Abstract—the paper discusses that Solid-state relays (SSR) are able to perform many of the same tasks as electromechanical relay (EMRs). The main difference is that SSRs have no moving mechanical parts within it. SSRs use a control circuit and a separate circuit for switching the load. The relay is energized by a light-emitting diode. The types of SSR are photo-coupled SSR, transformer-coupled SSR, and hybrid SSR.

I-INTRODUCTION: Solid State Relays (SSRs) are switching devices consisting of electronic components. The term "Solid State" means that these relays do not incorporate any moving parts in the load switching circuit. Often, solid state relays are used where the circuit under control must be protected from the introduction of electrical noises. [1]

II-DEFINITION: A solid state relay (SSR) is an electronic switch, which contains no moving parts. Essentially, it is an electronic device that relies on the electrical, magnetic, and optical properties of semiconductors and electrical components to achieve its isolation and relay switching function [2].The SSR requires relatively low control circuit energy to switch the output state from OFF to ON, or vice versa. Since this control energy is very much lower than the output power controllable by the relay at full load, "power gain" in an SSR is substantial--frequently much higher. Being solid-state devices, there are no moving parts to wear out, and they are able to switch on and off much faster than any mechanical relay armature can move. There is no sparking between contacts, and no problems with contact corrosion. [3]

III-PRINCIPLE OF OPERATION: These active semiconductor devices use light instead of magnetism to actuate a switch. The light comes from an LED, or light emitting diode. When control power is applied to the device’s output, the light is turned on and shines across an open space. On the load side of this space, a part of the device senses the presence of the light, and triggers a solid state switch that either opens or closes the circuit under control. [4]

IV-TRIACS: A triac is an electronic component approximately equivalent to two silicon-controlled rectifiers joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. This results in a bidirectional electronic switch that can conduct current in either direction. The triac is ideal for switching resistive AC loads. [5]
SSRs use a control circuit and a separate circuit for switching the load. The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. When voltage is applied to the input of the SSR, the relay is energized by a light-emitting diode. The light from the diode is beamed into a light-sensitive semiconductor that, in the case of zero-voltage crossover relays, conditions the control circuit to turn on the output solid-state switch at the next zero-voltage crossover. In the case of nonzero-voltage crossover relays, the output solid-state switch is turned on at the precise voltage occurring at the time. Removal of the input power disables the control circuit and the solid-state switch is turned off when the load current passes through the zero point of its cycle. [6]

• When applied properly, Solid State Relays (SSRs) will last millions of cycles due to the fact that no mechanical devices are included in the load switching circuit. The user will experience no arcing problems, and operation will be trouble free, even in polluted environments (dust, gasses, etc.) SSRs are a faster alternative to electromechanical relays because their switching time is dependent on the time required to power the LED on and off - approximately 1 ms and 0.5 ms respectively. Because there are no mechanical parts, their life expectancy is higher than an electromechanical relay.
• One significant advantage of a solid-state SCR or TRIAC relay 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 sudden magnetic field collapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching. [7]
• Additionally, SSRs are completely silent in their operation given their purely electronic nature.
• Increased lifetime due to the fact that there are no moving parts, and thus no wear
• Clean, bounce less operation
• Decreased electrical noise when switching
• Can be used in explosive environments where a spark must not be generated during turn-on
• Totally silent operation
• Smaller than a corresponding mechanical relay.
• Can continue to operate while subjected to severe vibration.

• One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to fail in the other mode, but these are the most common failures. Because a "fail-open" state is generally considered safer than a "fail-closed" state, electromechanical relays are still favored over their solid-state counterparts in many applications. [8]
• Fail short more easily than electro-mechanical relays
• Increased electrical noise when conducting
• Higher impedance when closed (-> heat production)
• Lower impedance when open
• Reverse leakage current when open (µA range)
• Possibility of false switching due to voltage transients
• Isolated bias supply required for gate charge circuit
• Higher Transient Reverse Recovery time (Trr) due to the presence of Body diode.

VII-TYPES OF SRR’s: It is convenient to classify SSR's by the nature of the input circuit, with particular reference to the means by which input-output isolation is achieved. Three major categories are recognized:
A-Reed-Relay-Coupled SSR’s in which the control signal is applied (directly, or through a preamplifier) to the coil of a reed relay. The closure of the reed switch then activates appropriate circuitry that triggers the thyristor switch. Clearly, the input-output isolation achieved is that of the reed relay, which is usually excellent.
B-Transformer-Coupled SSR’s in which the control signal is applied (through a DC-AC converter, if it is DC, or directly, if It is AC) to the primary of a small, low-power transformer, and the secondary voltage that results from the primary excitation is used (with or without rectification, amplification, or other modification) to trigger the thyristor switch. In this type, the degree of input-output isolation depends on the design of the transformer.
C-Photo-coupled SSR’s in which the control signal is applied to a light or infrared source (usually, a light-emitting diode, or LED), and the radiation from that source is detected in a photosensitive semi-conductor (i.e., a photosensitive diode, a photo-sensitive transistor, or a photo-sensitive thyristor). The output of the photo-sensitive device is then used to trigger (gate) the TRIAC or the SCR's that switch the load current. Clearly, the only significant “coupling path” between input and output is the beam of light or infrared radiation, and electrical isolation is excellent. These SSR's are also referred to as “optically coupled” or “photo-isolated. [9]

IX-Thermal considerations: one of the major considerations when using an SSR is properly managing the heat that is generated when switching currents higher than about 5 amperes (A). In this case, the base plate of the SSR should be mounted onto a good heat conductor, like aluminum, and used with a good thermal transfer medium like thermal grease or a heat transfer pad. Using this technique, the SSR case to heat sink thermal resistance is reduced to a negligible value of 0.1°C/W.

