A solid state relay (SSR) is a control device that contains no mechanical moving parts, unlike an electromechanical relay (EMR). It is composed of a sensor (for voltage or current) that receives an input signal and an electronic switching device.
The control signal activates the switch without moving parts. The current is conducted by a power transistor, an SCR (Silicon Controlled Rectifier), or a TRIAC (Triode for Alternating Current).
Unlike electromechanical relays (EMR), SSR produce no arcs or contact bounce and are immune to vibrations and oxide formation. Also, an SSR has a higher power gain than an EMR. Power gain is an important characteristic of the circuit, since the control signal has lower power than the controllable output signal. Furthermore, a solid state relay yields longer switching life.
Types of Solid State Relays
There are three types of SSR: (i) hybrid, (ii) transformed-coupled and (iii) photo-coupled.
Hybrid Solid State Relay (Reed-Relay-Coupled SSR). A control signal is applied to a reed relay which has an electromagnet that controls the reed switches. A reed switch is a device that is closed in the presence of a magnetic field and open otherwise, or vice-versa. The closure of the reed switch triggers the thyristor switch.
Transformer-Coupled SSR. In this type of SSR the input signal is applied to the primary of a low-power transformer. If this is an AC input, it must be applied through an AC-DC converter. The secondary voltage of the transformer is used to trigger the thyristor switch.
Photo-Coupled SSR. In a photo-coupled SSR, a control input is applied to a light or infrared source, whose radiation activates a photo-sensitive semiconductor (diode). The output signal of the photo-sensitive device triggers the TRIAC or the SCR that switches the load current. The diode may also switch the load through a MOSFET transistor.
Solid State Timers and Time Delay Relays
Time delay relays and solid state timers are devices used to provide a time delay. The most common types of timers are: (i) ON-delay and (ii) OFF-delay timers.
The application of a signal initiates the time delay. At the end of the delay, the load is energized or de-energized, depending on the normal state of the contacts (open or closed, respectively).
The removal of the input power causes the timer contacts to open or close, depending on their normal state.
Other types of solid state timers are:
Single-shot timers. The application of power causes the contacts to change position and remain so for the specified time delay. It is measured one output pulse for each rising-edge transition in the input.
Interval timers. The application of power initiates time delay, during which the output is energized.
Recycle timer. As long as power is applied the load is energized and de-energized at regular intervals.
Time delay relays and solid state timers are used in industrial control circuits, such as engine auto-start control, motor soft-start delay control and flashing light control.
GlobalSpec Supplier Catalog - Time Delay Relays and Solid State Timers
Precsion Timer Co., Inc - Solid State Timers
All About Circuits - Time-delay Relays
Electrical Construction and Maintenance Magazine - Basics of Time Delay Relays (TDRs)