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I am developing a PCB which is having control MCU circuit, it will control the relays. Relays output are connected to 230V AC mains load (~100W load). This PCB is mounted on a cabin where people cant access, and the risk of electrical shock is nill. So is it a good idea to connect the mcu with non isolated power supply(capacitor based) which will reduce the overall cost?

1) Compare to the isolated version this topology having any prons & cons?

2) I am scaring about the high common mode voltage which demands more clearance and creepage in PCB?

3) What about surge/spike compatibility. I need to add any protective devices additionally

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  • \$\begingroup\$ Perhaps read my question here. Also, there are very cheap, sealed DC power supplies with AC inputs that you might also consider. (The HLK-PMxx devices, for example.) \$\endgroup\$ – jonk Sep 18 at 17:14
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    \$\begingroup\$ @electron is this for a product or a one time thing? \$\endgroup\$ – Voltage Spike Sep 18 at 17:15
  • \$\begingroup\$ Small power supplies are cheap. What is your budget? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 18 at 22:43
  • \$\begingroup\$ @VoltageSpike; this is not for a single time use. This is going to be commercial product. Since this is industrial product and no safety concerns are there(no human access. interlocks there) what is the point of adding a isolation. other than operator's safety(Its important) will it help in any other way? \$\endgroup\$ – Electron Sep 19 at 16:05
  • \$\begingroup\$ I am by no means familiar with industrial power supply safety standards, I work on developing products. I do know that IEC61010 requires isolation or double insulation. I don't think you could build a doubly insulated supply inside of a product. The common way involves a line filter, fuses and a UL rated power supply to pass industrial or residential safety standards. \$\endgroup\$ – Voltage Spike Sep 19 at 16:09
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In short. An AC supply either needs to be double insulated, or have a transformer. Both need to be fused. Transformers isolate the supply from the grid. It would be difficult and costly to roll your own power supply.

PCB mounted power supplies are available that convert AC to DC, they have been tested and conform to IEC and UL standards. The other option is to build your own which would probably need to be tested to conform to those standards (even if you aren't selling a product for safety's sake).

I am scaring about the high common mode voltage which demands more clearance and creepage in PCB?

If you are using a PCB mounted power supply (or run any AC mains on a PCB), the AC and DC sections need to be separated and the AC mains side will need proper creepage and clearance between traces. The AC side will also need creepage and clearance from the DC side of the supply (or any other traces). This prevents arcing.

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Source: http://www.ni.com/white-paper/2871/en/

Compare to the isolated version this topology having any prons & cons?

I would say you need an isolating transformer minimum, I am not sure how you would design otherwise.

What about surge/spike compatibility. I need to add any protective devices additionally

Most of these are built into off the shelf power supplies, a line filter may be needed. You will need to fuse the device.

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  • \$\begingroup\$ Thanks for your support. I understood the clearance and UL standards. My question is pointing rather than safety aspects any EMI/EMC concerns for going with a isolated power supply. You know many of the LED Drivers (bulb) are coming without isolation. \$\endgroup\$ – Electron Sep 19 at 16:08
  • \$\begingroup\$ It really depends on what safety requirements your product needs to conform to. LED bulbs have a completely different set of standards than the ones I'm used to designing with. \$\endgroup\$ – Voltage Spike Sep 19 at 16:17

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