Crouzet Solid State Relay Knowledge Base

Crouzet Solid State Relays, including the new Cooltech family of DIN Rail mounted SSRs, incorporate innovative design features and components to provide enhanced immunity and protection from radiated and conducted voltage transient phenomenon. The Crouzet SSR design optimizes performance and immunity by close coordination of circuit design and component parameters.

Optical Couplers
The optocoupler(s) in a solid state relay serves two basic purposes; they provide electrical isolation between the input circuit and the AC mains, and they “switch” the output semiconductor when an input signal is applied to the relay. The former is achieved through the air gap between the input and output of the coupler. The latter is acheived when input current flows through a LED internal to the coupler. The LED illuminates the base/gate of a photo-transistor, or similar device internal to the coupler. This allows a small amount of “bleed-off” current (usually <100ma) to flow into the gate of the SSR's output semiconductors, which effecitvely turns on the solid state relay.

This makes the optocoupler one of the most critical components within any solid state relay. Unfortunately, it is also one of the most sensitive to electrical transients. Noise and/or transients on the AC mains can directly impact the performance of a coupler, as they are connected to the AC line in series with the gate of the output semiconductor. A failure in the coupler will usually result in the shorted solid state relay. That is, the solid state relay will turn the load on continuously, regardless of whether there is an input signal applied.

The two most common couplers used in solid state relays are photo-transistors and photo-triacs. Transistor couplers have a much higher gain and transfer ratio (ratio of input signal to switched output current) than do triac couplers. This enables them to turn on the output of solid state relays with only a few hundred µA of input current. They are therefore somewhat desirable in applications where the control signal is supplied by a very “weak” power source. However, this also makes them more susceptible to radiated or conducted transients. Especially in encapsulated relays that have higher stray capacitance associated with the potting material.

Conversely, the photo-triac couplers used in Crouzet's solid state relays do not have the problems associated with the high-gain output of photo-transistors. This provides the solid state relay with a higher level of noise immunity and serves a crucial role in the products compliance with the EMC Directive. Also, solid state relays incorporating triac drivers require fewer components in the output circuit, which simplifies the design and enhances overall reliability. However, they do require more input current (5ma to 10ma) in order to trigger the output of the solid state relay.

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