Whether it be Opto Coupler or Solid state Relay (SSR) or Mechanical Relay, all of the find application in industrial environments or any harsh environments where your on-board drive circuitry needs an isolation from the external environment. Current requirements, isolation requirements drive the selection and when we say isolation CTR in opto couplers is an important specification. Relays can be used for power isolation as well as signal isolation. Based on application , they are categorized as signal relays and power relays. The other category of relays is latching and non-latching.
Low currents:
If the load requirement is a low current, then a simple opto coupler shall help. The currents we are talking here are in the low mA. Opto coupler shall have a photo transistor at the output and the current carrying depends on the collector current capability. Let us look at the opto coupler MCT2E and it's collector current specifications to get an idea.
High Currents:
If the load requirement is a high current, then a simple opto coupler may not help and we might have to move to either Solid State Relay (SSR) or a Mechanical Relay. The currents we are talking here are in the low Amperes and can go up to 100A. Solid state Relay still has Opto coupler mechanism as basic principle but with current carrying capability much more than simple Relay.
Let us look at the Solid State Relay HSR312 and it's specifications to get an idea.
Take the case of G3VM-61FR1(TR05), which is another solid state relay (SSR) from OMRON which has high current specifications.
Let us take a look at Relay which is mechanical based and has capability of high currents at output side. For example, HY1-5V Relay from Panasonic can operate up to a current of 2A.
To summarize,
Opto coupler: used for low currents
Solid State Relay (SSR): Good switching characteristics, reliable, high current, good life, opto coupler based
Mechanical Relay: Rugged, high current, may deteriorate as mechanical contact is involved
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