Power Supply Unit - Panel Mounted Transformers

[josh993]

Good Evening,

 

I am currently designing how I am going to wire up a new 7mm layout - As part of this I would like to build a floor standing power supply unit with one 230V plug with various power rating outputs, which then has a single multicore lead that connects to the layout. This keeps the mains power separate from the layout, reduces the overall layout weight and also allowing it to be used with other layouts in the future.

 

In doing so, I am looking at building a self enclosed metal box which which will contain panel mounted transformers to give these various outputs.

 

I appreciate this is always a weary subject and am fully aware of the risks with mains voltage, but the equipment will be tested by competent electricians before use.

 

For the layout I think I need the following supplies:

 

Tortoise Point Motors - 15v AC

DC Controller - 16 to 18v AC (1 to 1.5 Amps)

DCC Controller - 10 to 15v DC (2 to 3 Amps)

Stepper Motor - 12v DC (2 to 3 Amps)

Electromagnets - 12v DC

 

Possibly a 12v DC for Buildings Lights? I don't mind supplying the fascia lights with it's own 230v Plug.

 

I am just wondering how many of these panel mount transformers can be fed safel  from one 230v supply - I presume if wired in parallel to the supply, (which I plan to protect with a 3A fuse the input voltage side) there would be no limit? I also plan to fuse the output channels aswell.

 

If I can get twin wound transformers, these is no reason why I should not need anymore than 3.

 

1 with 15V output for the DC Controller and Point Motors

1 with 12v for the Stepper motor and Electromagnets

1 with 12v Output for NCE DCC Controller

 

I would like to use individual outputs to ensure each accessory has its own supply, to avoid and minimise interference especially as the Controllers, lights and stepper motor may be in continuous use and a dip in power would be evident.

 

I look forward to any thoughts.

 

Thanks in Advance

 

 

 

 

 

 

[28XX]

If you are using DC analogue controllers you need a transformer for each one, or transformers with multiple electrically separate windings.

 

Why do you need DCC as well?

 

Each transformer should be individually fused on the mains input side. These fuses can be mounted inside the box. You cannot rely on the fuse in the plug to protect the transformers, it will only protect the mains cable.

 

The outputs should be fused as well. Panel mounted fuse holders are convenient. Fit slow blow so in the event of a short, the controllers overload protection operates first. This is called discrimination.

 

I used a big steel box from an old burglar alarm for mine. The box from a piece of 1970's hifi is another option.

[cliff park]

A very approximate starting point could be:- 5 supplies, 18 v each (erring on the high side) at 2 amps (average) =5X18X2=180 Volt Amperes (VA) If you said 200 watts you can see that this is less than 1 Amp at 240V. Bear in mind that not all of the outputs will be going full tilt all the time and you can see the whole lot will easily run off 3 Amps. Obviously one transformer could well supply two or three of the outputs, but that doesn't change the overall figures

[josh993]

If you are using DC analogue controllers you need a transformer for each one, or transformers with multiple electrically separate windings.

 

Why do you need DCC as well?

 

Each transformer should be individually fused on the mains input side. These fuses can be mounted inside the box. You cannot rely on the fuse in the plug to protect the transformers, it will only protect the mains cable.

 

The outputs should be fused as well. Panel mounted fuse holders are convenient. Fit slow blow so in the event of a short, the controllers overload protection operates first. This is called discrimination.

 

I used a big steel box from an old burglar alarm for mine. The box from a piece of 1970's hifi is another option.

 

Thanks 28xx - I am only planning on using one Analogue DC Controller or one DCC Controller. It is just to allow flexibility for other people to run their stock on the layout.

 

I had planned to fuse them individually inside the box as I appreciated I could not rely on the fuse in the plug. Would I need to use say a 5A for the Plug and 3A for the individual channels to avoid the weakest fuse blowing first as if I used 3A inside and 3A in the plug I presume this would be the outcome? I presume there is no reason these could not be done with panel mounted holders either? Perhaps on the a different side of the box and clearly labelled though so they were easily identifiable from the output channels.  

 

A very approximate starting point could be:- 5 supplies, 18 v each (erring on the high side) at 2 amps (average) =5X18X2=180 Volt Amperes (VA) If you said 200 watts you can see that this is less than 1 Amp at 240V. Bear in mind that not all of the outputs will be going full tilt all the time and you can see the whole lot will easily run off 3 Amps. Obviously one transformer could well supply two or three of the outputs, but that doesn't change the overall figures

 

Thanks for the calculation Cliff, do you think I could get away with 1A fuses for the individual inputs then and then a 3A in the mains plug?

