Illuminated switches - common return - different voltages ??

[Bideford Light]

Illuminated switches - common return - different voltages ?? I was told it should work!
Basically that's my problem. I've built a control panel with rocker switches to isolate track sections, these have a 12v illumination built in and having only three terminals are common return. But the track supply is up to 20v. The 12v & 20v are supplied by completely separate transformers, connected via the common return.8|
No track power lights are fine, but turn the controller up and in one direction the lights brighten, then the other direction they dim out?
Can any one say what I've got wrong?
8|

 

[andyspencer]

Hi there, I'm not sure how to explain it to you, but I don't think this would work. Maybe if the voltage was AC or constant polarity DC, but with you reversing polarity on DC, you common return would now be half a return and half a positive.
I'm sure someone will reword the above if it doesn't make sense...

 

[Bideford Light]

Andy,

Thanks that what I would have thought logically. But over the years I've built and worked on dozen of layouts using combined common return on the track and point motor supply with mulitple controllers but no similar problem seems to arise as you switch directions on the various controllers.
Also the switches which simple miniature rocker type are rated for mains voltage plus 12v illumination only have three connectors so it must be possible somehow!

 

[ntpntpntp]

Can you post a diagram of how you've wired this up?
Are these switches definitely wired with the illumination on a separate pole?
You're correct in using totally separate supplies with common return, but it sounds like the illumination is somehow on the common side of the circuit and is seeing both voltages across the bulb.

 

[Bideford Light]

Hi, I've attached a drawing. hope it makes sense! Thanks for your time.

 

[Gizzy]

Hi, I've attached a drawing. hope it makes sense! Thanks for your time.

As wired this isn't common return. I think you have made an assumption that wiring both negative terminals (or both positive terminals) is common return.

What you have done is wired 2 different supplies together on the 'return' line, but this doesn't work for the reasons below.

In the case below, the LGB controller reverses polarity, so you could have controlled 0 to +20V from the LGB and +12V from the Hornby combining as per this diagram.

If the LGB controller is reversed then the polarity is also reversed and thus you will get a current flow from the positive of the Hornby unit to what is now a negative of the LGB one, as per below.

That explains why your indicators brighten with the controller in one direction and dim when in the other. I suggest that you do not use 'common return' in this case using 2 different power units but wire the track and the lighting circuits seperately....

 

[Gizzy]

This is how I would wire the track and indicator circuits.

Note that each power unit has its own wiring.

I've added another switch and indicator to your diagram to show that you can have 'common return' for each of the track power and indicator circuits.

'Common Return' is really just a way of reducing wiring and saving on costs and copper. There is nothing to stop you wiring each indicator and switch directly to the power unit terminals, but the cost of extra wiring will be 25-50% more....

 

[steve parberry]

And i was told DCC was complicated:wits::wits::wits:

 

[Bideford Light]

Steve, Generally the old analogue approach is easy if you see http://rail.felgall.com/crw.htm this gives a nice clear explanation of common return in model railways. But for some reason it doesn't work here :wits:

Gizzy, My switches only have three connections so I can't split the track & light circuits. Based on my experience and the above site I would have assumed the light in my case would have worked like another train on a linked track section, but obviously not! I'm now wondering if the easiest thing would be to just run a constant 12v through the switches and use a relay to switch the track power.
Any thoughts?

 

[Gizzy]

Steve, Generally the old analogue approach is easy if you see http://rail.felgall.com/crw.htm this gives a nice clear explanation of common return in model railways. But for some reason it doesn't work here :wits:

Gizzy, My switches only have three connections so I can't split the track & light circuits. Based on my experience and the above site I would have assumed the light in my case would have worked like another train on a linked track section, but obviously not! I'm now wondering if the easiest thing would be to just run a constant 12v through the switches and use a relay to switch the track power.
Any thoughts?

From your diagram I thought you were using double pole switches so there should be at least 4 connections, normally 6? (Effectively these are 2 switches linked together.) Certainly, you should have 2 wires for the track and 2 for the indicator as per the diagram you posted. So I'm confused now!

Maybe you need to confirm that you have the right switches installed before we can progress any further. Do you have a part or cat number for these switches so I can check please....

 

[Gizzy]

Just pondering about this whilst on the Night shift;

if you swap the wires on the terminals of the Hornby Unit, what are the results?

Same or different?

And do you have a multimeter?

I reckon you'll be needing one....

 

[Gizzy]

I've been trying to fathom this out again today, now that I've slept and swung my body clock back onto day shifts!

I now understand that the switch you have drawn is one unit, with 3 terminals. I believe this switch is the cause of your problem.

The linky you provide to Steve's Railway Page has the following paragraph;

The single supply system has a number of disadvantages. Firstly you can not use the common return system with single supply because the power to the controllers is common and with more than one common section in the circuit trying to run two trains in opposite directions will give a short circuit. Each section of track will therefore need its own separate return wire and this means more wire more connectors and double pole instead of single pole switches to switch the sections between controllers.

This is what I reckon you have done using the switches you've fitted. As I said earlier in post #7, a Double Pole type switch is what is required with separate feeds to each pole from each supply.

