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Solid State Relays Basic Principles

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Solid State Relays and Contactors: how does it work?

What is a Solid State Relay (SSR)?

It is a switching device using electronic components. A SSR has absolutely no moving parts. Instead, it uses solid-state semiconductors’ electrical and optical properties in order to perform their input to output insulations and switching functions.

This technology is completely different from electromechanical relays (EMR). In order to switch on and off, EMRs use coils, magnetic fields, springs and mechanical contacts.

What does a celduc® Solid State Relay consist of?

Diagram showing what a Solid State Relay Consists of


A Solid State Relay is made of two main parts :

1- the input circuit

2- the switching circuit

An electrical insulation between the input and output contacts is also used.

1- The Input Circuit

The input circuit uses an optocoupler (or opto isolator).

An optocoupler is made of one (or more) infra-red light-emitting diode (corresponding to the transmitter), and a photo sensitive device (also named as photo triac, corresponding to the receiver) within a single housing (image below).


The role of the optocoupler is to isolate the input from the output. Celduc® made sure to select the best optocouplers on the market.

2- The Switching Circuit

The switching circuit is a component allowing the electrical power to be switched to the load.

  • For AC Solid State Relays, it can either be a triac or back-to-back thyristor (or Silicon Controlled Rectifier, SCR). celduc®’s back-to-back thyristors are using TMS² Technology, allowing celduc® SSR to have the best life-time expectancy compared to the majority of products on the market. This high life expectancy is also reachable thanks to celduc®’s Research & Development department who makes regular improvements on the production process, constantly increasing the cycle numbers.
  • For DC Solid State Relays the switching circuit can either be a bipolar transistor, a MOS transistor or a IGBT transistor.

The matching circuit guarantees the desired switching mode by processing the input signal received and switching the output circuit.

celduc®’s range of Solid State Relays offers different switching modes. Zero Cross, Random or Peak switching.

  • Concerning Zero Cross Relays (or Zero Voltage Control), the power only switches at the beginning of the alternation, once the control has been applied. Indeed, the switching only occurs when it is close to zero volts. It is recommended to use zero-cross relays for resistive or capacitive loads, as they can limit the di/dt disturbances on the network and increase both life expectancy of the load and the relay.

Waveforms showing Zero Cross Over


  • Concerning Random Relays (or Instantaneous control), the power only switches once the control voltage has been applied (with a turn-on time below 100μs). It is recommended to use random relays for all inductive loads when the phase shifts between voltage and current can be problematic with zero-crossing relays.
    Random Relays are also used when precise control of power to the load is required (i.e. phase-control applications).

    Waveforms of Random Relays (or Instantaneous control)
  • Concerning Peak Starting Relays, the power only switches when the voltage peak of the half-cycle following the application of the control is reached. It is recommended to use peak starting relays to control transformer type of inductive saturating loads. It will then prevent the transformer from being saturated and from enduring significant magnetizing current peaks.

Waveforms showing Peak Starting Relays


The advantages of celduc® Solid State Relays (SSR) over Electro-Mechanical Relays (EMR).

  • LONG LIFE EXPECTANCY: Solid State Relays are not subject to wear and tear, or deformation as they do not have any moving mechanical. When they are used correctly, SSR have a 200 time higher service life compared to an electromechanical relay (EMR).
  • PRECISE SWITCHING TIME: SSR has the ability to switch “OFF” AC loads when zero load current point is reached. It is then completely eliminating the arcing, electrical noise and contact bounce occurring with EMR and inductive loads.
  • LOW ENERGY CONSUMPTION: A Solid State Relay only needs a low drive power in order to switch heavy power loads.
  • VERY HIGH SWITCHING FREQUENCY: A Solid State Relay allows fast switching and reduced hysteresis (thanks to its great switching capacity). It can then be used for a very accurate temperature control.
  • SILENT OPERATION: A Solid State Relay does not generate acoustic noise during the changing state of outputs. It is indeed very important when it comes to domestic and medical uses.
  • SHOCK AND VIBRATION RESISTANCE: Thank to its high resistance to shock and vibration, there are no risks of accidental switching with Solid State technology.
  • OFFERS MORE FUNCTIONS: Solid State Relays offer more possibilities in terms of diagnostics, protection and communication.

What needs to be considered when using a Solid State Relay.

In order to work correctly, a Solid State Relay needs to sufficiently cool down so the junction temperature at the core of the power element does not exceed the specified values. These values usually go up to 125°C or 150°C depending on the power components.

Cooling will prevent the SSR from reaching heatsink temperatures that are too high (90 or 100°C).

In order to select the appropriate heatsink for your needs, you can either use a calculation or refer to the graphs provided by celduc® relais in the technical data sheets of the products, all available on RS Components.

SSR mounting on a heatsink

celduc®’s wide range of Solid State Relays

celduc® manufactures a wide range of Solid State Relays up to 125 Amps. celduc®’s range includes Single Phase, Two-Phase, or Three-Phase SSR, I/O interface modules, SSRs for printed circuits, diagnostic relays and Solid State Relays for motor control loads.

600V ac 75A RelayDIN Rail Relay with HeatsinkI/O interface module





Typical Applications for celduc® Solid State Relays

celduc® SSR are used in a wide range of applications thanks to their reliability, silent operation, and long life expectancy.


  • Commercial food processing machines
  • Plastic injection molding / extrusion
  • Furnaces
  • HVAC
  • Textile
  • Residential heating
  • Infrared heating
  • Drying
  • Thermoforming
  • Soldering equipment


  • Pumps
  • Compressors
  • Conveyor systems
  • Fans
  • Elevators
  • Lifts
  • Hoists
  • Motorized exercise equipment


  • Heaters
  • Municipalities
  • Cinemas and stages
  • Airport runways
  • Streets and roadways
  • Warehouses
  • Office spaces
  • Hazardous location and beacons


  • Automation interfaces
  • Heating element control
  • Electrovalves
  • Contactor coils
  • Sensor optical insulation.


  • Power transformers
  • Electromagnets
  • Switching power supplies
  • Regulators
  • Inverters
  • Power converters
  • Uninterruptible power supplies
  • Power factor correction capacitors
  • Solenoid Valves

For further information, all of celduc®'s catalogues and application guides are available here:

With nearly 200 full-time employees in 4 countries on 3 continents, celduc® relais is a leading player in the global market for industrial automation and magnetic proximity sensors. Founded and located in Sorbiers, France, celduc® relais, is known as the expert in solid state relays and magnetic sensors. Every year, we are proud to manufacture more than 1 million relays sold worldwide, helping to promote French industrial expertise and “100% made in France” quality.
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