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Topic: Safety of AC/DC Adapter in embedded project (Read 237 times) previous topic - next topic

osmosis311

Hi All,

I came across this tutorial this weekend for a really cool project: a Nextion touchscreen display + Wedmos D1 Mini + AC/DC adapter, inside a 3D printed wall switch box, that goes into the wall replacing a light switch, so you have a nice, flush-mounted "mains-powered" touch screen controller.

It looks very cool and I'm excited to try it out.  However, in another forum today, someone was cautioning me against using the AC/DC adapter (I know that's not the right term, sorry), which is a Mean Well IRM-03-5.  I know there are a ton of variations on this design and lots of different types/models available that all basically do the same thing.

This person warned me that using this type of device on a custom PCB without proper "clearance" (?) could be dangerous.

Was just curious what this group thought about this project overall, and its safety (two young kids at home, that's my #1 concern), and when/where it's appropriate to use one of these AC/DC PCB-mounted converters in a project.

Thanks!

funman1

You have a link to the project to look at?

osmosis311

https://community.home-assistant.io/t/ha-switchplate-diy-lcd-touchscreen-wall-switch-replacement/

adwsystems

This person warned me that using this type of device on a custom PCB without proper "clearance" (?) could be dangerous.
The two 'clearances' I can think of the person could be warning you about are:
1. Cooling. Making sure you have enough clearance and free air to provide the adequate cooling to keep everything in it safe operating range. Last thing you want is something to start smoldering in the wall in your house.

2. PCB Design. There are specifications on trace to trace and pin to pin separation and trace widths and wire gauges on PCBs when routing power, especially at higher voltages (including 120VAC) and/or at higher currents.

The latter is more likely to what they were referring, but the first is also important to keep in mind when you stuff a lot of stuff that is capable of generating heat in a small box and then place it in a (potentially) insulated environment (like an insulated wall) such that only the face provides any heat transfer area.

LesserMole

If I were you I would look into the regulations regarding wiring in your area. It may be that something mounted in the wall receptacle would be regarded as a fixture (part of the house wiring) and be subject to the wiring laws, as distinct from something plugged into an outlet.

It's not impossible that you could run foul of such regs and in the process be seen to have made an unauthorised change to the wiring, with who knows what insurance and liability issues. (Not saying it is the case, but I would be very inclined to check....)


ardly

I found this datasheet for the adapter.
http://www.farnell.com/datasheets/2547884.pdf

It says it is cooled by "free air convection" and that its no load power consumption is <0.1W.

Presumably though the the touch screen and the Wedmos are going to be drawing power all the time, so "no load" will not apply. You need to know how much heat all the components are going to generate.

Heat build up in a flush mounted solution is a risk, as could be the possibility of anything becoming live. Will the 3D printed wall switch box be made of metal or plastic. If its plastic can the material handle heat, will it burn, or give off fumes. A metal box might help the heat problem but makes earthing etc. more important.

I don't like leaving ordinary wall warts plugged in. I switch them off and unplug them. Building them into the wall seems like a strange idea to me.


"Facts do not cease to exist because they are ignored" - Aldous Huxley

wvmarle

Major heat source will be the power supply, especially if it involves a linear regulator. The WeMOS when doing WiFi transmissions may peak at 500 mA. Even at 100 mA average, if you have a 5V to 3.3V regulator, that's 170 mW, plus the 330 mA of the WeMOS itself - most of the power used by the device ends up as heat. The touch screen will draw significant power, too.
In a small enclosed box even as little as 1W of heat production can get it pretty warm, to the extent of overheating your components (normally rated to work up to 85 C.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

TomGeorge

#7
Feb 22, 2018, 08:06 am Last Edit: Feb 22, 2018, 08:14 am by TomGeorge
2. PCB Design. There are specifications on trace to trace and pin to pin separation and trace widths and wire gauges on PCBs when routing power, especially at higher voltages (including 120VAC) and/or at higher currents.

The latter is more likely to what they were referring, but the first is also important to keep in mind when you stuff a lot of stuff that is capable of generating heat in a small box and then place it in a (potentially) insulated environment (like an insulated wall) such that only the face provides any heat transfer area.
The site linked by the OP has a 3D graphic of the assembly.
It uses protoboard like this;

The article does not give mains potential warning nor does it have a fuse.

The powersupply is an Encapsulated Powersupply


Personally I would not put any circuitry or power supply in my walls even approved units.
Just wires, switches and lights fitted by licensed electricians, that's enough to worry about.

Fitting devices in walls appears here at odd times, and I ask have you contacted your insurance company and asked them if you are covered if you fit something like that.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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