 

Indeed, I am just worried about interference for using one channel for two or three outputs, as I have had lamps flicker before when using electromagnets and likewise when using a controller....

[Penrhos1920]

For all of your dc requirements and you tortoise I'd use a PC psu. Much more efficient and compact.

[Junctionmad]

Really can't see why any layout needs AC , rather then transformer , use switched mode power supplies for DC, far more efficient

[brigo]

Really can't see why any layout needs AC , rather then transformer , use switched mode power supplies for DC, far more efficient

 

Some DC controllers require AC inputs to work, such as those that use thyristors, or some use the 50 Hertz to generate pulsed outputs.

 

From a safety standpoint I'd much prefer a simple transformer with a split winding bobbin than a switch mode power supply of dodgy far eastern origin. I've had a few power supplies for computers and accessories go down in the last couple of years, even from companies like WD and Fujitsu.

 

Brian

Edited July 27, 2017 by brigo

[28XX]

Switch mode PSUs are unnecessarily complex. They do not survive use with inductive loads such as solenoid point motors. Any inefficiency will just warm the railway room up a bit. Scarcely noticeable in the grand scheme of things.

[Junctionmad]

Switch mode PSUs are unnecessarily complex. They do not survive use with inductive loads such as solenoid point motors. Any inefficiency will just warm the railway room up a bit. Scarcely noticeable in the grand scheme of things.

I cannot agree , mine have lasted ages. Point motors properly quenched cause no issues.

 

But to each his own

[Junctionmad]

Some DC controllers require AC inputs to work, such as those that use thyristors, or some use the 50 Hertz to generate pulsed outputs.

 

From a safety standpoint I'd much prefer a simple transformer with a split winding bobbin than a switch mode power supply of dodgy far eastern origin. I've had a few power supplies for computers and accessories go down in the last couple of years, even from companies like WD and Fujitsu.

 

Brian

The issue remains , generating regulated DC with any large current requirements isn't that simple or efficient using transformers and it's the reason they have virtually disappeared in commercial PSUs

 

As for dodgy far eastern designs , this is true of any component. Buy good recognised brands.

 

Funny, had loads of bits of PCs failing over the years but the PSUs are usually bomb proof in my experience ( and make quite good layout power supplies )

 

Edited

 

The op has indicated he needs in excess of 5A ( possibly more ) of dc current , he has not indicated how he intends to go from the AC output of a transformer to regulated ( and properly short circuit protected ) DC, at the desired current ( plus some headroom ) There's a lot more to this then just a Traffo

Edited July 27, 2017 by Junctionmad

[john new]

If you are using DC analogue controllers you need a transformer for each one, or transformers with multiple electrically separate windings.

 

Why do you need DCC as well?

 

Each transformer should be individually fused on the mains input side. These fuses can be mounted inside the box. You cannot rely on the fuse in the plug to protect the transformers, it will only protect the mains cable.

 

The outputs should be fused as well. Panel mounted fuse holders are convenient. Fit slow blow so in the event of a short, the controllers overload protection operates first. This is called discrimination.

 

I used a big steel box from an old burglar alarm for mine. The box from a piece of 1970's hifi is another option.

 

Your reference to a big steel box for a transformer housing confuses me. In the past electricians have told me you can't use metal housings for transformer boxes, everything that could be a conductor has to be enclosed. My proposed solution when I recycle the control panel components from The Classic Train Set (Layout now dismantled) was to put the transformers on the floor in a large plastic container, with them mounted internally on a wooden plank and with that bolted through with plastic car number plate bolts so no metal to the outside. Power box to sit on the floor with low voltage fly leads up to the layout panel. (Analogue systems 12V DC and 15V AC)  Housing probably a plastic tool box, lid cable tied down for safety and with louvered plastic panels added for ventilation.

 

What are the rules for making mains power boxes?

 

I fully appreciate need for safety and wish to be compliant as I don't want to build something that a future exhibition venue might prevent me from using. The panel and incorporated power box for the CTS, I made circa 1998 when I stopped using my original 1950s HD gear, was safe as per my thinking back then (still is for home use) but I doubt it is compliant with modern regs.

