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Solid-state relay

From Wikipedia, the free encyclopedia

A solid-state relay (SSR)is an electronic switching device that switches conduction states when a small

external voltage is applied along itsn-typeandp-typejunctions. 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. Therelaymay be designed to switch either AC or DC to the load. It serves the same

function as an electromechanicalrelay,but has nomoving parts.

Solid-state relays are composed ofsemiconductormaterials, includingthyristorsandtransistors,and have

current ratingsthat extend from a few microamps for low-power packages up to around a hundred amps for

high-power packages. They have extremely fast switching speeds usually ranging between 1 to 100

nanoseconds and are not easily affected by contact wear. Solid-state relays have several shortcomings: a

high susceptibility to damage (a relatively high vulnerability to overloads in comparison to

electromechanical relays); limited switching arrangements (SPST switching); a need for finer tuning due to

high "on" resistances.

Contents

1 Coupling

2 Operation

3 Parameters

4 Advantages over mechanical relays

5 Disadvantages

6 Images

7 See also

8 References

9 External links

Coupling

The control signal must be coupled to the controlled circuit in a way which providesgalvanic isolation

between the two circuits.

Many SSRs use optical coupling. The control voltage energizes an internalLEDwhich illuminates and

switches on aphoto-sensitive diode(photo-voltaic); the diode current turns on a back-to-backthyristor,

SCR,orMOSFETto switch the load. The optical coupling allows the control circuit to beelectricallyisolatedfrom the load.

Operation

A SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array, can work well for DC loads.

MOSFETs have an inherent substrate diode that conducts in the reverse direction, so a single MOSFET

cannot block current in both directions. For AC (bi-directional) operation two MOSFETs are arranged back-

to-back with their source pins tied together. Their drain pins are connected to either side of the output. The

substrate diodes are alternately reverse biased to block current when the relay is off. When 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.

http://en.wikipedia.org/wiki/Extrinsic_semiconductor#N-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#N-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#N-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#P-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#P-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#P-type_semiconductorshttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Current_ratinghttp://en.wikipedia.org/wiki/Current_ratinghttp://en.wikipedia.org/wiki/SPST_switchhttp://en.wikipedia.org/wiki/SPST_switchhttp://en.wikipedia.org/wiki/SPST_switchhttp://en.wikipedia.org/wiki/Solid-state_relay#Couplinghttp://en.wikipedia.org/wiki/Solid-state_relay#Couplinghttp://en.wikipedia.org/wiki/Solid-state_relay#Operationhttp://en.wikipedia.org/wiki/Solid-state_relay#Operationhttp://en.wikipedia.org/wiki/Solid-state_relay#Parametershttp://en.wikipedia.org/wiki/Solid-state_relay#Parametershttp://en.wikipedia.org/wiki/Solid-state_relay#Advantages_over_mechanical_relayshttp://en.wikipedia.org/wiki/Solid-state_relay#Advantages_over_mechanical_relayshttp://en.wikipedia.org/wiki/Solid-state_relay#Disadvantageshttp://en.wikipedia.org/wiki/Solid-state_relay#Disadvantageshttp://en.wikipedia.org/wiki/Solid-state_relay#Imageshttp://en.wikipedia.org/wiki/Solid-state_relay#Imageshttp://en.wikipedia.org/wiki/Solid-state_relay#See_alsohttp://en.wikipedia.org/wiki/Solid-state_relay#See_alsohttp://en.wikipedia.org/wiki/Solid-state_relay#Referenceshttp://en.wikipedia.org/wiki/Solid-state_relay#Referenceshttp://en.wikipedia.org/wiki/Solid-state_relay#External_linkshttp://en.wikipedia.org/wiki/Solid-state_relay#External_linkshttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/LEDhttp://en.wikipedia.org/wiki/LEDhttp://en.wikipedia.org/wiki/LEDhttp://en.wikipedia.org/wiki/Photodiodehttp://en.wikipedia.org/wiki/Photodiodehttp://en.wikipedia.org/wiki/Photodiodehttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/Electrical_isolationhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Photodiodehttp://en.wikipedia.org/wiki/LEDhttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/Solid-state_relay#External_linkshttp://en.wikipedia.org/wiki/Solid-state_relay#Referenceshttp://en.wikipedia.org/wiki/Solid-state_relay#See_alsohttp://en.wikipedia.org/wiki/Solid-state_relay#Imageshttp://en.wikipedia.org/wiki/Solid-state_relay#Disadvantageshttp://en.wikipedia.org/wiki/Solid-state_relay#Advantages_over_mechanical_relayshttp://en.wikipedia.org/wiki/Solid-state_relay#Parametershttp://en.wikipedia.org/wiki/Solid-state_relay#Operationhttp://en.wikipedia.org/wiki/Solid-state_relay#Couplinghttp://en.wikipedia.org/wiki/SPST_switchhttp://en.wikipedia.org/wiki/Current_ratinghttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#P-type_semiconductorshttp://en.wikipedia.org/wiki/Extrinsic_semiconductor#N-type_semiconductors

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It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if

switching a DC load. Usually a network is provided to speed the turn-off of the MOSFET when the control

input is removed.

