Solid State Relay

A solid-state relay is an ON-OFF control device in which the load current is conducted by one or more semiconductors

e.g., a power transistor, an SCR, or a TRIAC.

Types of SSR’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:

Reed-Relay-Coupled SSR’s 

When 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.

Transformer-Coupled SSR’s 

When 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.

Photo-coupled SSR’s

When 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 photo-sensitive semiconductor (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 infra-red radiation, and electrical isolation is excellent. These SSR’s are also referred to as “optically coupled” or “photo isolated”.