal Cutoff Switch & Fuses

• Thermal Cutoffs: As opposed to most other circuit protection devices, thermal cutoffs rely on mostly external (non-circuit) input to trigger circuit protection. Thermal cutoff fuses often rely on a one-time fusible link that, when exposed to ambient temperature over the melting temperature of the link, will fail to protect the circuit. There are also thermal cutoffs, called thermal switches, that operate similarly to a resettable fuse or a reed switch. These thermal cutoffs restrict the flow of current in the event of specified ambient temperatures. They will continue normal operation once the conductor reaches the desired temperature.