Power Relays
Original article date: September 1997
Despite its common usage the power relay is rarely seen as a complex and important component.. Schrack’s NORBERT STERL introduces the different contact magnetic and mechanical systems and how to classify them.
Using new and advanced contact materials the switching capacity of relays has either been maintained or increased and the use of new plastic materials has improved the insulation properties. Sensitivity – the ratio of input coil power to switching capacity – has also increased.
Relays are electromagnetically-operated switches. An actuating current on a coil operates one or more galvanically separated contacts or load circuits. The electromechanical relay is a remote-controlled switch capable of switching multiple circuits either individually simultaneously or in sequence.
The capability to operate the power contacts and switch high currents with low input power on the primary side provides the amplifier function of the relay. However unlike the transistor the relay is not an analog amplifier but a digital-type amplifier (1-0 ON-OFF) Input signals can be amplified by a factor of up to 105or reduced as in signal relay applications.
Relays have the same sub-systems and principals of operation regardless of whether they are electromechanical or electronic or designed to switch signal or high power loads. These principal functions are:
- Conversion of an electrical input signal on the primary side to an intermediate and non-electric physical signal
- Reconversion of this non-electric physical signal to operate a switching element (secondary side) such as contacts which switch and conduct electrical current (output load current)
- The use of the non-electric signal between primary and secondary side provides the necessary galvanic separation between input and output circuits.
According to the nature of the secondary switching element basic relay types are:
Electromechanical relays:- this is the most widely used type of relay design. In this case the switching element is a mechanical contact. Standard yoke-type designs as well as reed relays and others are part of this group. The principal internal functions of the electromechanical relay are:
- Conversion of electrical current (input coil current) to a magnetic field
- Conversion of the magnetic field into a mechanical force
- This force operates the contacts (secondary side)
- Contacts switch and conduct electrical current (output load current)
Electronic relays:- this type of relay uses electronic switches such as transistors triacs etc. as the main switching element.
Relays can also be classified according to the nature of the intermediate signal between the primary side and the switching element. For example in electromagnetic relays a magnetic field by the output signal operates on the mechanical contacts. Examples are the standard yoke-type relay or the reed relay.
In optocouplers the electronic switching element is controlled optically. On the primary side an optical signal is transmitted by means of a light-emitting diode while a phototransistor acts as a receiver and controls the switching element. In this case the optical connection provides the galvanic separation.
In solid-state relays the signal from the primary side can also be transmitted by other means for example by frequency modulation or capacitive effects.
Other more exotic relay designs include thermoelectric relays where the input energy on the primary side heats a bimetal part which actuates the contacts mechanically. In piezo relays the contacts are operated mechanically by the piezoelectric effect.
Relay designs can also be divided according to their switching capacity. Signal relays cover signal data and voice-switching applications up to approximately 2A resistive load. Typical applications are measurement systems computer interfaces and telecommunications. Designs for signal relays include standard yoke-type relays reed relays and solid-state relays.
According to IEC 255 power relays are those which switch up to 600V and 100A. The high current area overlays with the switching range of contactors. Generally power relays switch up to 16A for PCB types and up to 30A for industrial types. The most common design for power relays is the standard yoke-type relay though for some applications solid-state power relays may be used.
Although contactors are not strictly relays they have the same design principles. Like relays they are remote-controlled switches. The difference is the considerably higher input power consumption higher switching capacity and larger size.
Power relay types can be divided according to their mechanical features: miniature PCB industrial. etc. The term miniature refers to physical size. However as relay technology has used this term for some time it does not necessarily give an indication of absolute size. PCB power relays are designed for mounting onto printed circuit boards. Industrial relays are typically used in switching cabinets by control panel builders.
Primary functions of a relay
- The galvanic separation of the primary or actuating circuit and the load circuits
- Single input/multiple output capability
- Separation of different load circuits for multi-pole relays
- Separation of AC and DC circuits
- Interface between electronic and power circuits
- Multiple switching functions (eg delay signal conditioning)
- Amplifier function
Typical applications for relays
- Laboratory instruments
- Telecommunications systems
- Computer interfaces
- Domestic appliances
- Air conditioning and heating
- Automotive electrics
- Traffic control
- Lighting control
- Building control
- Electrical power control
- Business machines
- Control of motors and solenoids
- Tooling machines
- Production and test equipment
Schrack’s Norbert Sterl has written the first dedicated power relays book for over a decade. Costing (UK pounds)25 the 142 pages contain detailed information on contact magnetic and mechanical systems as well as the equally important and interrelated electrical mechanical and chemical effects.
- Schrack Components
Haydon House
296 Joel Street
Pinner HA5 2PY - Tel: 0181 868 1211
- Fax: 0181 866 2221
September 1997