Form 859-990514

SOLID-STATE RELAYS

DATA SHEET

page 9/12

Opto 22 • 43044 Business Park Drive • Temecula, CA 92590 • Phone: (909)695-3000 • (800)321-OPTO • Fax: (909)695-3095 • Internet: www.opto22.com

Inside Sales:(800)452-OPTO •Product Support: (800)TEK-OPTO•(909)695-3080•Fax:(909)695-3017 •E-mail:support@opto22.com•FaxBack:(800)474-OPTO

Heat Sink Calculation

Like all semiconductor devices, SSR current

ratings must be based on maximum junction

temperature. All Opto 22 SSRs operate

conservatively at maximum junction temperatures

of 110° C. Determining an adequate heat sink

for a given SSR conducting a given current is

very simple.

Note: Thermally conductive grease must be used

between the relay base and the heat sink.

Sample Calculation Given:

120-Volt, 20-Amp Load

50° C Ambient Air

ChooseModel120D25SSR.

Calculate dissipation as: 20 amps x 1.3 Watts per amp = 26 Watts

Calculate temperature rise junction to SSR base as: 26 Watts x 1.2° C per Watt = 31.2° C

Calculateallowabletemperatureofheatsinkbysubtracting31.2°Cfrom110°Callowablejunctiontemperature:

110° C-31.2 = 78.8° C

The heat sink is in a 50°C ambient, therefore, allowable temperature rise on heat sink is: 78.8° C-50° C = 28.8° C

If heat sink is allowed to rise 28.8° C above ambient, then the thermal resistance of the heat sink is simply the 28.8° C risedivided by the 26

Watt. Any heat sink having a thermal resistance less than 1.1° C per Watt will be adequate.

Duty Cycle Calculation

When solid-state relays are operated in an on/off mode, it may be advantageous to calculate the RMS value of the current through the SSR

for heat sinking or determining the proper current rating of the SSR for the given application.

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Tips

Applications