Image credit: Allen-Bradley / Rockwell Automation | Opto 22 | CCI Thermal Technologies Inc.
A solid state relay (SSR) is an electrically operated switch with no moving components.
Understanding Solid State Relays
Solid state relays are electrically operated switches used to isolate circuits or batteries, detect faults on transmission and distribution lines, and control large amounts of power using a smaller control signal. Unlike electromechanical relays, solid state relays are constructed of solid components such as transistors, resistors, and capacitors. Solid state relays consist of two circuits: a low voltage control circuit and a higher voltage load (switching) circuit.
Relay schematic showing the input and load circuits. Image credit: Futurlec
Because they have no mechanical contacts to wear out, solid state relays are well-suited to applications requiring frequent switching, such as use in a temperature controller. Solid state relays are also quieter than electromechanical devices due to their silent operation, and do not generate sparks when switching. They do tend to be more expensive than other relay types and often generate heat, sometimes requiring the use of a corresponding heatsink.
Type and Application
Solid state relays may be classified by device type as well as intended application.
Solid state relays may be optically isolated or non-isolated. An optically isolated relay does not have a direct electrical contact between the input (control) and output (switching) circuits, whereas a non-isolated devices does have this connection.
Aerospace / MIL-SPEC
Relays may be manufactured for aerospace and defense applications, and may also meet a number of U.S. military standards (MIL-SPEC). Common standards for relays include MIL-PRF-39016, M83536, and M83726.
Various types of relays may be applied to different uses, including automotive, general purpose, and latching applications.
Solid state relays may be mounted using a number of different methods.
Bracket (or flange) mounted relays are equipped with a flange for mounting. The flange is typically installed by bolting the device to a matching flanged which is then welded to a corresponding wall.
DIN rail mounted devices are equipped with a fastener capable of mounting on a DIN rail. DIN rails are mounting devices standardized by the Deutsches Institut fur Normung (DIN).
Panel mount relays are manufactured for mounting to an electrical panel.
PCB relays are mounted on printed circuit boards (PCB) using through hole contacts or surface mount technology (SMT). Socket relays are mounted to PCBs using pin sockets.
Specifications related to a solid state relay's switches are important to consider when selecting a product.
Solid state relays may switch if one of several conditions is met:
Instant on devices immediately turn on the load when a pickup voltage is present, and are often used to control inductive loads.
Peak switching devices turn on when the control voltage is present and the pickup voltage peaks. They are typically used to control heavy duty inductive loads.
Zero switching relays switch at the point where the AC load current passes through zero as it changes polarity. These relays are used to control resistive loads.
Poles and Throws
The term "pole" describes the number of separate circuits controlled by a switch. The number of circuits controlled by the relay determines the number of switch contacts, which in turn determines the poles needed to make or break the contacts. Switches typically have between one and four poles.
The image series below illustrates, from left to right, a single pole (SP), double pole (DP), and triple pole switch (3P). Note that, in the last image, the switch is connected to three separate circuits and has three contacts.
Image credit: Enasco | Skycraft Surplus | Frank Alapini
It is also important to consider a relay switch's throws, or the number of distinct positions a switch is capable of.
Single throw (ST) switches are open in one position and closed in another. For example, a single pole single throw (SPST) switch is a simple on-off switch, such as a light switch. A double pole single throw (DPST) switch is an on-off switch that opens and closes two contacts with a single motion.
Double throw (DT) switches are two-way devices. Double throw relays have three contacts and two positions: in the first position, contacts 1 and 2 are in contact, but the third remains open. In the second position, this connection is reversed to contacts 2 and 3.
Solid state relay switches can be classified into three different types or "forms", based on the pole and throw information above.
Form A switches are SPST (single pole, single throw) and normally open (NO).
Form B switches are SPST and normally closed (NC).
Form C devices are SPDT (single pole, double throw) and are changeover (CO) switches.
Important input specifications include pick-up and dropout voltage.
When discussing solid state relays, input refers to the current used to switch the device on and off when applied. Relays can use either alternating current (AC) or direct current (DC) voltages as inputs. Buyers may be advised to enter the pick-up voltage range when selecting a product. This is the range of voltages that will maintain an "on" position for the relay's switch. The maximum end of this range also indicates the absolute voltage limit that the relay can withstand without damaging its components.
Conversely, dropout voltage is a single voltage point; when the input voltage reaches or drops below this point, the relay is guaranteed to be in an "off" position. Dropout voltage is also known as turn-off voltage.
Solid state relays output "on" or "off" switching signals using one of a variety of different internal components.
Metal-oxide-semiconductor field-effect transistors (MOSFET) are frequently used to switch electronic signals. MOSFETs feature a metal-oxide insulating layer between a gate electrode.
Bipolar junction transistors (BJT) are three terminal devices in which the base current controls the emitter current. BJTs have higher output resistance than MOSFETs and are a good choice for demanding analog circuits.
Silicone controlled rectifiers (SCR) are three terminal thyristors which act as gated diodes. The gate terminal allows current to pass from cathode to anode and turn on the device. SCRs are useful in high power and high voltage applications such as motor control.
Triacs are gate-controlled thyristors which are similar to SCRs but can conduct in both directions. Low power triacs are often used in applications such as light dimming, and in small and major appliances.
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