By Thomas Blaha
President, Memory Protection Devices
Certain electronic devices require additional protection in the form of a fuse, a sacrificial device that provides increased protection against electric overcurrent or surges.
Patented by Thomas Edison in 1890 to provide circuit protection for his municipal electric distribution system, fuses are now commonplace, available in all shapes and sizes, including fuses for electronic circuitry.
The essential component of a fuse is a metal wire or strip that melts when too much current flows, thus interrupting the circuit to help protect sensitive circuitry against overcurrent damage as well as to help prevent excessive heat build-up that can potentially lead to electrical fires.
Fuses are designed with varying current and voltage ratings to meet application-specific and/or regulatory requirements. At the same time, fuses should be carefully selected so as to avoid needless interruptions caused by blown fuses. The fuse is typically constructed with a metal strip or wire fuse element mounted between a pair of electric terminals, which are enclosed within a non-combustible housing. The size and construction of the element allows for a certain maximum temperature that, if exceeded, causes the element itself or a solder joint to melt, thus opening the circuit. Certain types of fuses allow for short-term overload and are self-reset to restore the circuit after the overload has cleared.
Fuses are attached to electronic devices using a variety of mounting methods. For example, 2AG and 3AG cylindrical glass fuses, which are commonly used in consumer electronics and high fidelity audio equipment, can be off-board mounted, with the fuse inserted into an inline fuse holder. These fuses can also be on-board mounted, with the fuse resting in a clip or cradle. Fuse clips are very economical to produce, and allow for a high degree of interchangeability between fuse brands. Fuse clips are typically made of nickel-plated brass with pins inserted through the pc board that are soldered.
Multiple 2AG and 3AG fuse clips can be combined to create a fuse block, which allows multiple fuses to be connected to one common ground to save valuable pc board real estate. Fuse blocks are commonly available in 2-position, 4-position, 6-position, 8-position, 10-position and 12-position configurations.
In certain situations, the fuse may need to be panel mounted, where a hole is punched in a chassis and a fuse holder is hard mounted to the outside of the case for easy access and wired discretely. This form of attachment typically involves the use of a wiring harness, plastic body, screw-on cap and a spring that ensures the electrical connection. As a result, these systems are far more expensive to manufacture than standard fuse clips, which limits their overall demand.
Blade-type fuses are highly popular for automotive systems. Available in both standard and a mini-blade sizes, blade-type fuses are extremely rugged, color coded for easy identification, and allow for easy insertion and extraction in challenging environments with an inexpensive extraction tool. Blade-type fuse holders are usually mounted vertically with pins inserted into the pc board, allowing the fuses to stand straight up for easy replacement by the consumer. Multiple blade fuse holders can be combined in a plastic frame to create a fuse block that takes the guesswork out of component spacing and makes insertion into the pc board virtually goof-proof.
The pricing outlook for fuse holders appears to be relatively stable over the next 18 months, as resins and metals such as copper, brass and steel are currently trading in a relatively narrow band on the futures markets.