I have designed an Embedded System in which the PCB has both Non-Isolated and Isolated DC DC converter.
I am taking a 24V DC Supply from a PLC Device. Converting it to 3.3V with a Non -Isolated DC DC Converter. However, I want to control a Relay (Output) and also Sense for Contact Closure (Voltage free input/Alarm input) with in the same device.
As shown in the block diagram, I have Isolated the 3.3V using a separate unregulated Isolated DC DC converter for Digital IO operations. I am using an optocoupler to isolate the Digital IO from the microcontroller circuitry.
My questions are:
- I want to Control the Relay using the 24V tapped from the input of Non-Isolated DC DC converter. Does this configuration provide Isolation? or Should I use only the Isolated 3.3V to control the relay? (which won't catch relay startup voltage)
- Since I have potential free Inputs, I can use either ground or positive voltage to sense the NC contacts of a relay. I think the 24V tapped from the Inputs will provide long distance sensing proper. But what happens to the Isolation?
- This device will be DIN Rail mountable. If I have to use chassis ground, which ground should I share with Chassis ground? or is it even necessary as we will use only plastic enclosure?
Thanks in advance for any suggestions!