Hi. I have this final attempt at a relay chain circuit. Could someone check on the component values and circuit to make sure it will work? Here is the idea:

The micro-controller will use two digital out pins that will trigger a SSR. I calculated that the current limiting resistors would be 1050 ohm. This is from ohms law with 2.1V across the resistors and 1.2V across the leds inside the SSR. Is this right? Do I need anything else at this part of the circuit?

The outputs of the SSR will act as a SET momentary button and a RESET momentary button to turn on/off an OMRON relay. It is a latching circuit setup. The SSR outputs are switching +30V DC. Does this circuit part look ok? Are my values ok? Can someone recommend a diode part number for the flyback diode across the OMRON relay?

Let me know what you think of this. Thanks a lot! See attached picture and SSR spec sheet.

 

Looks ok. It's ok to round the resistors down to the much more common 1.0 K value.

But why? Since the relay gets you galvanic isolation from the load, why not just drive its coil with a 60 V rated medium power transistor or power darlington transistor directly from the uC, and let the uC handle the set/reset logic?

Or is the relay GND at a different potential from the uC ground? If so, you still need only one opto channel, letting the uC handle the logic.

ak

 

Looks ok. It's ok to round the resistors down to the much more common 1.0 K value.

But why? Since the relay gets you galvanic isolation from the load, why not just drive its coil with a 60 V rated medium power transistor or power darlington transistor directly from the uC, and let the uC handle the set/reset logic?

Or is the relay GND at a different potential from the uC ground? If so, you still need only one opto channel, letting the uC handle the logic.

ak

Ya, different. The uC is running on 5.25V.

I didn't think a transistor would be isolated enough. I also thought this SSR would be less parts. It also comes with the NC ouput. If I was using the uC logic and transistor to have a NC wouldn't it be wasting more power to stay closed?

Do you know what size diode should be on the Omron relay?