Enclosure for an open frame transformer

[Arthur Budd]

I bought a Gaugemaster T1 open frame transformer and a Maplin power supply box to mount it in. Unfortunately, owing to all the internal ribs and ventilation slots, the box is fractionally too small to take the transformer. This is the largest PSU box that Maplin do and grinding away the ribs would almost certainly weaken the vents and make it unsafe to handle. Has anyone any ideas where I can get a slightly bigger box? I've looked at RS but can't find anything suitable on their website, however I may have missed something.

 

Regards

 

Arthur

[griffgriff]

I bought a Gaugemaster T1 open frame transformer and a Maplin power supply box to mount it in. Unfortunately, owing to all the internal ribs and ventilation slots, the box is fractionally too small to take the transformer. This is the largest PSU box that Maplin do and grinding away the ribs would almost certainly weaken the vents and make it unsafe to handle. Has anyone any ideas where I can get a slightly bigger box? I've looked at RS but can't find anything suitable on their website, however I may have missed something.

 

Regards

 

Arthur

 

 

Many people use tool boxes as a 'power supply unit' and then link that with the layout through a suitable 'umbilical'.

 

Lockable boxes obviously can be sealed with a padlock to stop tampering etc. I myself by in sealed units so I don't have anything more to worry about than 24v

[noiseboy72]

cpc.co.uk do a range of boxes. They do a large ABS enclosure designed for external mains connections. It has 4 cable inlets along one side of the box, an integral, fused 13A 4 way block and a screw down lid. Junction boxes might be more suitable than project boxes for your application.

[Arthur Budd]

Thanks guys; something to think about there.

 

Regards

 

Arthur

[RedgateModels]

Just don't contemplate a wooden box if you are intending to exhibit - Manager's PAT testers consider this a complete no-no as wood gets wet and then doesn't insulate as well as you expect

[Arthur Budd]

Just don't contemplate a wooden box if you are intending to exhibit - Manager's PAT testers consider this a complete no-no as wood gets wet and then doesn't insulate as well as you expect

 

Yes, I'd heard about the "new" thinking in this regard. Good job I retired The Brickworks when I did as the power supply box for that was built in the early '90s out of plywood, following an article by Iain Rice. We exhibited for over 11 years using this box without any comment from exhibition managers or their electrical experts. But that's why I want to mount this transformer in accordance with today's thinking, just in case the new layout should ever venture beyond my study walls.

 

Regards

 

Arthur

[Gordon H]

Just don't contemplate a wooden box if you are intending to exhibit - Manager's PAT testers consider this a complete no-no as wood gets wet and then doesn't insulate as well as you expect

So how did the Southern get away with wooden shoe beams for so long I wonder?

[AMJ]

In the past I have mounted the transformer onto the underside of the baseboard. Secured the mains cable with a clamp similar to those fitted in 13A plugs. It is then protected by a plastic sandwich box (screwed to baseboard). This has had ample ventilation holes drilled in it. Not sure what the man with a PAT tester would think but it's safe.

 

Most kit I use these days use sealed unit power supplies. Whilst working at one place our in house sparks used to do PAT tests and so the laptop PSU got tested a few times. We had a few new LCD screens that failed the test (working fine) so the supplier agreed to exchange as they had failed the PAT test.

[noiseboy72]

I would not be too happy about the transformer screwed to the baseboard, it being wood that could get damp etc. Maybe insulate it with the lid of the box ? I would also fit a thermal fuse & an inline 1A fuse just for piece of mind. Don't forget the plug fuse is just to protect the cable, not the appliance.

 

Some laptop and lcd screens PSUs will fail PAT due to low insulation readings if not correctly tested. This is due to the use of a switched mode power supply and its requirement for a functional earth. This means that the earth wire is used to disipate electrical "noise" generated by the PSU in normal operation. Modern PAT machines have a setting to correctly interpret the results when testing them.

 

www.cpc.co.uk do a huge range of encapsulated PSUs, including a range with screw tags to make it very easy to mount a number of them into a box or directly onto the layout. About £6 and upwards.

[brixham]

I have used old pc psu cases...they have an IEC mains input socket, and sometimes an on/off switch and a switched ac outlet.

Your local pc repair shop should have a steady supply!

 

Remove all the internal gubbins, some of the wire can be reused. Fit a fuseholder, and fix the transformer, and earth the case. Use insulating boots over the live components, IEC's/switch and fuseholder. Check that a finger cannot pass through the hole where all the output cables used to be..blank off if necessary.

For outputs I've used 4mm terminals, or spring loaded 'loudspeaker' terminals that Maplin ( and others ) have

I add rubber feet so as not to mark any surface, also adds stability.

 

If the power unit is for high power, either a for a dcc power brick, or for O/G garden railways, then the fan can also be used. I've made several for LGB/G, using 18 volt ac ( 2 x 9volt ) and fitting a bridge rectifier and output leds. Also ac versions for lighting/point operation.

The fan runs happily on a single 9 v winding with a rectifier.

 

Malcolm

[bertiedog]

As usual with electricity if you do not know the answer, don't do it, it could be be your last action!!!

 

On cases, old tower units make reasonable PSU cases, but do not be tempted to use switched computer PSU's for trains, they do work, but are not correctly loaded, or rated for such model uses. (After saying this I use them at home for bench power supplies, but they are modified to load them correctly). I am not going to say how to do it, look it up on the net if you want, it is then at your risk, not mine.

 

I would be very surprised if switched supplies, even modified, would be acceptable at exhibitions, apart from commercially sourced items designed with model use in mind.

 

PAT testing would be required for such casings for exhibition uses. As for mounting on wood, don't....it is asking for a test failure, use SRB board, Glass fibre sheet, Bakelite sheet, (yes it's still made),, or Tufnol or Perspex, (acrylic), sheet, all resist damp. Do not use thin polystyrene sheet, too flammable, and distorts with heat, but thick sheet ABS may be used for transformer mounting.

 

If the transformers has appropriate mounting holes, mount the transformer on posts, away from the surface to promote cooling. Toroidal transformers have central post mountings made as well. Good ventilation is vital.

 

The transformer should be in the middle of the case, evenly spaced as far as practical, and proper grade cables brought in via protected grommeted and sheathed entries, taken to proper switching and fuses etc., via tough grade cable, preferably in sheathing as well.

 

The cable should be clearly marked as suitable for mains, and kept well way from the DC outputs, (and any AC feeds out). I would take the outputs via separate internal trunking to the power terminals out, clearly separated from any mains cable to the maximum clearances the box will allow.

 

You cannot overdo the extras like sheathing, trunking, and sheet insulation, after all it only has to be done once, and if done competently will pass all tests for years to come. As always, if in doubt, use a trained professional electrician.

 

Stephen.