SPST vs DPDT

[Chevron]

Can some one explain to me or show me in a wiring diagram for trains how each of these would work or what one is used for 2 controllers on a DC layout

 

for this post im using black and red for each rail

 

A SPST will break a single side of rail ? example either black or red depending on what's wired.

so only 1 wire is wired to the switch.

 

 

A SPDT will switch a single side of rail to a second controller? example either black or red depending on what's wired.

so only 1 wire is wired to both switch

By using this then the second wire is always fed through both controllers. - Would this cause a short circuit if both control a common wire together?

 

and

a DPDT will switch both rails to a second controller? example Both black and red.

so both wires are wired to both switch

which would swap complete control of both rails to each controller?

 

Do i have this correct?

 

If so im guessing you all use DPDT

and can someone show me or explain how that wiring would work.

 

Regards

Danny

[Broithe]

This is the DPDT situation you mention, I think. The SPDT is like one half of the DPDT switch.

 

 

You can have three-position centre-off switch or just a two-position switch, which has no 'off' position.

[irishthump]

If you simply want to switch between the 2 controllers then you can just use a SPST wired to the red rail as in Broithe's diagram. Then you wire both controllers using the black wire as a common return. This won't cause a short.

[Broithe]

If you simply want to switch between the 2 controllers then you can just use a SPST wired to the red rail as in Broithe's diagram. Then you wire both controllers using the black wire as a common return. This won't cause a short.

 

That's right - an DPDT switch is really just two SPDT switches that move together - in the diagram, the top part is one SPDT switch and the bottom is another, the fact that they move together is what makes them a DPDT switch. You don't (usually) need both poles in this sort of situation, but it does no harm to have both, and costs little extra in materials or effort.

 

It can be a matter of preference whether you have the centre-off position or not - it does no harm and can be useful, too.

[Chevron]

Thanks Broithe that diagram is perfect

 

I was planning on going with a on-off-on switch.

 

I think for ease of wiring i might go with a SPST switch and keep a common black.

[Broithe]

I was planning on going with a on-off-on switch.

 

That's what I would do.

 

There's usually little or nothing in the cost and the centre-off position can be useful - just don't accidentally leave it there and confuse yourself, but you'll only do that once...

[patrick]

If you simply want to switch between the 2 controllers then you can just use a SPST wired to the red rail as in Broithe's diagram. Then you wire both controllers using the black wire as a common return. This won't cause a short.

 

No, the system works. SPST switches using one rail as common return requires less wiring though and I strongly suggest using center off three position switches so sections can be isolated allowing for parking locomotives and trains on any section of the layout even when two other trains are being run. Actually the more I think about it using two position switches is a really bad idea.

[Chevron]

No, the system works. SPST switches using one rail as common return requires less wiring though and I strongly suggest using center off three position switches so sections can be isolated allowing for parking locomotives and trains on any section of the layout even when two other trains are being run. Actually the more I think about it using two position switches is a really bad idea.

 

Exactly what i was thinking

 

Having a neutral like a brake instead of a always on whether you like it or not lol

[Broithe]

Centre-off is almost always preferable, you don't have to use the 'off', but you can.

 

These Hornby controllers don't have a centre-off for the direction switch, like the Gaugemaster ones do, and this can cause strange issues requiring cuts in the track that aren't necessary if you use a Gaugemaster.

[Chevron]

Centre-off is almost always preferable, you don't have to use the 'off', but you can.

 

These Hornby controllers don't have a centre-off for the direction switch, like the Gaugemaster ones do, and this can cause strange issues requiring cuts in the track that aren't necessary if you use a Gaugemaster.

[ATTACH=CONFIG]18787[/ATTACH]

 

 

And speaking of which.... should i just get a second basic controller or should i buy a dedicated 2 controller like that ?

 

as this is what im using just now

[Broithe]

For a DC controller, the Gaugemaster is hard to beat. Available in single, double and quadruple housings.

 

You can always put two singles together, but you can't move a double apart....

 

If a single dies, you still have one that works.

 

A double will be cheaper than two singles.

 

It's up to you and you requirements...

[Chevron]

Any recommendations on a dual track controller ? as in where to buy one?

[Broithe]

Marks carry Gaugemaster - and Hattons do.

 

The 'dual' is a good device - The Model D.

 

[Broithe]

Marks carry Gaugemaster - and Hattons do.

 

The 'dual' is a good device - The Model D.

 

A quick look on eBay finds a good few, but at more than the Hattons price!

[Chevron]

yea had a look at ebay and was lost . so had to check back here as to what to buy.

 

funny how cosmetics can play a part.

 

the first one you pictured looks better for me .. but i can go with the model D

[Broithe]

yea had a look at ebay and was lost . so had to check back here as to what to buy.

 

funny how cosmetics can play a part.

 

the first one you pictured looks better for me .. but i can go with the model D

 

The Gaugemaster has the centre-off direction switch, the Hornby doesn't, which can cause issues.

 

Gaugemaster will basically fix it, should it go wrong, for the rest of time, even if you bought it second-hand (because they very rarely do fail). I've never had direct experience of one failing, anyway..

