Solid state contactor PCB mount solid-state DIL relay A solid state relay (SSR) is an electronic switching device that switches on or off when an external

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

(AC or DC) is applied across its control terminals. They serve the same function as an

electromechanical relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...

, but as

solid-state electronics Solid-state electronics means semiconductor electronics: electronic equipment using semiconductor devices such as transistors, diodes and integrated circuits (ICs). The term is also used as an adjective for devices in which semiconductor electr ...

contain no

moving parts Machines include both fixed and moving parts. The moving parts have controlled and constrained motions. Moving parts are machine components excluding any moving fluids, such as fuel, coolant or hydraulic fluid. Moving parts also do not include ...

and have a longer operational lifetime. 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 A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

devices such as

thyristor A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...

s and

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

s, to switch currents up to around a hundred

amperes The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...

. 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, as well as having a higher "on" resistance.

Operation

In AC circuits, SCR or

TRIAC A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised tradem ...

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 A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

around the inductance. With the addition of a zero-point detector (and no adverse circuit inductance and resultant back-e.m.f.), 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 The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

, 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. 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.

Timing

Selection of the appropriate type of SSR 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.

Coupling

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

galvanic isolation Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.John Huntington ''Show Networks and Control Systems: Formerly Control Systems for Live E ...

between the two circuits. Many SSRs use optical coupling. The control voltage energizes an internal

LED A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...

which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back

(

), SCR, or

to switch the load. The optical coupling allows the control circuit to be electrically isolated from the load.

Characteristics

Most of the relative advantages of solid state relays over electromechanical relays are common to all solid-state devices when compared to electromechanical devices. * Totally silent operation. * 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. * 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. * Clean, bounceless operation.

Parameters

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 Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable ...

or resistance affecting output current,

thermal resistance Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. * (Absolute) thermal resistance ''R'' in kelvin ...

, and thermal and electrical parameters for

safe operating area For power semiconductor devices (such as BJT, MOSFET, thyristor or IGBT), the safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage. SOA is usually presented ...

(e.g.,

derating In electronics, derating (or derating) is the operation of a device at less than its rated maximum capability to prolong its life. Typical examples include operations below the maximum power rating, current rating, or voltage rating. In electron ...

according to thermal resistance when repeatedly switching large currents). SSRs can also include

zero crossing A zero-crossing is a point where the sign of a mathematical function changes (e.g. from positive to negative), represented by an intercept of the axis (zero value) in the graph of the function. It is a commonly used term in electronics, mathemat ...

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).

References

{{refs

External links

A DC Fault Protection Circuit for Audio Amplifiers

National Instruments Solid State Relays

Solid State Relays vs Electromechanical Relays
Solid state switches Relays de:Relais#Halbleiterrelais