Solid-State Relays

Opto 22 • 43044 Business Park Drive • Temecula, CA 92590-3614 • www.opto22.com

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Applications: Tips

Heat Sink Calculation

Like all semiconductor devices, SSR current ratings must be based on maximum internal junction temperature. All Opto 22 SSRs

operate conservatively at maximum internal junction temperatures of 110 °C. Use the equation below to calculate the maximum

allowable heat sink thermal resistance for your application. It is good engineering practice to provide a margin for error instead of

running the application right at the limits. If your application is near the thermal limit, it can be helpful to add a fan to move air

across the heat sink.

IMPORTANT: Thermally conductive grease must be used between the relay base and the heat sink.

Sample Calculation 1

120-volt, 20-amp load; 50 °C ambient air temperature

Model: 120D25 SSR.

See the last two columns of the table on page 4 for thermal resistance and dissipation values for the 120D25. Also, see the note at

the bottom of the table.

Dissipation: 1.3 watts/amp

Thermal resistance: 1.2 °C/watt

Maximum junction temperature: 110 °C

The calculation would be as follows:

This calculation indicates that you

should select a heat sink with a thermal

resistance of less than 1.1 °C/watt.