[Chevron]

sounds good to me.

 

Cheers broithe that's one added to my shopping list.

Edited April 20, 2015 by Chevron

[Junctionmad]

If you simply want to switch between the 2 controllers then you can just use a SPST wired to the red rail as in Broithe's diagram. Then you wire both controllers using the black wire as a common return. This won't cause a short.

 

huh, what happens when you reverse one controller and not the other, in a rail common situation

[Chevron]

huh, what happens when you reverse one controller and not the other, in a rail common situation

 

 

thats a good question id like to see answered.

Cant test as i only have one controller at the minute.

[Broithe]

huh, what happens when you reverse one controller and not the other, in a rail common situation

 

Well, nothing, other than the train on the track that is now being fed from the 'reversed' controller will go the other way, as was required.

 

All that matters to any loco is the difference between the two rails that it's on.

[Junctionmad]

Well, nothing, other than the train on the track that is now being fed from the 'reversed' controller will go the other way, as was required.

 

All that matters to any loco is the difference between the two rails that it's on.

 

hmm. if you wire standard DC controllers through a SPDT switch and common the other wire ( as per the diagram shown previously ) , these controllers , reverse by using their own DPDT switch, so if one is reversed and the other is left in forward. Then you will have the positive of one controller directly connected to the 0VDC of the other. (via the common unswitched wire )

 

You cant use common rail in this situation IMHO

 

( it can be fudged by adding the reversing DPDT switch after the SPDT controller selector switch of course, but thats not typically how this DC controllers are put together )

Edited April 21, 2015 by Junctionmad

[irishthump]

Well, nothing, other than the train on the track that is now being fed from the 'reversed' controller will go the other way, as was required.

 

All that matters to any loco is the difference between the two rails that it's on.

 

But if you have one controller set forward and the other set backward if both are turned on you will have a short at the common rail.

You will have to be sure to never have both controllers powered on at the same time.

By powered on I mean with the throttle above zero.

[Junctionmad]

But if you have one controller set forward and the other set backward if both are turned on you will have a short at the common rail.

You will have to be sure to never have both controllers powered on at the same time.

By powered on I mean with the throttle above zero.

 

Im trying to gently get him to see the error of his ways

 

In fact you dont need both controllers powered on,( i.e. throttle advanced ) because in a standard emitter follower ( series pass) DC controller, there is a constant path to the OV line, even one controller has its throttle to zero, the other controller will be staring into a 0V line dead short.

 

fundamentally , there is no common rail in a DC system, both rails can assume both polarities ( at different times )

 

DCC is different, it will work in that case ( though its not of course necessary )

Edited April 21, 2015 by Junctionmad

[Dhu Varren]

huh, what happens when you reverse one controller and not the other, in a rail common situation

 

As long as both controllers are fed from separate transformers, and are not feeding the same track section at the same time, there is not a problem. However, if both controllers are fed from the same transformer, then a short circuit will occur with a common rail setup when one controller is reversed.

[Junctionmad]

As long as both controllers are fed from separate transformers, and are not feeding the same track section at the same time, there is not a problem. However, if both controllers are fed from the same transformer, then a short circuit will occur with a common rail setup when one controller is reversed.

 

you cant even be sure , in that case, as most transformers, have the 0vDC lines referenced/connected to earth to prevent floating common mode voltage issues. Hence there is a common path for the short through the mains earth wire. IN double isolated systems it will technically work, but its really bad practice, because the 0VDC of one of the controllers will be lifted up by the voltage from the other isolated controller and so will anything connected to that 0Vdc ( accessories etc )

 

 

basically it not a workable solution, use DPDT switches

[Chevron]

So once i switch the reversing switch it changes the polarity of the rails as in

 

if black was positive and red was negative it would be

black is now negative and red now positive

 

so if both controllers had power on the rail it would short due to one trying to make red positive and the other trying to making negative?

 

so there is no shorting of the wirings its a case of must have 2 wires from each section to swap the rails polarity back and forth.

[irishthump]

Im trying to gently get him to see the error of his ways

 

In fact you dont need both controllers powered on,( i.e. throttle advanced ) because in a standard emitter follower ( series pass) DC controller, there is a constant path to the OV line, even one controller has its throttle to zero, the other controller will be staring into a 0V line dead short.

 

fundamentally , there is no common rail in a DC system, both rails can assume both polarities ( at different times )

 

DCC is different, it will work in that case ( though its not of course necessary )

 

I'm glad I run DCC!

[Junctionmad]

I'm glad I run DCC!

 

truly DCC is much simpler for any sort of decent sized layout. DC requires too many switches and connections etc to try an simulate prototype movements

[Dhu Varren]

basically it not a workable solution, use DPDT switches

 

Strange then, that common return wiring has been used successfully on model railways for donkeys years.

 

http://rail.felgall.com/crw.htm

[Joe Keegan]

''most transformers, have the 0vDC lines referenced/connected to earth '' Junction mad

 

If you live in Ireland / UK you would trip/operate the RCD/ELCB on your socket circuit using what you described.

