This essay on relays is one of a series on electro-mechanical engineering. For a complete treatment of motors, generators, transducers, actuators, and other electro-mechanical engineering devices read UNDERSTANDING ELECTRO-MECHANICAL ENGINEERING, AN INTRODUCTION TO MECHATRONICS.> (It is included by McGraw-Hill as a selection in their Electronic Engineers' Book Club.) Other essays in the series deal with electro-mechanical actuators, transducers, amplifiers, and other devices.
Ye-e-e-ch! If there is one thing that electronic engineers despise (and fear) it is moving parts. They are obsolete, or ought to be, and either have been, or soon will be, replaced by electronics. Dirty words. (Your car and the buttons on your keyboard don't count, of course.).
The electro-mechanical relay dates from the early telegraph in the 1840's so it certainly is a candidate for history's scrap barrel, an elecrical buggy whip. And, in fact, many have been replaced by transistors and SCRs in both logic and power switching. So why are there still dozens of companies making and using them in the thousands?
A relay is one or more switching contacts operated by an electromagnet and restored by a spring, gravity, or a second electromagnet. Big relays are called "contactors" and some are called "motor starters" or "Circuit breakers." (Circuit breakers are relays used instead of fuses, so they can be electrically restored, and protect circuits from a few watts to public utility circuits of many megawatts.)
The contacts are usually of metal, although carbon is sometimes used because it will not weld from arcing heat. To prevent surface oxides from being insulating films between the contacts, "noble" metals are usually used. Silver and its alloys are most common, because inexpensive, but gold, platinum, palladium, and rhodium are all used. Mercury, sealed in a tube, is a commonly used and highly reliable contact material. Some contacts are enclosed in vacuum, some in gas, some in oil. Magnetic reed switches were originally developed as relay contacts, surrounded by a coil, for telephone switching. Contact design is still largely empirical.
Contact loads extend from microwatts to megawatts, from microamperes to megamperes, from microvolts to megavolts, from DC to microwaves, from resistive to capacitive to inductive. Any combination may be handled by a single relay. The different contacts on the same relay may have completely different voltage and current ratings. It is common for high power contacts for load control to be operated by the same electromagnet as low power contacts for information feedback and logic.
Relay electromagnets may be either AC or DC and may require any power from a milliwatt to many watts, depending on the size and number of the contacts it operates. There are also relays operated by electric motors and by compressed air cylinders. Certain relays which respond to specified electrical parameters use watthour meter motors. Simpler relays may be used to operate at particular values of current or voltage.
A single relay may have any number of contacts from one to a hundred.
There is complete electrical insulation between the electromagnet operating coil and the contacts and between different contacts. Dielectric strength of the insulation is commonly many kilovolts but may be made with many megavolts. There is no limit to the available electrical resistance of the insulation. Capacitive and inductive couplings among coil and contacts may be made as little as desired.
Operating and release times are usually greater than one millisecond and are commonly ten or more milliseconds. This time can be a serious limitation in logic circuits. On the other hand, relays can be made with substantial operating and release times if so desired.
Within a single relay the closing and opening sequence of different contact pairs can be set, which is a benefit in certain logic circuits.
Mechanical latching can be provided so that power failure in the coil circuit does not change the contact closures. Unlatching may be either by a separate electromagnet, by hand, or by another mechanism. Circuit breakers are typically latching relays.
When a pair of relay contacts separate and interrupt a current there is an arc between them as the inductive energy stored in the circuit is dissipated. The arc may be infinitessimal in a logic circuit or it may be a big flaming explosion in a utility circuit breaker. Large transient voltages may be induced in associated circuits. Many ways are used to minimize arc damage.
When contacts close they bounce like impacting billiard balls. Circuits must be made to tolerate this bounce or mechanical energy absorbers may be provided to prevent bounce.
Relays are provided with a full spectrum of environmental resistance, including shock and vibration, corrosion, and dirt.
The best sources of information are manufacturers' catalogs, for both relays and contacts. Thomas Register is the best source of lists of manufacturers.
Other essays in this series deal with electro-mechanical actuators, amplifiers, and other devices.
For a full treatment, please see my book:
"Understanding Electro-Mechanical Engineering" published by IEEE Press. It is included by McGraw-Hill in their Electronic Engineers' Book Club. Click here.
My other books are:
"Designing Cost-Efficient Mechanisms" published by SAE Press. This is the book which introduced and explains Minimum Constraint Design. Click here.
"Real-World Engineering" Published by IEEE Press. How to be a successful engineer. Click here.