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DC-AC Solid State Relay or SSR - del30008

These are Thyristor, Triac and Zero-Crossing Optocoupler based Relays. They do not have wear-tear. But they dissipate some heat. They are rugged but have to be used with external snubbers when switching highly reactive loads. The designs shown here are for heaters and non-inductive loads. They were sealed in thermally-conductive epoxy. 

One application is in cooling and heating control - Temperature Control in Plastic Molding

It is more expensive than a relay. But is good for on-off time-cycling proportional control. The control circuits are here Analog Circuits. This is a DC Control AC Load SSR, This Solid state relay is the most popular and standard. The  Motorola MOC3041 is a 5KV Opto-Isolated Zero-Cross Triac Driver, These circuits is based on such application notes. This was made by me in nice numbers for plastic and rubber molding machine, temperature controllers.

If you have a proportional controller and you need a 2-3 second cycle time, using relays is not good, they will not last. Here SSR wins, with resistive heaters it will work very well.

Read about the Zero-Crossing-Triac-Driver-MOC3041 an important part of this circuit.

These can turn 230V AC equipment on and off. The output is like a NO normally open contacts of a relay and have to be in series with the Load like any other switch. This should not be used for large inductive loads like big motors. The Q1 transistor limits the current thru the LED by providing an alternate path for more current. The DC input can be from 3V to 20V. The Triac can be chosen depending on current in the load.

DC-AC Solid State Relay or SSR

PNG-SchematicNew-Circuit   PCB - L1  L2  SS  M1  M2
DC-AC Solid State Relay or SSR - del30008

This a DC Signal Controlled AC SSR, common usage in controlling Lights, Motors and Heaters. The Number of operations are virtually unlimited if the Load is within limits and avoid power surges or wrong connections.


The Facia shown below was on the top of the SSR, the enclosure was a high-temperature plastic in which a nickel  plated aluminum plate of 4 mm which was insert molded. This plate was to transfer the heat, making this enclosure was  costly and complex.   


Below you will find a Contacter replacement, it will switch R-Y-B phases. It is a high power SSR. In some places it is unwise to use an SSR, a Electromechanical Contacter will perform better. The merit of SSR is fast switching, no wear and tear. No dust humidity influence.

Three Phase SSR