How Relays Work Relays Relays are switches that open and close circuits electromechanically or electronically. Relays control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams show, when a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is Normally Closed (NC), there is a closed contact when the relay is not energized. n either case, applying electrical current to the contacts will change their state. Relays are generally used to switch smaller currents in a control circuit and do not usually control power consuming de!ices e"cept #or small motors and $olenoids that draw low amps. Nonetheless, relays can %control% larger !oltages and amperes by ha!ing an ampli#ying e##ect because a small !oltage applied to a relays coil can result in a large !oltage being switched by the contacts. &rotecti!e relays can pre!ent e'uipment damage by detecting electrical abnormalities, including o!ercurrent, undercurrent, o!erloads and re!erse currents. n addition, relays are also widely used to switch starting coils, heating elements, pilot lights and audible alarms. Solid-state relay F rom Wikipedia, the free encyclopedia Jump to: navigation, searchA solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. SSRs consist of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate this switch without mechanical parts. he relaymay be designed to switch either A! or "! to the load. #t serves the same function as an electromechanical relay, but has no moving parts. $ac%aged solid-state relays use power semiconductordevices such asthyristorsand transistors, to switch currents up to around a hundred amperes. Solid-state relays have fast switching speeds compared with electromechanical relays, and have no physical contacts to wear out. Application of solid-state relays must consider their lower ability to withstand momentary overload, compared with electromechanical contacts, and their higher &on& state resistance. 'nli%e an electromechanical relay, a solid-state relay provides only limited switching arrangements (S$Sswitching).
RelaysRelays are switches that open and close circuits electromechanically or electronically. Relays
control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams
show, when a relay contact is normally open (NO), there is an open contact when the relay is not
energized. When a relay contact is Normally Closed (NC), there is a closed contact when the relay
is not energized. n either case, applying electrical current to the contacts will change their state.Relays are generally used to switch smaller currents in a control circuit and do not usually control
power consuming de!ices e"cept #or small motors and $olenoids that draw low amps. Nonetheless,
relays can %control% larger !oltages and amperes by ha!ing an ampli#ying e##ect because a small
!oltage applied to a relays coil can result in a large !oltage being switched by the contacts.
&rotecti!e relays can pre!ent e'uipment damage by detecting electrical abnormalities, including
o!ercurrent, undercurrent, o!erloads and re!erse currents. n addition, relays are also widely used
to switch starting coils, heating elements, pilot lights and audible alarms.
Solid-state relayFrom Wikipedia, the free encyclopedia
Jump to: navigation, search
A solid-state relay (SSR) is an electronic switching device that switches on or off when a
small external voltage is applied across its control terminals. SSRs consist of a sensor which
responds to an appropriate input (control signal), a solid-state electronic switching device
which switches power to the load circuitry, and a coupling mechanism to enable the control
signal to activate this switch without mechanical parts. he relay may be designed to switcheither A! or "! to the load. #t serves the same function as an electromechanical relay, but
has no moving parts.
$ac%aged solid-state relays use power semiconductor devices such as thyristors and
transistors, to switch currents up to around a hundred amperes. Solid-state relays have fast
switching speeds compared with electromechanical relays, and have no physical contacts to
wear out. Application of solid-state relays must consider their lower ability to withstand
momentary overload, compared with electromechanical contacts, and their higher &on& state
resistance. 'nli%e an electromechanical relay, a solid-state relay provides only limited
Coupling[edit]he control signal must be coupled to the controlled circuit in a way which provides galvanic
isolation between the two circuits.
any SSRs use optical coupling. he control voltage energies an internal *+" which
illuminates and switches on a photo-sensitive diode (photo-voltaic) the diode current turnson a bac%-to-bac% thyristor , S!R , or S+ to switch the load. he optical coupling
allows the control circuit to be electrically isolated from the load.
