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ikcdab

12v ring main?

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for accessory supply (not traction current, that is separate), i intend to install a 12v bus line under the baseboard and then branch off from this for various accessories such as servo drivers, LEDs, relay power etc.

Thinking i know how my house is wired, i think i could continue the bus line back to the power supply to complete a ring - as below.

Untitled-1.jpg.06ef4d5c69bca4e50330a0782c379921.jpg

 

the total length of the ring would be about 18m a nd i have a reel of 2 x 0.75 multistrand to use for it. Online calculators show a voltage drop of around 1 to 1,5 volts if it was a single "open-ended" cable, but i have no idea how this works it i loop the end back to the power supply.

 

so, questions, good buddies - 

1. is it ok to loop the end of my 12v bus line back to the power supply - i dont think i am creating any short circuits and it seems to match my house wiring

2. will a ring as opposed to an "out and back" reduce voltage drop  - i guess by 50%?

 

Thanks

Ian

 

 

Edited by ikcdab

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Other then the consumption of wire there is no other drawback to a low voltage ring main , however twice the number of connectors  etc give opportunities for poor installation and hence unexpected resistances 

 

if you size your wire right , you don’t get any issues with voltage drop anyways and wire is expensive , ring mains are essentially simply doubling the conductor area 

 

( the ring main has largely been replaced by radials ) 

Edited by Junctionmad

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There is a fairly simple way of calculating all the volt-drops around a ring circuit, which depend upon the size and position of the loads around the ring, but for a rough-and-ready check you can add up all the loads, assume they are all at the furthest point, calculate volt-drop for a single pair of wires, and divide that by two.

 

If you want to visualise how volt-drop works in a ring main, imagine a clothes-line tightly strung between two pole, the height of the fixings on each pole above ground being the same. Level, straight line. Voltage constant around the circuit under no-load condition.

 

Now hang a weight somewhere along it, representing a load; now a different weight somewhere else; and a third weight etc. The clothes-line will now form a series of straight-lines at different angles to the ground ........ that is your voltage profile around the circuit.

 

1 hour ago, Junctionmad said:

the ring main has largely been replaced by radials

 

Not so. Rings and double-end feeds are used very widely indeed in electrical distribution.

 

 

  • Informative/Useful 1

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Thanks for the replies. I was concerned that taking a piece of cable out of a 12v supply, round in a 18m loop and straight back in again would create a problem.

Ian

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Think about whether you should put a fuse or circuit breaker on the feed to this circuit at the supply, and make sure you connect it all up very carefully and switch off the supply when not in use.

 

12V might not sound very exciting because it is safe to touch, but if the supply can feed say 1A continuously into the circuit, and there is a dodgy joint or substantial leakage, that current is quite enough to cause sufficient heating to start a fire in the nice, dry, fuel-rich environment of a model railway.

Edited by Nearholmer

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2 hours ago, ikcdab said:

Thanks for the replies. I was concerned that taking a piece of cable out of a 12v supply, round in a 18m loop and straight back in again would create a problem.

Ian

Nope, nothing wrong at all.

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1 hour ago, Nearholmer said:

Think about whether you should put a fuse or circuit breaker on the feed to this circuit at the supply, and make sure you connect it all up very carefully and switch off the supply when not in use.

 

12V might not sound very exciting because it is safe to touch, but if the supply can feed say 1A continuously into the circuit, and there is a dodgy joint or substantial leakage, that current is quite enough to cause sufficient heating to start a fire in the nice, dry, fuel-rich environment of a model railway.

Very wise,  thank you.  Now I don't know what size fuse.  Info online suggests my 9g servos consume 200ma running  and 6ma at idle and I'll have 30 or so. I only intend to activate max 5 at a time,  mostly 2s and 3s.  So 5 x 200ma max power?

My leds are also roughly 20ma, say there's 40 of those,  so 40 x 20ma.

So does that mean a  2amp fuse,  or have I got that wrong? 

Thanks

Ian

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2A sounds right.

 

What is the power supply rated at, and does it have built-in protection (fuse or circuit breaker)?

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I like the washing line analogy for a ring connection. The 2 basic ways of connecting multiple circuits to a single power supply are ring and star. Star is where each circuit, or group, is separately fed from the supply. Each has their advantages and disadvantages, e.g. with the start configuration you can have different fuses so they are tailored for each group of circuits. I don't think there is a clear winner. Your plan for a ring system looks fine.

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1 hour ago, Nearholmer said:

2A sounds right.

 

What is the power supply rated at, and does it have built-in protection (fuse or circuit breaker)?

Here's some pics of it.

I accidentally shorted it and it did cut out promptly!

The two led voltmeters survived the process.

Ian

20200916_194636.jpg.f22eafd5450933bdd11245b702e186b3.jpg

The little bit of black tape is just covering up the "power on" led which was dazzling the picture. 

