240D25-17 Datasheet, PDF (9/12 Pages) List of Unclassifed Manufacturers – SOLID-STATE RELAYS

DATA SHEET
Form 859-990514
SOLID-STATE RELAYS
page 9/12
Applications
Tips
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
Choose Model 120D25 SSR.
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
Calculate allowable temperature of heat sink by subtracting 31.2° C from 110° C allowable junction temperature:
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 rise divided 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.
I_RMS = RMS value of load or SSR
T₁ = Time current is on
T₂ = Time current is off
I_ON = RMS value of load current during on period
I_RMS =
2
(I_ON) x T₁
T₁ + T₂
Opto 22 • 43044 Business Park Drive • Temecula, CA 92590 • Phone: [phone] • (800)321-OPTO • Fax: [phone] • Internet: www.opto22.com
Inside Sales: (800)452-OPTO • Product Support: (800)TEK-OPTO • [phone] • Fax: [phone] • E-mail: [e-mail] • FaxBack: (800)474-OPTO