Fuses: Fuses are current sensitive devices that are designed to be an intentional weak link in an electrical circuit. Fuses contain a wire element, filament or thin film that melts under pre-determined fault or overcurrent conditions.
Fuses Unlimited offers a broad range of circuit protection devices. Our product offering includes both overcurrent and overvoltage components. Overcurrent products include fuses, fuse clips, fuseholders, fuse blocks, circuit breakers and positive temperature coefficient (PTC) resettable devices. Overvoltage products include Multilayer Varistors (MLV’s) and Metal Oxide Varistors (MOV’s), discreet transient voltage suppression diodes (TVS Diodes), TVS diode arrays, power thyristors (SIDACTORS®), switching thyristors (SCR’s, Triacs and Sidacs), electrostatic discharge suppressors (ESD) and gas discharge tubes (GDT’s).
Electronic fuses are typically rated at 300 volts or below and are designed to protect circuits in the limited space requirements of electronic equipment such as consumer electronics, computers, instrumentation, power supplies and telecommunication equipment. Miniature electronic fuses refer to the cartridge-style fuse type. The subminiature fuse category includes the Pico, Micro, 2.3 x 8mm cartridge, 3.6x10mm cartridge and the surface mount fuses.
Automotive style fuses are used extensively in cars, trucks buses and off-road transportation equipment to protect the cables, wires and electrical components that supply electrical power to operate lights, heaters, air conditioning, radios, power windows and other electrical accessories. They are also used in electric and hybrid vehicles.
Midget and power fuses are typically rated between 250 and 600 volts and are designed to provide circuit protection in various types of industrial equipment used in industrial and commercial buildings. Midget fuses are built in a 13/32” x 1-1/2” (10x38mm or 5AG) case size. Midget ftuses have ampere ratings up to 50 amps and power fuses can have ampere ratings up to 6,000 amps. Typical midget and power fuse applications include overload and short circuit protection in motor branch circuits, heating and cooling systems, control systems, lighting circuits and electrical distribution networks.
Semiconductor fuses are very fast acting current limiting fuses that offer low melting integral (I2t) values and peak let-through currents. Voltage ratings range from 130 to 1,500 volts. These fuses are designed to protect diodes, thyristors, triacs, transistors and other solid-state power semiconductor devices.
Medium Voltage Fuses re current limiting power fuses having voltage ratings from 0.6kV to 34.38kV. Continuous current ratings range from 0.5 to 1,200 amps. E-rated medium voltage fuses are general purpose fuses that are primarily used to protect transformers and provide both current overload and short circuit protection. R-Rated fuses are back-up fuses primarily used to protect motors and motor controllers. R-Rated fuses only provide short circuit protection. Other medium voltage fuses are designed to protect potential transformers, capacitors and distribution transformers. Medium Voltage fuses are designed in accordance with the American National Standards Institute (ANSI). Back-up fuses designed in accordance with IEC Standards are also available.
Telcom and Telpower fuses are designed to protect the photovoltaic (PV) circuits found in solar power systems. Solar fuses are rated at 600 or 1,000 volts and are built in Midget (10x38mm or 5AG), Class-R and Class J case sizes.
Special purpose fuses are fuses designed to operate in specific applications such as lighting circuits, fork lift truck circuits, explosive environments, cables and welding circuits.
PTC Devices are resettable overcurrent circuit protection devices that are typically used in applications where sensitive components are at risk from overcurrent conditions. The ability of a PTC Device to reset after exposure to an overcurrent condition is an attractive feature in circuits that are not easily accessible to a user or technician or where constant uptime is required. PTC Devices are offered in surface mount, radial leaded, and battery strap (axial-leaded) versions. Telecom versions are offered in surface mount and radial leaded packages. Littelfuse offers the entire range of PTC Devices using the brand name Polyfuse®.
