A Circuit Breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Generally, wherever precise and reliable circuit protection is required, a circuit breaker is specified.

Polymer suppression devices are typically designed for ESD protection. They have extremely low capacitance, very low leakage current and extremely fast turn on time. Due to these characteristics, this type of device is suited to very fast (high speed) applications. These type of ultra-fast applications must have low capacitance to keep from the accidental attenuation of high-speed data signals due to capacitive loading.

A fuse is an overcurrent protection component that is designed to be the intentional weak link in a circuit. A standard, one-time fuse, when the correct rating is chosen, will “blow” before the current gets to a level that will damage the circuitry or create a fire hazard. This event of the fuse blowing will actually break the circuit and thus no current will flow past that point. A fuse contains a short piece of wire made of an alloy that melts readily. The flow of current through a fuse causes the wire to heat up and melts when excessive current passes through the fuse. This action burns out the fuse and breaks the circuit.

The standard cartridge fuse will need something to mount it to the board; this is where a fuseholder, block or clip comes in. Each fuseholder, block or clip has a specific current rating that must be referenced when specifying. There are numerous fuse mounting options such as: Shock-Safe (panel mount and PC board types), Indicating, RF- Shielded, Traditional, Watertight, RF Shielded Watertight, Inline, Traditional Clips and Omni-Blok.

A (GDT) gas discharge tube, also called a Spark Gap Tube or Gas Tube Surge Arrester, acts like a switch when in its "on state" and shunts an over voltage condition to ground with very minimal voltage left in the circuit. It is made up of a ceramic tube or envelope with two or three electrodes lying opposite of each other separated by inert gas. The gas acts like an insulator and creates a very high impedance to the circuit, almost as if it were open. When the voltage reaches a certain "DC Sparkover", the inert gas ionizes creating an environment of very low resistance to the circuit. The ionized gas now allows the circuit to arc within the tube and between the electrodes, completing the circuit and shunting the over voltage to ground. GDTs are specifically designed to protect primary telecom systems from lightning surges.

A (MLV) multilayer varistor is a compact, surface mountable, chip that is voltage dependent (like a MOV) and bidirectional. It has an electrical behavior like a back-to-back diode, by offering protection in both forward and reverse directions. Like a MOV, the MLV stays in a very high impedance state when there is no over voltage condition then changes to a very conductive state when a condition arises. This allows the voltage to pass into the MLV where it is dissipated. MLVs are designed to suppress a variety of transient events, such as ESD, EMI and inductive switching. MLVs are typically applied to protect integrated circuits and other components at the circuit board level.

Things to Consider

• Operational usage: whether a one-time-use device is acceptable or if a resettable device is required
• Operating current/ hold current/ non-trip current: the maximum amount of current that a device can handle without blowing/tripping
• Trip Current
• Switching Temperature
• Voltage rating & Interrupt rating
• Resistance: the amount of resistance the component contributes to the circuit
• Agency approvals
• Mounting Option & Form factors

(MOV) metal oxide varistors are voltage dependent devices that have an electrical behavior similar to back to back Zener diodes. When exposed to high voltage transients, a varistor’s impedance (resistance within a circuit) changes from very high, almost an open circuit, to a highly conductive level (very low impedance to the circuit). The conductive state of the varistor is now much lower than any other path along the circuit and thus the energy flows into the varistor where it "clamps" the voltage by partly absorbing and partly shunting to ground the voltage above the clamping amount, thus dissipating the energy. MOV’s can be used in either AC or DC applications. MOV’s are intended for a comprehensive range of applications that typically are higher voltage and slower in nature such as lightning opposed to a faster event such as ESD.

Things to Consider

• Operational usage: whether a one-time-use device is acceptable or if a resettable device is required
• Operating current/ hold current/ non-trip current: the maximum amount of current that a device can handle without blowing/tripping
• Trip Current
• Switching Temperature
• Voltage rating & Interrupt rating
• Resistance: the amount of resistance the component contributes to the circuit
• Agency approvals
• Mounting Option & Form factors

A PTC thermistor is very similar in function to a resettable fuse. It also will create a very high resistance to the current at a particular temperature (caused by the additional current), called the switching temperature, that shelters the device that you are protecting from the overheat or over current condition. PTC thermistors are also like resettable fuses in the area of resettability. When the overheat or over current event is removed, the thermistor will cool down and reset just like the resettable fuse. A PTC thermistor is similar to a resettable fuse that at a certain temperature, caused by the flow of current, switches to a very high resistance state from a very low resistance state. It also “resets” itself after it cools down when the over current condition is removed.

A resettable fuse actually does not “blow” like a standard fuse. It actually creates a very high resistance that keeps the majority of the current from flowing past it into the device that you are protecting on the circuit. The resettable fuse then “resets”, as the name implies, once the overcurrent condition causing the event subsides. A resettable fuse is made up of a combination of carbon black and a special formulated polymer that has many conductive chains when in its “off” state. When a current reaches the specified rating, the resettable fuse will switch from a very low resistance to a very high resistance (called tripping). The earlier conductive chains in its off state expand apart leaving very few conductive chains for the current to follow. This creates a high enough resistance that very little current can pass through the resettable fuse. When the over current condition is taken away, the device cools down quickly, bringing the conductive paths back together and resetting itself for the next time.

Thyristors are silicon-based devices that protect a circuit from over voltage conditions by switching to a low on-state voltage and shunting the additional voltage to ground. Once the thyristor has "turned on", the current level must fall below a certain amount for it to turn off and reset. Most cases this is a low amount so in effect the circuit would probably need to shut down for the device to reset. Thyristors are designed to suppress lightning and other transients that are induced on telecommunications systems.

A TVS diode is a silicon based device that allows that allows current and voltage to only flow in one direction (unidirectional) when it reaches a certain “breakdown voltage” or in the case of bidirectional, will act like two back-to-back unidirectional diodes and will protect the line from either direction. It effectively clamps the voltage to a safe amount for the circuit.

Things to Consider

• Reverse standoff voltage
• Peak pulse current
• Maximum clamping voltage
• Breakdown voltage
• Maximum breakdown voltage temperature coefficient