Solid State Relays SSR

SSR Defined. 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. (The SCR and TRIAC are often called
“thyristors,” a term derived by combining thyratron and
transistor, since thyristors are triggered semiconductor
Like all relays, 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 than in an electromagnetic relay of comparable
output rating. To put it another way, the sensitivity of an
SSR is often significantly higher than that of an EMR of
comparable output rating.
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 (see figure 1), 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.
• Transformer-Coupled SSR's (see figure 2), 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.