OptoNews Tip: Choosing an SSR for your application

Posted by Jean Femia on May 15, 2017 12:27:13 PM

Over 40 years ago, Opto 22 started manufacturing solid state relays (SSRs). With the wide variety we've built over the years, we probably have the SSR you need in stock right now. 

We also have some advice on choosing and running SSRs in specific applications.

Heating control

For example, a customer recently asked about SSRs for heating control. 

If you're driving a heater load that cycles on and off continuously, as in a temperature control application, you're probably aware that constant cycling can cause thermal fatigue. We use a thick copper lead frame to mount thyristor chips in our power series SSRs; the frame guards against thermal fatigue.

For complete reliability, we also recommend operating SSRs that cycle heater loads at 75% rated current. In the SSR Data Sheet (see page 19), you'll find the following table to guide you in selecting the proper SSR for your heater load. You'll also want to see where resistors are needed to avoid damage to the SSR, so check out the resistor calculation and diagram. 

Maximum Heater Current SSR Current Rating
 1.5 A 2 A
 2.5 A 4 A
 7.5 A 10 A
 18 A 25 A
 35 A 45 A
8 A 10 480V

Old-fashioned motor illustrationMotor control

For single-phase reversing motor control, the phase shift capacitor can cause twice-line voltage to appear across the open SSR. That means for a 120 volt line, you'll need to choose a 240 volt SSR. See the SSR Data Sheet for a diagram. 

For 3-phase motor control, see the wiring diagram in the SSR Data Sheet. Check the surge current drawn by the motor and make sure the SSR's surge current rating is sufficient. Here's a general guideline for choosing compatible SSRs based on the horsepower of your motor:

240 Volt 3-Phase Motor 
Motor HP SSR Model
 3/4 HP  Z240D10
 3/4 HP  240D10
 3/4 HP  240A10
 2 HP  240D25
 2 HP  240A25
 3 HP  240D45
 1-1/2 HP  480D10-12
 1-1/2 HP  480D15-12

For 3-phase reversing motor control, again, check the SSR Data Sheet's diagram for SSR and resistor placement. Here's a general guide for SSR and resistor selection:

Motor Rating,
Full Load
Opto 22
SSR Rating
Resistor for
120 V Line
Resistor for
240 V Line
 1.25 A  3 A 4 ohm 50 W  8 ohm 50 W
 5 A  10 A 1 ohm 100 W  2 ohm 100 W
 8 A  25 A 0.5 ohm 100 W  1 ohm 100 W
 16 A  45 A 0.25 ohm 150 W  0.5 ohm 150 W
 5 A  15 A 1 ohm 100 W  2 ohm 100 W

More information on choosing and using SSRs

This is just a taste of the detailed information in the SSR Data Sheet on selecting SSRs and using them in specific applications. You'll find SSRs for mounting on circuit boards, a wide variety of AC and DC-switching power SSRs, and even SSRs with an integral heatsink for easier installation. 

The data sheet also answers questions and helps you troubleshoot problems. Skim through it and make sure you see the sections that apply to your application.

Remember that "Optrol" SSR in the photo above? It was one of our original models. More than 10 years ago, that SSR was found in a machine that was being replaced. It still worked fine after 30 years of service. So when you need an SSR for your application, you can count on ours. 

And as always, contact our pre-sales engineers if you have any questions about which products to choose. 

Get the SSR Data Sheet

Topics: Tips, SSRs

Written by Jean Femia

Jean Femia writes about technical subjects and has focused on automation and control systems for more than 15 years. She likes learning about technology and taking corners in her Honda S2000.
Find me on:

    Subscribe to Email Updates

    Recent Posts

    Posts by Topic

    see all