Electrical Relays

Electrical Relays



Relays are electrically operated switches that open and close under the control of another electrical circuit. The current that flows through the coil of the relay leads to creation of a magnetic field that attracts a lever and changes the switch contacts. Relay switches have two switch positions as the coil current can be on or off.

A relay can also be considered as an electrical amplifier because it is able to control an output circuit of higher power than the input circuit.

Types of Electrical Relay
There are different types of relay available in the market depending on the end use. These are:

* Latching Relay
* Reed Relay
* Polarized Relay
* Mercury-Wetted Relay
* Machine tool Relay
* Relay Circuit
* Contactor Relay
* Solid-state contactor Relay
* Solid state Relay
* Overload protector Relay

Pole and Throw Configurations
Relay is a type of switch and thus can have the following configurations:

* SPST - Single Pole Single Throw
* SPDT - Single Pole Double Throw
* DPST - Double Pole Single Throw
* DPDT - Double Pole Double Throw
* QPDT - Quadruple Pole Double Throw

Specifications to be Considered
There are certain important specifications that must be considered while selecting an appropriate relay for a particular application. These are: Selection of an appropriate relay for a particular application requires evaluation of many different factors:

* Number and type of contacts: These can be normally open, normally closed, or changeover (double-throw).

* Rating of contacts: While small relays switch a few amperes, there are large contactors that are rated for up to 3000 amperes.

* Voltage rating of contacts: This can vary depending on the type of relay. While control relays are rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V.

* Package/enclosure: The packing or enclosures of relay can be open, touch-safe, double-voltage for isolation between circuits, outdoor, oil-splash resistant and explosion proof.

* Mounting: This can be plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment.

* Regulatory approvals: Relays approved from agencies assure high quality to customers.

Electrical Relays Applications
Relays are used in numerous applications including:

* Modem
* Automobiles
* Aerospace
* Telecommunications

Electrical Fuse

Electrical Fuse



A fuse is used for protecting electrical devices and components from over currents and short circuits. A electrical fuse works by interrupting the flow of current through the melting of an internal element.

General specifications
Listed below are common specifications that apply to electric fuses are:

* Fuse types: These include:
o Miniature: These fuses are available in dimensions of 5x20mm and 6.3x32mm.

o Subminiature or micro: Also known as micro fuses, these have very small dimensions (no principal dimension greater than 10 mm) and aresuitable for compact circuit board layouts.

o Midget: These refer to fuses that have 13/32" diameter.

o Automotive: These fuses are only used in automotive.

o Blade type: These fuses are typically used in low voltage, high current applications.

o PC board: These fuses are specifically used on PC boards.

o Protective: These fuses are used for protecting secondary circuits or low voltage ICs.

* Mounting: These include:
o Solderable or surface mount
o Solderable with leads
o Replaceable with holder or clips

o Performance Ratings
There are certain performance criteria that one must consider while selecting fuses. These are:
+ Voltage rating: It is the maximum voltage, up to which a safe fault current interruption will occur.

+ Current rating: It is the maximum continuous operating current of the circuit.

+ Rated breaking capacity: It is the short circuit current at which the fuse can blow without being destructed.

+ Interrupt rating: These can be high interrupt, medium interrupt, or low interrupt.

o Performance Characteristics
There are certain performance characteristics that one must consider while selecting fuses. These are:
+ Fast acting: These fuses are used in circuits that have inrush currents or where the over current must be quickly interrupted.

+ Time lag: These fuses are used in circuits where high starting inrush currents occur and decay gradually.
Operating temperature of fuse is another environmental parameter that must be considered.

* Material of construction: These include:
o Ceramic
o Sand