Solid State Relays from Sords Electric
What are they and how they work in industry.
A Solid State Relay (SSR) is an electronic power switch that is controlled by a separate control input.
With no control signal to the control inputs, the output switch will be open and the output circuit will be off. Therefore NO power will flow to the load.
When the proper control signal is applied the the control inputs, the output switch closes, completing the output circuit. Power will FLOW to the load.
This type of control of an SSR is known as On-Off control and is similar to how a mechanical relay operates. For example; Mercury relays and electro-mechanical relays (contactors).
However, Solid State Relays have several advantages over mechanical relays (contactors).
These advantages include: No moving parts, no audible noise, longer life, No arcing, higher switching speeds, and shock & vibration resistance.
On-Off Solid State Relays (SSRs) come in two types, Zero Switching (Zero Cross) and Instant On.
ZERO Switching SSRs switch ON at the first zero crossing point of the AC sine wave after the application of the control voltage. And conversely the SSR switches off at the first zero crossing after control power is removed.
Random switching of instant ON SSRs switch ON instantly with the control voltage is applied to the circuit no matter the place on the sine wave where is voltage is.
Another type of switching is Peak switching where the SSR (Solid State Relay) turn on at the first PEAK of the sine wave. Peak switching is mostly used for inductive loads and transformers.
Solid State Relays (SSR) can be used for DC switching as well. When voltage is applied the DC SSR switches on at the same instant as when the control voltage is applied to the SSR input. DC SSRs are used with resistive loads for the control of DC motors and valves.
ANALOG switching Solid State Relays are used where very tight control is needed in a process. These SSRs require an analog signal for switching. The most common signals are 4 to 20mA, 0-10 vdc, 0-5 vdc and 1-5 vdc. These SSRs are called phase angle solid stat relays and are commonly used in resistive or slightly inductive loads including lighting and HVAC loads. Soft start can be utilized with analog switching SSRs.
Another similar SSR firing method to analog switching is burst or distributed switching. This kind of switching can be used with a PID controller providing the control signal to the SSR. A number of full cycles are switched on over a predetermined amount of time. Bust or distributed firing limits the limits the seen voltage over time to better control the load and even it out.
There are some limitations for SSRs.
SSRs generate heat so precautions must be made to keep the SSRs from over heating and failing with the use of Heat Sinks. Please review the heat sink charts for the Solid State Relay you plan to use to get the correct amount of cooling required. Some heat sinks can come with a fan. Also some SSRs come complete with a integral heat sink for plug and play application.
Fuses should be used for over current protection on SSRs.
A semiconductor fuse is best for the SSR protection. This type of fuse provides Type 2 protection coordination and has the smallest let-through (I2t) value. Due to UL 508 requirements, fast acting Class CC or J fuses are the most types used for SSRs. These fuses only provide Type 1 protection coordination and have a high let-through (I2t) value. Circuit breakers are typically not suitable protection for Solid State Relays because f their relatively slow reaction speed.
Industrial control panels are covered by the requirements in the Standard for Industrial Control Panels, UL 508A.
216-765-4230
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