Edited July 28, 2017 by john new

[Dagworth]

Your reference to a big steel box for a transformer housing confuses me. In the past electricians have told me you can't use metal housings for transformer boxes, everything that could be a conductor has to be enclosed. My proposed solution when I recycle the control panel components from The Classic Train Set (Layout now dismantled) was to put the transformers on the floor in a large plastic container, with them mounted internally on a wooden plank and with that bolted through with plastic car number plate bolts so no metal to the outside. Power box to sit on the floor with low voltage low voltage fly leads up to the layout panel. (Analogue systems 12V DC and 15V AC)  Housing probably an plastic tool box, lid cable tied down for safety and with louvered plastic panels added for ventilation.

 

What are the rules for making mains power boxes? I fully appreciate need for safety and wish to be compliant as I don't want to build something that a future exhibition venue might prevent me from using.

Firstly, if you don't KNOW the rules then this is not the place to be asking, and secondly, don't even try building your own, get professional advice with anything to do with mains electricity

 

Andi

[john new]

Firstly, if you don't KNOW the rules then this is not the place to be asking, and secondly, don't even try building your own, get professional advice with anything to do with mains electricity

 

Andi

Advice understood Andi.

 

However, I am not stupid and am capable of building something that is safe for use at home, what I don't know is how to make sure it is both safe and fully compliant with current rules for use in an exhibition venue. If someone explains the rules, or gives a link to them, I can then see if I can make it compliant. If I can't I will buy in. As a retired Emergency Planning Officer I am already well above average when it comes to being risk-averse.

Edited July 28, 2017 by john new

[kevinlms]

Advice understood Andi.

 

However, I am not stupid and am capable of building something that is safe for use at home, what I don't know is how to make sure it is both safe and fully compliant with current rules for use in an exhibition venue. If someone explains the rules, or gives a link to them, I can then see if I can make it compliant. If I can't I will buy in. As a retired Emergency Planning Officer I am already well above average when it comes to being risk-averse.

Any device should be safe to use at home and away. There is no such thing as 'OK for home, but unsuitable for exhibitions'.

 

Reminds me of a former work colleague who complained for months that his ladder was unsafe and needed replacement. When it was condemned, his first question was 'Can I take it home'?

[john new]

Any device should be safe to use at home and away. There is no such thing as 'OK for home, but unsuitable for exhibitions'.

Reminds me of a former work colleague who complained for months that his ladder was unsafe and needed replacement. When it was condemned, his first question was 'Can I take it home'?

My earlier wording probably wasn't clear enough. I wasn't implying safety at home should be lower, just asking for a link to guidance to follow in future. The panel I will dismantle in due course is safe now, but with hindsight and more experience of assessing risks I can see where I could have made it safer. .One of those moves would be separating mains from the controls with a separated power box, the post was made as I read it that the OP was suggesting putting the transformer on the panel. Nb The heading implied that, the text doesn't.

 

The risks I could eliminate by making it safer when put onto the risk matrix fall in the area of extremely unlikely nevertheless any replacement will also cut out those. Anything constructed will be PAT certified before going out on the road.

 

Won't be relevant to me for well over 12 months anyway at minimum as any replacement HD layout is a long way off. For my current Pebbles End layout I use a Gaugemaster (as I already owned it) and current build Plum Hollow will use the Combi coming because I subscribed to BRM.

Edited July 28, 2017 by john new

[Crosland]

The op has indicated he needs in excess of 5A ( possibly more ) of dc current , he has not indicated how he intends to go from the AC output of a transformer to regulated ( and properly short circuit protected ) DC, at the desired current ( plus some headroom ) There's a lot more to this then just a Traffo

 

Not from a single output. Three Amps is easily doable with large reservoir caps, linear regulators and lots of heatsinking.

 

Personally, I wouldn't bother, just buy suitable item(s) off the shelf.

[dasatcopthorne]

Have a look at the power supplies on the Brimal website.

 

Possibly put them inside a plastic Project Box.

 

Dave

Edited July 29, 2017 by dasatcopthorne

[Junctionmad]

There's no

Issue with metal cases being used , conventional earth bonding should be employed

 

The " rules " can be complex because they cover many cases but a safe solution is well within the reach of a competent DIYer providing they have a reasonable understanding of the basics.

[28XX]

Personally, if anything goes BANG I'd like it to be in a big, well earthed steel box than anything flammable like plastic or wood.

 

Incidentally the ex burglar alarm box had a cable restraint and a fused-phase, earth and neutral terminal block ready fitted.