 

And the ratings

 

20200916_194657.jpg.cfeeaf7cfc012a6c62411a3256faa40e.jpg

Edited by ikcdab

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14A @  +12V DC .......... you definitely need to put selected protection on the individual circuits that you feed with that, because feeding 14A into a poorly-made joint or a leakage path will quickly cause potentially dangerous overheating, and you need to be certain that the wires you use on the sub-circuits are fit for the full let-through currents of whatever fuses or circuit-breakers you use - you don't want the wire blowing before the fuse does.

 

The 5V and 3.3V outputs even more so - those are impressive currents for in a model railway environment!

 

Have you checked the ratings of the wires you've used to bring the various voltages out to the chocolate blocks? I'd want to be sure that the wires won't fuse (burn through) at currents that the PSU will happily sustain, that the chocolate blocks themselves are good for the currents involved, and that you've minimised the chances of accidental short-circuits on that whole board.

 

To be clear, its fire-risk I'm concerned about here much more than electric shock.

 

 

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22 minutes ago, Nearholmer said:

Have you checked the ratings of the wires you've used to bring the various voltages out to the chocolate blocks?

Those look like the output wires that came as part of the PSU to me.

 

I use an AT power supply on my large layouts, 8amps at 12v and 20amps on the 5volt. Wish the minus 12volt rail had a bit more oomph than the 1/2 an amp it's rated at though as I find the split power supply superb for Tortoise power.

 

Andi

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4 minutes ago, Dagworth said:

Those look like the output wires that came as part of the PSU to me.

 

Does the PSU have individual circuit protection inside it for each wire? 

 

Taking the orange 3.3V wires as an instance, is each one protected at 7A? 

 

Because if it doesn't, and they are simply connected together inside the unit, it will be capable of delivering the entire 28A, continuously, through one wire if the external board is arranged with terminals for the individual wires. In ikcdab's case, if the PSU doesn't contain protection for each individual wire then the full 28A could be supplied via two wires.

 

That's why I'm concerned - those wires look a bit lean for either 28A or 14A to me, although it is impossible to be sure from a photo, especially with the insulation in situ.

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14 minutes ago, Nearholmer said:

 

Does the PSU have individual circuit protection inside it for each wire? 

 

Taking the orange 3.3V wires as an instance, is each one protected at 7A? 

 

Because if it doesn't, and they are simply connected together inside the unit, it will be capable of delivering the entire 28A, continuously, through one wire if the external board is arranged with terminals for the individual wires. In ikcdab's case, if the PSU doesn't contain protection for each individual wire then the full 28A could be supplied via two wires.

 

That's why I'm concerned - those wires look a bit lean for either 28A or 14A to me, although it is impossible to be sure from a photo, especially with the insulation in situ.

No computer PSU that I've ever seen has protection for each individual wire, they tend to have all the output wires for each voltage soldered into a matrix of holes very close together with one huge mass of solder over the lot.

 

The backup PSU I use as a bench supply has 16AWG wires for each voltage, good for 15a as single wires in free air. https://www.engineeringtoolbox.com/wire-gauges-d_419.html

 

Andi

 

Andi

Edited by Dagworth

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They aren't seperated, there are no fuses inside

I have dismembered a few in my time, they rely on an electronic overload shut down and all the wires for each output are commoned.

IMHO they are not a good idea for model railway use. i.e I wouldn't use one.

Edited by melmerby
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I've got a 16v AC 80VA supply running around under my layout using 1.5mm blue/brown cable, it is fused at the supply

I use it for many things.

I even have an Arduino Nano based point decoder operating some MPB point motors running from it.

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That's what I thought - I found a schematic on-line for a sister of the one in question, and it looks to be exactly as you describe: a single output per voltage and an electronic shutdown, which I would guess is designed to be very "sharp-edged", allowing full design current flow, then cutting almost instantaneously if that is exceeded by even a tiny amount.

 

I would advise bringing all the wires for each voltage out to a decently-sized single terminal (rather than splitting them across a chocolate block), and having a decently-sized busbar with the individual sub-circuit protective devices on it. That way, the circuit up to the individual sub-circuit protection should be fit to carry the full rated current.

 

(As a personal/professional opinion, I regard it as "slightly dodgy practice" to use multiple cables in parallel on the output - best practice would surely be to have a single cable of suitable rating, terminated in decent lug. But, if that's the way the computer industry does it, its a miracle they don't experience fires in PSU connections from time-to-time.)

 

 

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I would favour using a 12v 5A PSU like a computer one using fancy circuitry they do not chuck out as much heat. Then use a voltage regulator in the feed to low volt devices 5v and 3.3v . for a big layout us two keep the positive lines separate but link the 0v  lines together to give a common ground. 

