Understanding Solid State Relays (SSR)

Both zero-cross and random solid state relays are used for industrial applications. What are the differences between these two types of switching? Here are some answers.

Zero-crossing Solid State Relays

For ZERO VOLTAGE CONTROL (OR ZERO-CROSS RELAY), power switching only takes place at the beginning of the alternation after the control has been applied. In fact, switching the power component only takes place at close to zero volts.

For resistive or capacitive loads, it is preferable to use zero-cross relays which can limit the di/dt, disturbances on the network and increase the service life of the load and the relay.

Switching only at zero crossings will minimize or eliminate line and radiated noise resulting from initiating or terminating the AC output waveform in mid-cycle. However, designers need to be aware that zero-crossing SSRs may not be able to turn off with highly inductive loads. To account for this, celduc relais offers random-switching SSRs that turn on/off at the instant called for by the input transition. Again, the designer must understand the load and choose the appropriate SSR from celduc’s catalog.

Random Solid State Relays

For INSTANTANEOUS CONTROL (OR RANDOM RELAY), power switching takes place as soon as the control voltage has been applied (turn on time less than 100μs).

This type of control is used for all inductive loads where  the phase shift between voltage and current can cause problems with zero-crossing relays. It is also used in applications where precise control of power to the load is required (phase-control applications).

celduc range of Solid State Relays

celduc relais offers both zero-cross and random solid state relays.

SO8, SA8, SMT8, …. : Zero-Cross SSRs for all loads / heavy duty loads

SO9, SA9, SMT9, … : Zero-Cross SSRs for standard industrial loads / Resistive Loads aC-51

SO7, SA7, SGT7, … : Random SSRs