To continue to use the switches you have brought, then it looks like you will have to go with your suggestion of using a relay to switch the track power. This effectively also divides the two circuits (track power & indicator power) as I drew in my modified diagram of your own drawing....

 

[Bideford Light]

Gizzy,
Many thanks for your thoughts. As I don't want to replace the switches I'll insert a switching relay so I don't mix different voltages and polarities.

 

[don9GLC]

'When you have eliminated the possible, all you are left with is the impossible' or so Sherlock Holmes is claimed to have said.

Your circuit should work, so its impossible. Except that in electrical circuits there is usually a fault or a mistake if things don't work.

Quite clearly your indicator lights are being powered by both supplies. There must be an unintended connection somewhere. It could be within the power supplies (unlikely but possible), or in the internal connections of the switches, or the way the other switches are connected together. Or something else!

It should not be necessary to connect relays to make the circuit work. Since there is something wrong with the circuit, there is no certainty that adding relays will resolve everything. There is a small but serious risk of fire if there is a fault.

The simplest way to resolve this is with a multimeter. If you don't have one, and cannot borrow one, it may be cheaper to purchase a basic meter. Maplin have a basic model for just less than £10.

The first thing I would check is that the outputs of the power supplies are actually 'floating'. Measure between earth and each terminal with nothing else connected. The actual voltage will depend on 'stray' resistance but a zero reading would indicate that there is an internal connection to earth. I would expect a similar reading from each terminal.

I am a bit dubious about your rocker switches. Mains rated switch with 12V indicator? It depends how you connect them, but that sounds like it could contravene safety regulations. Are you absolutely sure that you have correctly identified the connections, and that the internal connections are as you have shown? As Gizzy mentioned, what make and part number?

And finally how have you connected the other switches and track sections? Disconnect all switches and track sections and then try connecting one, test for correct operation, then add the next switch and track section. And so on.

Sorry that this is not a clear simple answer, but you have not asked a clear simple question!

In the news this week a care worker used a manual key to open a lift door, pulled a wheelchair into the empty lift shaft and fell seriously injuring herself and killing the patient. It was 'well known' that there was a problem with the lift. Its a lot less dramatic, but if you have an electrical fault that is not understood, you risk a serious fire.

 

[don9GLC]

Oops, best to engage memory (and brain) before posting.

First year circuit analysis needed. One circuit, two supplies, should be calculated as two separate circuits and the results added, the Principle of Superimposition.

Effectively the 24V powers the track AND the indicator light, and the 12V powers not just the indicator light but contributes to the track power.
In short, this circuit will not work as expected. There is also the issue that 36V is possible, exceeding safety guidelines.

I could attempt a calculation using assumed values but it's far from simple. Motors and indicator lights do not have a linear voltage / current characteristic and electronic regulators are designed for certain behaviors. In any case, the answer is that the circuit does not work as required.

Using a relay is probably the simplest way of separating the two supplies if you want to keep the same switches.

 

[ntpntpntp]

I just went through a 5 minute online tutorial about superposition for circuit analysis, to try and refresh my extremely faded memory!

I would have thought that if this is the case (LGB controller contributing power to the light and/or the Hornby controller contributing power to the track) there must therefore be some appreciable current path (significantly lower than infinity resistance) between the output terminals of the other controller? Otherwise there's no path round the other branch of the circuit.

How about simply disconnecting the Hornby controller and seeing if the indicator light comes on when the LGB controller is turned up gently?

Must admit it kind of goes against what I remember being taught about separate power sources being separate circuits when used in common like this, which is why in one of my earlier posts I did comment maybe the indicator light is somehow in the common side of the circuit? Another possibility could be a bad connection between the common pole of the switch and the common connection between the two controllers - that would divert track current back through the indicator light?

Anyway it clearly doesn't work as-is. Must admit I'd have stayed clear of using common return and used 4-pole switches and separate indicator lights on a completely separate circuit, as Gizzy suggested earlier.

 

[Zerogee]

While IANAE*, I do recall reading VERY STRONG WARNINGS IN BIG LETTERS on all LGB power supplies and instruction leaflets, that under NO circumstances should the power outputs of two such supplies be connected to each other....

Jon.

*I Am Not An Electrician, as opposed to the more common internet disclaimer IANAL (I Am Not A Lawyer)!

 

[don9GLC]

I would have thought that if this is the case (LGB controller contributing power to the light and/or the Hornby controller contributing power to the track) there must therefore be some appreciable current path (significantly lower than infinity resistance) between the output terminals of the other controller? Otherwise there's no path round the other branch of the circuit.

When using the Principle of Superimposition, as I remember it, you replace a constant voltage source with a short circuit when calculating the path through other power sources. Hence there will be a relatively low resistance throughout the other controller, roughly the equivalent internal resistance of the power supply.

This is not the best forum (technically) for posting circuit diagrams or mathematical equations, but here goes. The resultant currents in each connection are the sum of the red and blue currents.

And even though he may not claim be a lawyer, Jon is completely correct that two power supplies should not be connected together (without some powerful electrical magic!) And if you don't know what is needed you DEFINITELY should not connect them!

Don