Circuit Breakers are are mechanical overcurrent circuit protection devices. They trip or open a circuit when excess current runs through the device. Circuit Breakers are used to protect a broad range of equipment and systems such as motors, transformers, telecommunications equipment, computersm industrial automation, air-conditioning, power distribution, variable speed drives, indistrial control panels and other industrial equipment systems. They are also used in low voltage battery powered distribution circuits found in heavy duty vehicles, cars, boats, aircraft and off-road vehicles. There are three basic types of circuit breakers.
Hydraulic Magnetic Circuit Breakers have an electromagnetic coil, an iron core sealed in a tube filled with hydraulic fluid, a spring loaded actuator and two electrical contacts. When excessive current from an overload runs through the device, the electromagnetic field in the coil is increased which moves the iron core through the tube and causes the spring loaded actuator to trip, separating the contacts and opening the circuit. Opening speed is determined by the viscosity or thickness of the hydraulic fluid.
Thermal Circuit Breakers are tripped by a thermally sensitive bimetal strip. Heat from a current overload deforms the metal strip and trips an actuator which opens the circuit.
Thermal Magnetic Circuit Breakers utilize a bimetal strip to open the circuit in an overcurrent condition and an electromagnetic coil to open the circuit in a short circuit condition.
Disconnect Switches provide a means of quickly disconnecting mechanical or electronic systems from their primary power source safely. These switches operate both automatically, to protect against circuit faults, as well as manually in case an emergency stop or when planned maintenance is required. Industrial disconnect switches are typically installed in control panels and used to protect heavy machinery. Battery disconnect switches are installed in an electrical system that runs on a battery.
Bus Bars are electrical devices that are designed to distribute a large positive to individual circuit protection devices that feed a number of smaller wires. Bus Bars can also collect negative conductors and step them up to larger ones. Bus Bars are used in a number of electrical circuits in boats, trucks, and busses. It is impractical to attach wires from numerous loads in a vehicle or boat directly to the battery terminal or battery switch terminal. Large electrical systems may use several layers of progressively smaller Bus Bars.
MLV and MOV Varistors are voltage sensitive devices designed to protect circuits against transient voltage surges. Surface mount varistors utilize a ceramic multilayer-layer construction (MLV) and are used to protect circuit boards in small electronics from transients generated from electrostatic discharge (ESD), inductive load switching and lightning surge remnants. Metal Oxide Varistors (MOV’s) are epoxy encapsulated zinc oxide discs with radial or axial leads. MOV’s are mid-range devices used to protect small machinery, power sources and components. Industrial MOV’s are larger rigid terminal mount devices made with zinc oxide discs enclosed in epoxy polymer cases to provide complete electrical isolation. The industrial MOV’s are used in high energy industrial applications.
Sensors include the reed switch, the reed sensor, the reed relay, the Hall Effect sensor, the temperature sensing thermistor and RTD's. Typical applications include the sensing of proximity, liquid level, liquid flow, speed and temperature.
Thyristors are solid state switches that are normally open (high impedance) circuits, capable of withstanding a rated blocking voltage until triggered to an on state. Once triggered, thyristors become a low impedance current path until the main current either stops or drops below a minimum holding current. Thyristors are normally two or three terminal devices for either uni-directional or bi-directional circuit configurations. An SCR is a uni-directional device. A Triac is a bi-directional device. Thyristors are typically used in home appliances such as lighting (dimmer switch), heating and temperature control, alarm activation and fan speed control. They are also used in power tools for controlled actions such as motor speed, a stapling event or battery charging. Outdoor equipment including water sprinklers, gas engine ignition, electronic displays, area lighting and sports equipment also use thyristors.
TVS Diodes are discreet, fast switching, semiconductor devices that can respond to overvoltage events faster than most other types of circuit protection devices. TVS Diodes feature low clamping voltage transients such as lightning, inductive load switching and electrostatic discharges. TVS stands for discrete transient voltage suppression. They can be used in a wide variety of applications such as protecting data and signal lines, microprocessors, MOS memory and AC power lines. Industry segments where TVS diodes are typically used are in telecommunications and industrial equipment, computers, peripheral devices and consumer electronics.