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A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function as an electromechanical relay, but solid-state electronics contain no moving parts and have a longer operational lifetime. Solid state relays were invented in 1971 by the Crydom Controls division of International Rectifiers.[ non-primary source needed ] [1]
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 AC or DC loads. Packaged SSRs use power semiconductor devices such as thyristors and transistors, to switch currents up to around a hundred amperes. SSRs have fast switching speeds compared with electromechanical relays, and have no physical contacts to wear out. SSRs are unable to withstand a large momentary overload the way an electromechanical relay can, and have a higher "on" resistance.
In AC circuits, SCR or triac 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 magnetic field around the inductance. With the addition of a zero-point detector (and no adverse circuit inductance and resultant back EMF), the individual SCRs can be switched back on at the start of a new wave. This feature is called zero-crossing, or zero-crossover, switching.
An 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.[ vague ]
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.
SSRs for DC switching applications may use MOSFETs or IGBTs.
Selection of the appropriate type of SSR [2] is important, especially when the application calls for time critical On/Off condition with no variation.
Applications which require Time Critical On/Off switching, should use Transistor or MOSFET design types as they are not subject to the inherent Zero Cross variations that SCR or TRIAC devices will exhibit.
The control signal must be coupled to the controlled circuit in a way which provides galvanic isolation between the two circuits.
Many SSRs use optical coupling. The control voltage energizes an internal LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor (TRIAC), SCR, or MOSFET to switch the load. The optical coupling allows the control circuit to be electrically isolated from the load.
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, 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). SSRs can also include zero crossing 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).
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily forming an electronic switch. It was developed to combine high efficiency with fast switching. It consists of four alternating layers (NPNP) that are controlled by a metal–oxide–semiconductor (MOS) gate structure.
A thyristor is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high power applications like inverters and radar generators. It usually consists of four layers of alternating P- and N-type materials. It acts as a bistable switch. There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its gate lead controls the larger current of the anode-to-cathode path. In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large. The thyristor continues conducting until the voltage across the device is reverse-biased or the voltage is removed, or through the control gate signal on newer types.
A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum, Goldey, and Holonyak of Bell Laboratories in 1956. The practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.
A TRIAC is a three-terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised trademark.
The DIAC is a diode that conducts electrical current only after its breakover voltage, VBO, has been reached momentarily. Three, four, and five layer structures may be used. Behavior is similar to the voltage breakdown of a triac without a gate terminal.
A voltage regulator is a system designed to automatically maintain a constant voltage. It may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
Zero-crossing control is an approach for electrical control circuits that starts operation with the AC load voltage at close to 0 volts in the AC cycle. This is in relation to solid-state relays, such as TRIACs and silicon controlled rectifiers. The purpose of the circuit is to start the TRIAC conducting very near the time point when the load voltage is crossing zero, so that the output voltage begins as a complete sine-wave half-cycle. In other words, if the controlling input signal is applied at any point during the AC output wave other than very close to the zero-voltage point of that wave, the output of the switching device will "wait" to switch on until the output AC wave reaches its next zero point. This is useful when sudden turn-on in the middle of a sine-wave half-cycle could cause undesirable effects like high-frequency spikes, for which the circuit or the environment is not expected to handle gracefully.
A power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics. Such a device is also called a power device or, when used in an integrated circuit, a power IC.
Power electronics is the application of electronics to the control and conversion of electric power.
A snubber is a device used to suppress a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems or excess force or rapid movement in mechanical systems.
An electronic component is any basic discrete electronic device or physical entity part of an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements. A datasheet for an electronic component is a technical document that provides detailed information about the component's specifications, characteristics, and performance. Discrete circuits are made of individual electronic components that only perform one function each as packaged, which are known as discrete components, although strictly the term discrete component refers to such a component with semiconductor material such as individual transistors.
Phase-fired control (PFC), also called phase cutting or phase-angle control, is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase angle of the applied waveform.
A buck converter or step-down converter is a DC-to-DC converter which decreases voltage, while increasing current, from its input (supply) to its output (load). It is a class of switched-mode power supply. Switching converters provide much greater power efficiency as DC-to-DC converters than linear regulators, which are simpler circuits that dissipate power as heat, but do not step up output current. The efficiency of buck converters can be very high, often over 90%, making them useful for tasks such as converting a computer's main supply voltage, which is usually 12 V, down to lower voltages needed by USB, DRAM and the CPU, which are usually 5, 3.3 or 1.8 V.
An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. The name is derived from its common schematic diagram representation, with four switching elements configured as the branches of a letter "H" and the load connected as the cross-bar.
In electronics, an electronic switch is a switch controlled by an active electronic component or device. Without using moving parts, they are called solid state switches, which distinguishes them from mechanical switches.
A QUADRAC is a special type of thyristor which combines a DIAC and a TRIAC in a single package. The DIAC is the triggering device for the TRIAC. Thyristors are four-layer (PNPN) semiconductor devices that act as switches, rectifiers or voltage regulators in a variety of applications. When triggered, thyristors turn on and become low-resistance current paths. They remain so even after the trigger is removed, and until the current is reduced to a certain level. Diacs are bi-directional diodes that switch AC voltages and trigger triacs or silicon-controlled rectifiers (SCRs). Except for a small leakage current, diacs do not conduct until the breakover voltage is reached. Triacs are three-terminal, silicon devices that function as two SCRs configured in an inverse, parallel arrangement. They provide load current during both halves of the AC supply voltage. By combining the functions of diacs and triacs, QUADRACs eliminate the need to buy and assemble discrete parts.
The following outline is provided as an overview of and topical guide to electronics:
This glossary of power electronics is a list of definitions of terms and concepts related to power electronics in general and power electronic capacitors in particular. For more definitions in electric engineering, see Glossary of electrical and electronics engineering. For terms related to engineering in general, see Glossary of engineering.