Simple AC-DC embedded power supply

I'm working on a design for a remote power control application involving a wireless embedded computer tied to a relay. The device will be mounted inside of a NEMA-rated outdoor enclosure and AC main power will be available within the box. What is the best way to provide regulated, 5V DC current to my embedded device in this scenario? I am currently prototyping using a standard wall wart power supply, but space may be tight in my enclosure, and is a wall wart transformer really the best in this case?

I will be designing my own PCB for the embedded device. Should I design a complete AC-DC power supply on this same PCB? Or should AC power be more isolated from my embedded system?

Thanks for any help you can provide! Jon

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I would stay away from the wall wart, they are mainly designed for a controlled environment - indoors. There are plenty of 5V PS bricks, or DIN rail mount supplies with proven temperature ranges, and high MTBF. You did not mention the current requirements, but a low I unit around 2A should run you about $50. Try Newark electronics, DigiKey, Jameco, or a host of others...

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probably a wall-wart switcher is better.

Does it need isolation? how much current does it need ?

Will doing that save money? It seems to me using the pre-aprooved PSU is easier.

Bye. Jasen

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The wall wart has the advantage that it's already tested and approved to local safety standards. If this is going to be a commercial product or used by people other than yourself that might be a factor. Just check it doesn't overheat in the enclosure. If money no object go for a quality unit with plenty of spare capacity so it runs cooler.

Another possibility if cost isn't a factor might be to use a modular power supply like these:

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Again you may need to think about the safety implications - might need a cover over any exposed mains to protect anyone servicing the unit?

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My preference would be for a standard AC/AC adapter (i.e. simple transformer) wall-wart. Other components can be on the PCB. Your options are open; you can rectify/filter the low voltage AC and run a commercial DC/DC converter as a high quality switching supply, or for low power just use a linear regulator.

As others have noted, safety requirements dictate a certified unit for your connection to the line power; it's easy to get precertified units, but only if you keep the specifications simple. Nothing's simpler than a current-limited transformer.

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Thank you all for your responses! I truly appreciate your input.

These units are meant for production, so safety/regulatory standards are certainly a concern. Cost is also a concern. I like the pcb- mount AC/DC power supplies, and had looked into them before, but for ~ $50-$70... the power supply would be the most expensive component on these boards. I'd rather avoid that kind of expense, if possible. A simple switching DC-DC power supply on the board could be a few bucks, but doesn't take care of the AC part. If I went with a wall wart, the total cost should be below the cost of the pcb-mount or din-rail type power supplies. But, it seems that might be sacrificing quality. Some of have said the wall wart should be fine, others not.

If I went the AC/AC adapter (wall transformer) direction, and rectified / filtered the low-voltage AC current on the board, what, if any, concerns do I have regarding safety / regulatory standards?

Thanks again for the help!

-- Jon

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