New here - and to electronics in general. So far I am having a blast 
But I am having a hard time figuring out how best to power Particle projects from 120/240V AC.
But I am mainly interested in ways to do it without an external wall wart, and as compactly as reasonable, for the purpose of prototyping mains-connected products.
It would be really nice with a write-up of the options and pros and cons of different approaches.
I’ll typically use small board mounted isolated AC/DC converters to power projects from AC. I’ve used RECOM devices, but CUI appears to have several good options also … both available from Digikey … +5v or +3.3v available (or other voltages) with various output currents.
Thanks, that was a good lead.
Where does one buy such things in Europe?
Unfortunately I don’t know too much about European distribution, but I thought Digikey had a presence there. Have you checked the major distributors like Digikey or Mouser … I had thought it fairly easy to source from them in Europe (but I could be mistaken). You might also just take a look at the RECOM international page.
Try Mouser, RS Components, Farnell. I use Recom AC/DC too and they are great.
I found another thread where a buck converter was suggested. The item description says low and high voltage sides are isolated, am I correct in assuming this indicates it should be safe to use?
It is a lot cheaper than what I found on Digikey – are there any downsides to be aware of?
Yes… a lot cheaper. Probably too much.
Do your choice !
Disclaimer: in case of electrocution, don’t come back here to complain 
Looks like an interesting possibility @aal, and something to perhaps look into further. (Too bad the shipping is almost as much as the device itself … hopefully this is not on a per piece basis). Even a bit smaller than the RECOM units. It’s an interesting find … might even pick up a couple myself to evaluate.
It’s good that it’s noted as isolated as this is preferred from a safety standpoint. (Probably good to at least verify isolated input/output with a multimeter though). That said, the AC is exposed, and of course appropriate caution is always very important any time working with line voltage … even using isolated converters does not ensure you can’t get hurt … proceed with caution!