One significant advantage of a solid-stateSCRorTRIACrelay over an electromechanical device is its

natural tendency to open the AC circuit only at a point of zero load current. Because SCR's and TRIAC's are

thyristors, their inherent hysteresis maintains circuit continuity after the LED is de-energized until the AC

current falls below a threshold value (the holding current). In practical terms what this means is the circuit

will never be interrupted in the middle of a sine wave peak. Such untimely interruptions in a circuit

containing substantial inductance would normally produce large voltage spikes due to the suddenmagnetic

fieldcollapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This

feature is called zero-crossover switching.[1]

Parameters

This section requiresexpansion.(September 2010)

SSRs are characterised by a number of parameters including the required activating input voltage, current,

output voltage and current, whether it is AC or DC,voltage dropor resistance affecting output current,

thermal resistance,and thermal and electrical parameters forsafe operating area(e.g.,deratingaccording to

thermal resistance when repeatedly switching large currents).

Advantages over mechanical relays

Most of the relative advantages of solid state and electromechanical relays are common to all solid-state as

against electromechanical devices.

Slimmer profile, allowing tighter packing.

Totally silent operation SSRs are faster than electromechanical relays; their switching time is dependent on the time needed

to power the LED on and off, of the order of microseconds to milliseconds

Increased lifetime, even if it is activated many times, as there are no moving parts to wear and no

contacts to pit or build up carbon

Output resistance remains constant regardless of amount of use

Clean,bouncelessoperation

No sparking, allows it to be used in explosive environments, where it is critical that no spark is

generated during switching

Inherently smaller than a mechanical relay of similar specification (if desired may have the same

"casing" form factor for interchangeability).

Much less sensitive to storage and operating environment factors such asmechanical shock,

vibration,humidity, and external magnetic fields.

Disadvantages

Voltage/current characteristic of semiconductor rather than mechanical contacts:

o When closed, higher resistance (generating heat), and increased electrical noise

o When open, lower resistance, andreverse leakage current(typically A range)

o Voltage/current characteristic is not linear (not purely resistive), distorting switched

waveforms to some extent. An electromechanical relay has the low ohmic (linear) resistance

of the associated mechanical switch when activated, and the exceedingly high resistance ofthe air gap and insulating materials when open.

o Some types have polarity-sensitive output circuits. Electromechanical relays are not affected

by polarity.

http://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/TRIAChttp://en.wikipedia.org/wiki/TRIAChttp://en.wikipedia.org/wiki/TRIAChttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Solid-state_relay#cite_note-1http://en.wikipedia.org/wiki/Solid-state_relay#cite_note-1http://en.wikipedia.org/wiki/Solid-state_relay#cite_note-1http://en.wikipedia.org/w/index.php?title=Solid-state_relay&action=edithttp://en.wikipedia.org/w/index.php?title=Solid-state_relay&action=edithttp://en.wikipedia.org/w/index.php?title=Solid-state_relay&action=edithttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Thermal_resistancehttp://en.wikipedia.org/wiki/Thermal_resistancehttp://en.wikipedia.org/wiki/Safe_operating_areahttp://en.wikipedia.org/wiki/Safe_operating_areahttp://en.wikipedia.org/wiki/Safe_operating_areahttp://en.wikipedia.org/wiki/Deratinghttp://en.wikipedia.org/wiki/Deratinghttp://en.wikipedia.org/wiki/Deratinghttp://en.wikipedia.org/wiki/Contact_bouncehttp://en.wikipedia.org/wiki/Contact_bouncehttp://en.wikipedia.org/wiki/Contact_bouncehttp://en.wikipedia.org/wiki/Shock_%28mechanics%29http://en.wikipedia.org/wiki/Shock_%28mechanics%29http://en.wikipedia.org/wiki/Shock_%28mechanics%29http://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Reverse_leakage_currenthttp://en.wikipedia.org/wiki/Reverse_leakage_currenthttp://en.wikipedia.org/wiki/Reverse_leakage_currenthttp://en.wikipedia.org/wiki/File:Wiki_letter_w_cropped.svghttp://en.wikipedia.org/wiki/Reverse_leakage_currenthttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Shock_%28mechanics%29http://en.wikipedia.org/wiki/Contact_bouncehttp://en.wikipedia.org/wiki/Deratinghttp://en.wikipedia.org/wiki/Safe_operating_areahttp://en.wikipedia.org/wiki/Thermal_resistancehttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/w/index.php?title=Solid-state_relay&action=edithttp://en.wikipedia.org/wiki/Solid-state_relay#cite_note-1http://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/TRIAChttp://en.wikipedia.org/wiki/Silicon-controlled_rectifier

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Possibility of spurious switching due to voltage transients (due to much faster switching than

mechanical relay)

Isolated bias supply required for gate charge circuit

Higher transient reverse recovery time (Trr) due to the presence of the body diode

Tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail

"open".

Images

Miniature solid state relay. Solid state contactor.

See also

Solid State Relay Inventor and Early Patents

Push-button telephone

References

1.

allaboutcircuits.com

External links

A DC Fault Protection Circuit for Audio Amplifiers

Solid state relay glossary of terminology

National Instruments Solid State Relays

Solid State Relays vs Electromechanical Relays

Categories:

Solid state switches

Relays

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