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Solid-state relays (SSRs) provide a means of combining electronics with the function of isolation in an electromechanical relay. Since the coil of an electromechanical relay is completely isolated from its contacts, the input signal to the relay coil may he 110 volts ac and the output signal that goes through the contacts can be as small as 3 volts or 5 volts logic level. Another way to use the isolation is through a small coil voltage such as 5 volts dc and allow the contacts to switch 110 volts ac. The solid-state relay is made in a similar design in that its input is isolated from its output. ill. 1 shows a diagram of two different types of solid-state relays. AMAZON multi-meters discounts AMAZON oscilloscope discounts |
Above: ill. 1:Solid-state relays (SSRs) that
use silicon controlled rectifiers (SCRs) and triacs for isolation. Newer
SSRs use optocoupler devices to provide isolation. These devices mimic
the action of a relay coil and its contacts.
The original types of SSRs use a silicon controlled rectifier (SCR) if it's rated for dc voltage usage, and it uses a triac if it's rated for ac voltage usage. The SCR and triac are discussed in detail in other areas of this site. These devices required a small gate voltage to operate. When the small gate voltage was applied, they could switch larger voltages. Newer SSRs utilize a photocoupled device that includes a light- emitting diode (LED) mounted so that its light shines directly on a photo device such as a photodiode or phototransistor. The LED and photo device are encapsulated into a single IC so that outside light does not interfere with their operation. When the photo device detects light shining on it, it will go into conduction and pass current and voltage to whatever is connected to it. Some devices use miniature relays as the output device to provide another layer of isolation, while other devices use solid-state components such as metal oxide semiconductor field effect transistors (MOSFETs). In each case the solid-state relay provides an effective means to produce an isolation circuit that can be used for logic functions.
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