230:, 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.
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SSRs consist of a sensor which responds to an appropriate input (control signal), an 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. They may be designed to switch either
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relays inherently switch off at the points of AC zero cross when there is zero load current. The circuit will never be interrupted in the middle of a sine wave peak, preventing the large transient voltages that would otherwise occur due to the sudden collapse of the
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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.
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SSRs switch faster than electromechanical relays; the switching time of a typical optically coupled SSR is dependent on the time needed to power the LED on and off - on the order of microseconds to milliseconds.
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hardware to only turn the voltage on or off when the AC voltage is at zero. Proportional SSRs can delay the onset of voltage after the zero crossing in order to lower the current output (phase angle control).
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Applications which require Time
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Most of the relative advantages of solid state relays over electromechanical relays are common to all solid-state devices when compared to electromechanical devices.
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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,
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Selection of the appropriate type of SSR is important, especially when the application calls for time critical On/Off condition with no variation.
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and have a longer operational lifetime. Solid state relays were invented in 1971 by the Crydom
Controls division of International Rectifiers.
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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.
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according to thermal resistance when repeatedly switching large currents). SSRs can also include
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High-power solid-state electronic switching device responding to an external control signal
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to switch the load. The optical coupling allows the control circuit to be
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Many SSRs use optical coupling. The control voltage energizes an internal
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522:Solid State Relays vs Electromechanical Relays
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512:Solid state relay glossary of terminology
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537:Solid state switches
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416:"Solid State Relays"
47:improve this article
480:"Solid State Relay"
375:safe operating area
217:zero-point detector
62:"Solid-state relay"
371:thermal resistance
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425:2023-02-01
402:References
326:Parameters
321:operation.
319:bounceless
184:thyristors
73:newspapers
278:thyristor
198:Operation
43:talk page
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390:See also
379:derating
257:Coupling
221:back EMF
146:) is an
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317:Clean,
192:amperes
152:voltage
87:scholar
542:Relays
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290:MOSFET
245:Timing
228:MOSFET
158:, but
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282:TRIAC
239:IGBTs
208:triac
94:JSTOR
80:books
455:OCLC
445:ISBN
186:and
66:news
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286:SCR
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270:LED
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