Earth connection for chassis mount power supply

[jfleming26]

Hi

First time posting on here, hopefully I've put this in the right place

I am helping someone out with the electronics side of their model railway layout, we are using a microcontroller system to control relays, servos etc. We are looking at powering this via a transformer which was recommended (link below, we have the 504-252 model) with a variable buck converter on each output. Looking at the datasheet it doesn't seem to have an earth connection, so if I were to wire a 3-pin UK mains plug to it in the 230V arrangement is there an earth point I have missed, or should I try and find a way to screw the earth wire to the housing? The whole thing will be inside a box or enclosure of some kind so it won't be exposed in any way
I've attached the datasheet

RS PRO 50VA 2 Output Chassis Mounting Transformer, 9V ac | RS

uk.rs-online.com

Thanks in advance

 

[Diver300]

Welcome to ETO.

There isn't really a housing. There are the dark laminations which are the steel bits that make up most of the weight of the transformer, and a silver coloured clamp that goes around the lamination. You should just take the earth wire to the clamp. If it's in a metal box, and the metal box is earthed, then the that's all you need to do.

If there's no way of making contact with the laminations or the clamp, for instance if it's in a plastic box and there aren't any metal bits that can be touched, you don't really need the earth at all.

You need to connect live and neutral to the outer two pins labelled 115 V, and join together the inner two. It is a good idea to cover all of those connections so that loose wires that are supposed to be low voltage can't touch the live, or neutral or the wire between the two inner terminals. That sort of fault is far more likely than the transformer failing to isolate the mains.

You can also earth the outputs at some point. If you've got rectifiers and some sort of regulator connected to the windings, than you would normally earth one side of the output of the regulator, often the negative. However, there is no problem running it without an earth. A transformer from RS will provide good isolation from the mains, but earthing the output will stop you getting a shock if there is a loose wire that touches something live.

Instead of that transformer, there is a slightly cheaper, lighter and easier to use alternative, a toroidal transformer. https://uk.rs-online.com/web/p/toroidal-transformers/6718952/

Just stick a mains plug on the brown and blue wires, and it's working and safe. Ok, if you short it's output out, it will overheat in a couple of minutes. But on the mains safety, there are no exposed metal bits.

The only danger for the unwary on toroidals is when mounting them, you must not put anything conductive through the middle that loops back around the outside. So a mounting bolt onto a metal box is fine, as long as the top of the box (or anything else conductive) does not touch the bolt head or the metal plate that is part of the mounting kit. There will only be a tiny voltage, probably less than 1/4 of a Volt, but if there is a conductive path, quite a lot of current will flow.

 

[JimB]

it doesn't seem to have an earth connection, so if I were to wire a 3-pin UK mains plug to it in the 230V arrangement is there an earth point I have missed, or should I try and find a way to screw the earth wire to the housing?

There are two mounting holes for the bolts which hold the transformer to the case or mounting plate or whatever.
Use a solder tag, connect the incoming earth wire from the mains plug to the solder tag, and pass one of the transformer mounting bolts through the solder tag.

JimB

 

[jfleming26]

Thank you both for your replies, Diver300 we're looking at this one as someone recommended it, as it's already been bought was hoping to be able to use it. Thanks for such a comprehensive answer, the connections will all be covered, and the transformer will be in a box, not sure if metal or plastic yet, if plastic I was looking to have an earth connection anyway for completeness even though the metal parts would be covered.

I will go down the route of connecting the earth wire using the mouting bolt, as that will work either way


Thanks again

 

[Diver300]

Thank you both for your replies, Diver300 we're looking at this one as someone recommended it, as it's already been bought was hoping to be able to use it.

The one you've got will work without any problems. Cover the terminals and earth the case, and it'll be fine.

 

[jfleming26]

OK, will do that, thanks again