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I'm designing a home automation controller in college. The system is on 24V (relays and sensors). I'm using TI Cortex M3 MCU and i want to control it over USB. This picture of the system is currently draft.

the system

I have some questions that I could not find any answers. The board needs 5V(3.3V LDO on there), so a 24V to 5V conversion needed.

  1. Is there any problem that the whole system is on 24V ground?
  2. If 1. problem. Do I need a isolated DC/DC converter?

The outputs externally wired and the inputs opto-isolated.

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You will have much less potential for problems if you use an isolated converter and keep the power stuff (relays) on the 24V side with opto-isolators. The downside is that it will be a bit more complex.

Relays do not really provide 100% isolation from the mains item being switched- noise feeds back through the relay coil which can cause the microcontroller to misbehave. For example, switching a motor load to close some drapes might cause a spike that causes the microcontroller to go amuck until the WDT or something like an address space violation resets it.

If you're using a multilayer commercially made board for the microcontroller and just building the power stuff you may come out okay if you're careful about how you do the interfaces.

However, since you are already putting opto-isolators on the inputs, I say use the isolated converter and opto-isolate the relay drivers. That should allow you to concentrate on developing the project rather than potentially troubleshooting at the last minute.

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  • \$\begingroup\$ thanks your answer. edit: and if i use non-isolated conv, 24V ground and the USB(PC) ground can create grounding loop? \$\endgroup\$ – kergtot Jun 7 '15 at 12:21
  • \$\begingroup\$ The USB is another reason to want isolation. As well as a ground loop-- if something went wrong you're much less likely to damage your computer if you isolate the 5V and 24V sections. \$\endgroup\$ – Spehro Pefhany Jun 7 '15 at 12:51
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If you need to use an isolated DC/DC converter, and you will also need an isolator for signals to/from the 24v and 5v system.

If you don't need isolation, then you can still use an isolated DC/DC converter if it's all you have available. Just link the ground of the 5v and 24v system. Just make sure that signals from the 24v electronics are limited to 5v if passed to the 5v system. If passing signals from the 5v system to 24v system, you may need a way of boosting the signal from 5v to 24v.

The reason you might want isolation is if transient spikes created by relays are interfering with the 5v system. However, it shouldn't be necessary; Spikes from relays can be suppressed using a diode across the relay like so:

schematic

simulate this circuit – Schematic created using CircuitLab

Does that answer your question? If not, perhaps you can give more information, such as what your 24v system does, what components it has, etc, and what information/signals flow from/to the 24v and 5v systems? Also, are there any interfaces to other systems? (e.g. do the relays control mains equipment, and are there any other connections between the 24v/5v system and other systems? Perhaps you can supply a complete circuit diagram of your entire proposed system, with examples of what the system is going to be automating?

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