Crouzet Solid State Relay Knowledge Base

Heating applications by their nature suggest repeated frequent load cycling. Although heating elements are predominately resistive, the wiring configuration may add significant stray capacitance to the load circuit, resulting in unexpected surge currents during each turn-on cycle. Zero-voltage turn-on solid state relays, along with the zero-current turn-off characteristic of all AC solid state relays, are ideal for resistive heating applications and serve to address inrush currents.

Resistive loads themselves pose no particular voltage related concerns for solid state relays. Electrical noise placed on the power lines by other devices may or may not effect the SSR. Transient protection provided internally or external for each solid state relay is advisable in applications where electrical noise is present.

In general, the main concern for SSR selection is derived from load current. Repeated load cycling typical of resistive heating applications places stress on the solid state relay's output semiconductor due to the initial surge currents of each turn on cycle, and the cumulative effects of thermal cycling created by each load cycle. Combined with typically high ambient temperatures, solid state relays used in heating applications are placed under more stress than would otherwise be apparent.

Selection of a solid state relay for heating applications should take into account the combination of repeated load cycles and ambient temperature, in conjunction with the surge current withstand characteristics of the SSR, and the available heat sink. It is always advisable to oversize the heat sinking where possible. However, it is common that the relays will be expected to operate at elevated temperatures, thus making them more susceptible to surge current induced failures.

It is therefore recommended that an SSR with a continuous rating of at least 2x the full-load current of the application be utilized in order to maximize the life expectancy of the solid state relay.

Solid State Relay Heating Applications;

Professional Cooking Equipment

Temperature Control Systems

Extrusion & Thermoforming Machinery

Packaging Equipment

Commercial Coffee Makers

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