0
\$\begingroup\$

I have designed an Embedded System in which the PCB has both Non-Isolated and Isolated DC DC converter.

enter image description here

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:

  1. 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)
  2. 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?
  3. 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!

Update: Here is the schematic of Input and Output: enter image description here

\$\endgroup\$
  • \$\begingroup\$ What relay? What do you mean by "tapped"? Isolation from what? Could you add your [potential] relay setup to the diagram? \$\endgroup\$ – DiBosco Jul 21 '17 at 9:26
1
\$\begingroup\$

I want to Control the Relay using the 24V tapped from the input of Non-Isolated DC DC converter. Does this configuration provide Isolation?

No it doesn't provide isolation because it bypasses the isolation provided by the opto-isolators. However, if you can rely on coil/contact isolation then you have isolation but, that then begs the question as to why you are using opto-couplers to drive relays.

or Should I use only the Isolated 3.3V to control the relay? (which won't catch relay startup voltage)

Try making your isolated DC/DC converter more like 24 volts or 12 volts and have this power the opto AND the relays. If you need feedback signals to the microprocessor then that might be a good reason for the opto-couplers.

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?

You lose isolation - if you want to feedback relay activities to your micro you need isolated feedback circuitry (maybe consider using the ADuM5401 range from ADI) or use the opto-couplers but you need an isolated supply to the relays: -

enter image description here

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?

If you were to use any external ground it has to be connected to your microprocessor ground for at least EMC reasons I would say. After all, your output relays are isolated and this means they are also isolated from ground unless wired that way in certain applications of your module.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.