A solid state relay is an ON-OFF type of electronic control device. The load current in a solid state relay is conducted by one or more semiconductors like a thyristor.

Specifying a Solid State Relay

Consideration must be given to:

  • Load Voltage - AC or DC, Line Voltage.
  • Load Current - AC or DC, Max and Min.
  • Input Voltage - AC or DC, Max and Min.
  • Ambient Temperature - Required for Derating and heatsink calculations.
  • Mounting Style - PCB, Chassis or DIN Rail Mounting
  • International Approvals - UL, CSA, VDE, TUV, etc.

Terminologies

SCR: A semiconductor which is made from silicon used to convert AC power to DC which can be used to control or rectify AC power semiconductors.

Diode: A semiconductor which allows electric current to flow easily in one direction but not in the other.

Rectifier: A semiconductor used to convert AC power to DC. It allows current to flow in one direction and prevents the flow of current in the opposite direction.

Discrete Device: An individual electrical component packaged or unpackaged, such as a resistor, capacitor, transistor, diode, etc.

Heat Sink: A material with good heat conducting or dissipating properties to which a packaged device is attached to cool and maintain the output semiconductor junctions within the rated temperature range.

Trigger: To turn on an SCR utilizing its gate.

Thyristor: A term for the general class of semiconductor devices which includes SCR's, triacs, alternistors, etc.

Anode: Positive terminal of an SCR or Diode. Positive with respect to cathode and or gate terminals.

Cathode: Negative terminal of an SCR or Diode with respect to the anode

Typical Applications for Solid State Relays

All motors need some form of switching for control. SSRs offer a high reliability alternative to electro-mechanical contactors.

Switching solenoids

Issuing of product
Issuing of cups
Issuing of liquids

Controlling Heaters

Commercial Coffee Machines
Commercial / Industrial Cooking Equipment
Plastics Industry (extrusion / thermoforming)
Plastics Industry (drying)
Packaging Industry
Heat Tracing
Industrial Cleaning Equipment
Solder Wave / Reflow Systems

HVAC&R

Compressor Control
Blower Control
Motorized Duct / Vent Control
Anti-Condensation Equipment

Lighting Control Systems

Dramatic Theatrical Effects
Mobile Stage Shows
Traffic Signal Systems
Motorway Information Systems
Lifts & Escalators
Automatic Door Operation

High Reliability

Medical Equipment
Low Switching Noise
Low Electromagnetic Interference