X-Load considerations: a leading cause of application issues with SSRs is improper heat sinking. Problems can also arise from operating conditions that specific loads impose upon an SSR. The surge characteristics of the load should be carefully considered when designing in a SSR as a switching solution.

XI-Resistive loads: Loads of constant values of resistance are the simplest application of SSRs. Proper thermal consideration, along with attention to the steady state current ratings, will result in trouble-free operation.

XII-DC loads: this type of load should be considered inductive, and a diode should be placed across the load to absorb any surges during turn-off.

XIII-Lamp loads: Incandescent lamp loads, though basically resistive, can present some challenges. Because the resistance of the cold filament is about 5 to 10 percent of the heated value, a large inrush current can occur. It is essential to verify that this inrush current is within the surge specifications of the SSR. One must also check that the lamp rating of the SSR is not exceeded. Due to the unusually low filament resistance at the time of turn-on, a zero-voltage turn-on characteristic is particularly desirable with incandescent lamps.

XIV-Capacitive loads: these types of loads can prove to be problematic because of their initial appearance as short circuits. High surge currents can occur while charging, limited only by circuit resistance. Caution must be used with low-impedance capacitive loads to verify that the di/dt capabilities are not exceeded. Zero-voltage turn-on is a particularly valuable means of limiting di/dt with capacitive loads.

XV-Motors and solenoids: Motor and solenoid loads can create challenges for reliable SSR functionality. Solenoids have high initial surge currents because their stationary impedance is very low. Motors also frequently have severe inrush currents during starting and can impose unusually high voltages during turn-off. As a motor’s rotor rotates, it creates a back EMF that reduces the flow of current. This back EMF can add to the applied line voltage and create an overvoltage condition during turn-off. Likewise, the inrush currents associated with mechanical loads having high starting torque or inertia, such as fans and flywheels, should be carefully considered to verify that they are within the surge capabilities of the SSR. A current shunt and oscilloscope should be used to examine the duration of the inrush current. [10]

XVI-APPLICATIONS: The SSR is usually used in applications where there is a need to separate high voltage circuits from the low voltage or low power circuits. By using SSR, circuits can exchange signals, and at the same time, they are galvanically isolated. The SSR allows for a safe interface between the high voltage and low voltage circuits. It breaks the ground loop to eliminate cross talk and interference between the high voltage and low voltage circuit.
The SSR has been extensively used in the telephone set, modems, fax machine, PBX or central office equipments. In telephone applications, it is always necessary to isolate the telephone equipments from the incoming telephone lines. Isolation is important to protect the electronic equipments from harmful voltages or current caused by lightning. SSR, with high input-output transient rejection specifications, can provide good isolation and surge protection.
Since its introduction, the SSR has gained acceptance in many areas that had previously been the sole domain of the EMR or the contactor. The SSR is increasingly employed in industrial process control applications, particularly temperature control, motors, lamps, solenoids, valves and transformers. The list of specific applications for the SSR is broad. [11]
Typical examples of SSR applications include:
• Industrial automation
• Electronic appliances
• Industrial appliances
• Packaging machines
• Tooling machines
• Manufacturing equipment
• Food equipment
• Security systems
• Industrial lighting
• Fire and security systems
• Dispensing machines
• Production equipment
• On-board power control
• Traffic control
• Instrumentation systems
• Vending machines
• Test systems
• Office machines
• Medical equipment
• Display lighting
• Elevator control
• Metrology equipment
• Entertainment lighting[12]
XVII-CONCLUSIONS: DC output solid state relays give designers a great deal of flexibility in the relative connections of control and load voltages. Care must be taken when a solid state relay is used to control and inductive load, to ensure that at turn off, the voltage rating of the solid sate relay is not exceeded. SSRs are highly susceptible to surge currents and damage when used at signal levels above their rating. Although there are no metal contacts to weld, damage to the MOSFET can render the relay unusable. SSRs are a nice alternative to mechanical relays but have higher path resistances and are not fully isolated between the contacts.

[1] Technical letters, National Plastic heater sensor and control Inc.
[2] By TJ Landrum, Product Manager, Eaton, Research book.
[3] Tony R. Kuphaldt, Lessons in Electric Circuits, Volume IV – Digital, ch 5 pg 142
[4] Anthony Bishop, Solid-state relay handbook with applications‎ - 224 page
[5] By TJ Landrum, Product Manager, Eaton, Research book.
[6] By TJ Landrum, Product Manager, Eaton, Research book.
[7] Tony R. Kuphaldt, Lessons In Electric Circuits, Volume IV – Digital, ch 5 pg 142
[8] Tony R. Kuphaldt, Lessons in Electric Circuits, Volume IV – Digital, ch 5 pg 143
[9] Research book, Relays defined and described by Crydom Corporation
[10] By TJ Landrum, Product Manager, Eaton, Research book.
[12] By TJ Landrum, Product Manager, Eaton, Research book.

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