Relay: Difference between revisions

Contact materials for relays vary by application. Materials with low contact resistance may be oxidized by the air, or may tend to "stick" instead of cleanly parting when opening. Contact material may be optimized for low electrical resistance, high strength to withstand repeated operations, or high capacity to withstand the heat of an arc. Where very low resistance is required, or low thermally-induced voltages are desired, gold-plated contacts may be used, along with palladium and other non-oxidizing, semi-precious metals. Silver or silver-plated contacts are used for signal switching. Mercury-wetted relays make and break circuits using a thin, self-renewing film of liquid mercury. For higher-power relays switching many amperes, such as motor circuit contactors, contacts are made with a mixtures of silver and cadmium oxide, providing low contact resistance and high resistance to the heat of arcing. Contacts used in circuits carrying scores or hundreds of amperes may include additional structures for heat dissipation and management of the arc produced when interrupting the circuit.<ref>Ian Sinclair, ''Passive Components for Circuit Design'', Elsevier, 2000 {{ISBN|008051359X}}, pp. 161–164</ref> Some relays have field-replaceable contacts, such as certain machine tool relays; these may be replaced when worn out, or changed between normally open and normally closed state, to allow for changes in the controlled circuit.<ref>{{cite book |first=Joseph E. |last=Fleckenstein |title=Three-Phase Electrical Power |publisher=CRC Press |date=2017 |isbn=978-1498737784 |page=321}}</ref>

* ''SPST-NO'' (Single-Pole Single-Throw, Normally-Open) relays have a single ''[[Form A (switches)|Form A]]'' contact or ''make'' contact. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total.

{{splittosplit|List of relay types|date=September 2021}}

The high speed of switching action of the mercury-wetted relay is a notable advantage. The mercury globules on each contact [[coalescence (physics)|coalesce]], and the current rise time through the contacts is generally considered to be a few picoseconds.{{cncitation needed|date=November 2023}} However, in a practical circuit it may be limited by the [[inductance]] of the contacts and wiring. It was quite common, before restrictions on the use of mercury, to use a mercury-wetted relay in the laboratory as a convenient means of generating fast rise time pulses, however although the rise time may be picoseconds, the exact timing of the event is, like all other types of relay, subject to considerable jitter, possibly milliseconds, due to mechanical variations.

The same coalescence process causes another effect, which is a nuisance in some applications. The contact resistance is not stable immediately after contact closure, and drifts, mostly downwards, for several seconds after closure, the change perhaps being 0.5&nbsp;ohm.{{cncitation needed|date=November 2023}}

A [[solid-state relay]] (SSR) is a [[solid state (electronics)|solid state]] electronic component that provides a function similar to an [[electromechanical]] relay but does not have any moving components, increasing long-term reliability. A solid-state relay uses a [[thyristor]], [[TRIAC]] or other solid-state switching device, activated by the control signal, to switch the controlled load, instead of a solenoid. An [[optocoupler]] (a [[light-emitting diode]] (LED) coupled with a [[photo transistor]]) can be used to isolate control and controlled circuits.<ref>{{Cite web|url=https://www.electronics-tutorials.ws/blog/optocoupler.html|title=Optocoupler Tutorial|date=16 September 2013 }}</ref>

Because relaysRelays are much more resistant than semiconductors to nuclear radiation, so they are widely used in safety-critical logic, such as the control panels of radioactive waste-handling machinery. Electromechanical [[protective relay]]s are used to detect overload and other faults on electrical lines by opening and closing [[circuit breaker]]s.

{{Main|protectiveProtective relay}}