5v inside a backbox

[XAPBob]

I’m looking to run some small electronics inside a backbox, with a partially perforated cover. Basically a temp sensor on an ESP8266 - it’ll run off a 5v supply (there is a built in 3.3v conversion on the board I am looking at) taking well under an amp.

There are a bunch of low power small psus available - some of which have CE markings (some of which are rather dubious).

Questions:
- would you put a fuse in-line before the PSU?
As far as I can tell that’s not happening in things like USB equipped sockets.

- is there a better option that I’m not aware of?

Running the same questions past the commercial electrician who is doing first fix at the moment as well, but more heads means additional different experiences of potential solutions.

cheers

 

[Simon47]

Chances are that with a small PSU like that, you won't be able to fit a meaningful fuse since the device itself will "fuse" long before the fuse.

From the way the question is phrased, it sounds like there'll be the electronics which is mains powered - but you haven't said what the electronics will do in terms of getting information out. Will it be sending a signal up an ELV cable to something, using radio, something else ? If it's sending a signal up a cable, I'd be inclined to power it from whatever is at the other end of the cable - using a higher voltage (e.g. 12V) and regulating it down if volt drop is likely to be a problem.
Doing that also avoids all the problems with having different parts of the system powered from different supplies.

 

[XAPBob]

Chances are that with a small PSU like that, you won't be able to fit a meaningful fuse since the device itself will "fuse" long before the fuse.

From the way the question is phrased, it sounds like there'll be the electronics which is mains powered - but you haven't said what the electronics will do in terms of getting information out. Will it be sending a signal up an ELV cable to something, using radio, something else ? If it's sending a signal up a cable, I'd be inclined to power it from whatever is at the other end of the cable - using a higher voltage (e.g. 12V) and regulating it down if volt drop is likely to be a problem.
Doing that also avoids all the problems with having different parts of the system powered from different supplies.

The ESP2866 is an arduino compatible microcontroller with WiFi on board, hence I only need the power - data will be RF.
Running a 12V DC network is technically possible - 12v would sit in the LV containment trays, I could probably get a DIN rail 12v supply to put in distribution boards... Then I’d just need a regulator to step it down.
But given that the 240v wiring is already there...

I think some of the PSUs have resettable fuses - don’t know of those are considered useful against anything other than basic overload, probably depends on exactly how they are fitted and what they actually control (just disconnecting the 5v would be an odd design decision to a layman like myself).

 

[XAPBob]

I suspect I might go a different way...
The Sonoff Mini, and the Shelly P1 are both miniature relay boxes designed to go into backbones, using the ESP2866, and obviously have rated 240V convertors built in.

If I can "abuse" the switch contacts on the Sonoff Mini in the way that I *hope* I can, then I can wire in a DS18B20 (one wire digital thermometer) really easily, and without opening the box (well, other than to switch it to DIY mode - a software option that takes it out of the cloud but needs a jumper moved on the board).

I don't need the relay in either case, though I might add a neon indicator light that I could switch on for debugging purposes (make sure I have the correct box in front of me)

 

[Lucien Nunes]

Fuse the PSU with whatever upstream fuse it is supposed to have. If it is a complete consumer-ready device with an inlet connector designed for a cordset for UK / EU use, it should accept a supply fused at 16A since that would be typical protection for a socket-outlet circuit in locations without fused plugs. If the available supply is a general-purpose socket-outlet circuit, the conventional way to supply it would be through a fused connection unit, giving the necessary fusing-down from a possible 32A circuit and DP isolation. If it is running on its own dedicated circuit suitably protected at the DB, that wouldn't be necessary. If it is a bare-bones PSU designed for integration within a product, you would need to check the specs.

Pay close attention to the electrical separation of the ELV side, if any part of it is accessible to the outside world. If there is mains kicking about in the enclosure, it will need to be constructed and insulated sufficiently for the purpose. For example, it would not be acceptable for the leads of an external temperature sensor (that are not insulated for 230V) to be able to contact basic insulation on the 230V circuit.