Operation[edit]An SSR based on a single S+, or multiple S+s in a paralleled array, can wor%
well for "! loads. S+s have an inherent substrate diode that conducts in the reverse
direction, so a single S+ cannot bloc% current in both directions. or A! (bi-
directional) operation two S+s are arranged bac%-to-bac% with their source pins tied
together. heir drain pins are connected to either side of the output. he substrate diodes arealternately reverse biased to bloc% current when the relay is off. /hen the relay is on, the
common source is always riding on the instantaneous signal level and both gates are biased
positive relative to the source by the photo-diode.
#t is common to provide access to the common source so that multiple S+s can be
wired in parallel if switching a "! load. 'sually a networ% is provided to speed the turn-off
of the S+ when the control input is removed.
#n A! circuits, S!R or R#A! relays inherently switch off at the points of ero load current.
he circuit will never be interrupted in the middle of a sine wave pea%, preventing the large
transient voltages that would otherwise occur due to the sudden collapse of the magnetic field
around the inductance. his feature is called ero-crossover switching. 012
Parameters[edit]
.his section re/uires e-pansion0 (September 2010)
SSRs are characterised by a number of parameters including the re3uired activating input
voltage, current, output voltage and current, whether it is A! or "!, voltage drop or
resistance affecting output current, thermal resistance, and thermal and electrical parameters
for safe operating area (e.g., derating according to thermal resistance when repeatedly
switching large currents).
Advantages over mechanical relays[edit]
.his section does not cite any sources0 lease help improve thissection y adding citations to reliale sources0 nsourced material
may e challenged and removed0 (February 2015)
ost of the relative advantages of solid state and electromechanical relays are common to all
solid-state as against electromechanical devices.
• (limmer prole, allo4ing tighter packing0
• .otally silent operation
• ((*s s4itch faster than electromechanical relays5 the s4itching time of atypical optically coupled ((* is dependent on the time needed to po4erthe 6$ on and o7 8 of the order of microseconds to milliseconds
• &ncreased lifetime, even if it is activated many times, as there are nomoving parts to 4ear and no contacts to pit or uild up caron
• Output resistance remains constant regardless of amount of use
• Clean, ounceless operation
• 9o sparking, allo4s it to e used in e-plosive environments, 4here it iscritical that no spark is generated during s4itching
• &nherently smaller than a mechanical relay of similar specication ifdesired may have the same ;casing; form factor for interchangeaility<0
• =uch less sensitive to storage and operating environment factors such asmechanical shock, viration, humidity, and e-ternal magnetic elds0
.his section does not cite any sources0 lease help improve this
section y adding citations to reliale sources0 nsourced material
may e challenged and removed0 (February 2015)
• >oltage?current characteristic of semiconductor rather than mechanicalcontacts:
o When closed, higher resistance generating heat<, and increasedelectrical noise
o When open, lo4er resistance, and reverse leakage current typically@" range<
o >oltage?current characteristic is not linear not purely resistive<,distorting s4itched 4aveforms to some e-tent0 "n
electromechanical relay has the lo4 ohmic linear< resistance of theassociated mechanical s4itch 4hen activated, and the e-ceedinglyhigh resistance of the air gap and insulating materials 4hen open0
o (ome types have polarity8sensitive output circuits0lectromechanical relays are not a7ected y polarity0
• ossiility of spurious s4itching due to voltage transients due to muchfaster s4itching than mechanical relay<
• &solated ias supply re/uired for gate charge circuit
• Aigher transient reverse recovery time .rr< due to the presence of theody diode
• .endency to fail ;shorted; on their outputs, 4hile electromechanical relaycontacts tend to fail ;open;0
![[ 3000 Series Time Delay Relays and Measuring Relays ... · [ 3000 Series Time Delay Relays and Measuring Relays ] ... Measuring Relays ] • Time Delay Relays ... Dear Reader, Dear](https://static.documents.pub/doc/80x56/5b85683b7f8b9aec488e43dd/-3000-series-time-delay-relays-and-measuring-relays-3000-series-time.jpg)