Don

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I have several computer PSU's stockpiled in my scrap drawer.    I am pretty cavallier about electrics but no way would I use anything like a 5 amp supply anywhere near model railway equipment. I have had wires glowing on a lot less than 5 amps and I wouldn't risk any more than a 1 amp power unit for accessories except possibly for throwing points with 1950s point motors and momentary contact.   Several separate 1 amp wall warts plugged into a proper 220/240 volt ring main properly installed by someone who knows what they are doing, (not necessarily a professional electrician as one nearly killed my son by wiring a metal patress box to a live feed) has to be the better solution. Even better use model railway power units with a decent 1 amp cut out not a modern 2.5p  1 amp Polyswitch.    If you must use a 5 amp monster then 0.25 amp Polyswitches on the individual feeds should delay the onset of the fault which burns the shed down.

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17 hours ago, ikcdab said:

for accessory supply (not traction current, that is separate), i intend to install a 12v bus line under the baseboard and then branch off from this for various accessories such as servo drivers, LEDs, relay power etc.

Thinking i know how my house is wired, i think i could continue the bus line back to the power supply to complete a ring - as below.

Untitled-1.jpg.06ef4d5c69bca4e50330a0782c379921.jpg

 

the total length of the ring would be about 18m a nd i have a reel of 2 x 0.75 multistrand to use for it. Online calculators show a voltage drop of around 1 to 1,5 volts if it was a single "open-ended" cable, but i have no idea how this works it i loop the end back to the power supply.

 

so, questions, good buddies - 

1. is it ok to loop the end of my 12v bus line back to the power supply - i dont think i am creating any short circuits and it seems to match my house wiring

2. will a ring as opposed to an "out and back" reduce voltage drop  - i guess by 50%?

 

Thanks

Ian

 

 

 

Hi Ian,

 

I'm not sure you even need 12 volts. Servos only need five volts and modern LEDs are usually far too bright at 20mA. I think you could feed the whole lot from 5 volts at even less than 5 amperes. As others have already said, that PSU is much too powerful to be used without individual current limiting devices on each output. That amount of energy dumped into a resistive short-circuit can generate more than enough heat to start a fire,

 

As for a ring, don't waste your time. It's only going to make it much more difficult to identify any problems. (Very few countries ever adopted the "ring mains", for good reason.)

 

Cheers,

Andy

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3 hours ago, AndyID said:

 

Hi Ian,

 

I'm not sure you even need 12 volts. Servos only need five volts and modern LEDs are usually far too bright at 20mA. I think you could feed the whole lot from 5 volts at even less than 5 amperes. As others have already said, that PSU is much too powerful to be used without individual current limiting devices on each output. That amount of energy dumped into a resistive short-circuit can generate more than enough heat to start a fire,

 

As for a ring, don't waste your time. It's only going to make it much more difficult to identify any problems. (Very few countries ever adopted the "ring mains", for good reason.)

 

Cheers,

Andy

Hi Andy, that's a good point, but I am using MERG servo4 drivers which need 12v. I know they are developing a 5v version, bit I already have 14 of the 12v versions ready to go. I have also installed 12v relays for polarity switching. So it's just seemed best to use 12v for everything.

Here are my switch panels: 

 

 

 

Ian

Edited by ikcdab

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9 hours ago, Donw said:

I would favour using a 12v 5A PSU like a computer one using fancy circuitry they do not chuck out as much heat. Then use a voltage regulator in the feed to low volt devices 5v and 3.3v . for a big layout us two keep the positive lines separate but link the 0v  lines together to give a common ground. 

Don

There's no fancy circuitry about a computer PSU. They are just another switched mode power supply which are  readily available for all sorts of supply voltages

If you wanted just12v DC a dedicated 12v SMPS would be a lot smaller than a computer PSU.

 

9 hours ago, Nearholmer said:

(As a personal/professional opinion, I regard it as "slightly dodgy practice" to use multiple cables in parallel on the output - best practice would surely be to have a single cable of suitable rating, terminated in decent lug. But, if that's the way the computer industry does it, its a miracle they don't experience fires in PSU connections from time-to-time.)

 

 

But they have to be seperate wires in a PC as each one goes to a specific item via a suitable plug/socket combination, be it HDD, graphics card, motherboard etc. All have various combinations of supply voltage requirement

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Inside a PC I get the logic of doing it the way it’s done, and I would surmise that each of the sub-assemblies being fed incorporates local electronic over current (and possibly direct thermal) protection.

 

But, I thought this unit was being sold as a general purpose PSU for use outside of a PC too. Is that not the case?

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9 hours ago, melmerby said:

I've got a 16v AC 80VA supply running around under my layout using 1.5mm blue/brown cable, it is fused at the supply

I use it for many things.

I even have an Arduino Nano based point decoder operating some MPB point motors running from it.

Hi Mr M, what size fuse did you use?

Ian

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13 minutes ago, ikcdab said:

Hi Mr M, what size fuse did you use?

Ian

it's 5A fast blow but the cable is rated way above that.

It feeds the  LDT occupancy detectors & turntable decoder plus some other 15v accessories.

 

Edited by melmerby
wrong info

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