 

UK / European standards are double insulation transformers where there is no risk of electric shock on the low voltage output. The mains winding and the low voltage winding does not touch . The power is generated by electro magnetic field of the mains winding to the low voltage winding .

 

Check a H&M , Gaugemaster & Morley twin controllers where you will find that 1) the DC supply for the track is not connect to earth , 2) each supply for each controller is fed from a separate winding from the the transformer .

 

Dhu Varren is corrected '' as long as both controllers are fed from separate tranformers '' and this has always the work practice of common return wiring . If memory serves me correct the Rev Peter Denny used this method along with John Ahearn back in the 40's /50's .

Edited April 21, 2015 by Joe Keegan

[Junctionmad]

''most transformers, have the 0vDC lines referenced/connected to earth '' Junction mad

 

If you live in Ireland / UK you would trip/operate the RCD/ELCB on your socket circuit using what you described.

 

UK / European standards are double insulation transformers where there is no risk of electric shock on the low voltage output. The mains winding and the low voltage winding does not touch . The power is generated by electro magnetic field of the mains winding to the low voltage winding .

 

Check a H&M , Gaugemaster & Morley twin controllers where you will find that 1) the DC supply for the track is not connect to earth , 2) each supply for each controller is fed from a separate winding from the the transformer .

 

Dhu Varren is corrected '' as long as both controllers are fed from separate tranformers '' and this has always the work practice of common return wiring . If memory serves me correct the Rev Peter Denny used this method along with John Ahearn back in the 40's /50's .

 

 

 

Sorry, let me state first that Im a practising electronics engineer, and I write this surrounded by a considerable rack of test equipment

 

Firstly its not all all uncommon , in the absence of double insulated transformers, that the 0vdc is connected internally to the mains protective earth

 

Contrary to you comments, since no current path to earth exists , then this wiring will NEVER affect the RCD, in normal non fault situations . ( The RCD detects imbalances in Live and Neutral , and is not affected by DC currents on the secondary side of a transformer

 

There is NO requirement in the EU , that double insulated transformers HAVE to be used in low voltage power supplies . Some are and some are not. typically in small PSUs, that power iPhones and the like, you will get double insulated, but any PSU, with a metal earth pin, is likely not to be double insulated. larger switched mode power supplies are often not double insulated

 

The reason that the earth is connected to 0VDC is to ensure that a reference ground is maintained. This prevents the 0VDC line from floating above earth potential, which can cause common mode voltage issues, where you connect devices that are powered by their own transformers, together via a DC connection , a classic example would be comms busses etc. This is why the ATX spec for PC psu's had DC GND connected to earth

 

so you may find both types.

 

 

As to feeding with different transformers, yes no problem, but not as was suggested with SPDT feeding a SINGLE DC Track section, which anyway is not something you want to so anyway

[Junctionmad]

Strange then, that common return wiring has been used successfully on model railways for donkeys years.

 

http://rail.felgall.com/crw.htm

 

 

it traditionally was used with rheostat controllers, its far less useful with solid state devices. It relies on the transformer being truly floating , so that in effect one transformers GND can be lifted up above the other to allow the correct currents to flow to the respective controllers. Persoanlly I think its a nonsense process, when a simple DPDT switch will keep everything seperate.

 

It also generates issues when you have a common power supply, so again you have to check and KNOW that the PSU in a double controller etc is actually seperate and not commoned

[Mayner]

I hope the argument about common Vs separate return wiring is not confusing or putting our less experienced members off.

 

In my experience once the controllers are fed through separate transformers or transformer windings common return wiring using single pole switches is perfectly adequate for most layouts, the system works fine with resistance, variable transformer or electronic controllers including modern Gaugemaster

 

Common return wiring has been the standard in magazine the majority of magazine articles and books on layout wiring publish in the UK and United States during the last 50-60 years an it work.

 

The majority of section switches supplied by manufactures like Peco, Hornby are single pole intended for common return wiring.

 

I have built worked on several layouts large and small which worked fine with common return. The positive and negative flowing from separate transformers through a "common return" does not create a short circuit or any noticeable interference between controllers.

 

I use a 40+ year old H&M Safety Minor unit with variable transformer speed control, a 20 year old electronic hand held controller built as part of a MRSI club project and a new Gaugemaster electronic hand held unit. I usually power one of the hand held unit off the 15V ac auxilary supply on the Safety Minor, the second hand held off a separate 15V ac power supply.

 

Common return may be unsuitable for DC layouts where signalling is used to control train movements and is not recommended for large DCC layouts that are divided into booster or circuit breaker protected sections to prevent the entire layout shutting down if a loco derails or there is a short in one section.

Edited April 22, 2015 by Mayner

[Junctionmad]

As long as you ensure the transformers are truly floating

 

Its fine generally on older gear which was where the practice starts.

 

You can also get faults modes that beverage 2x track voltage ( mind you you get that in dcc )

 

I'm a member of merg. Common rail issues are a fairly common source of comments

 

In my view the practice stemed from dual rheostat type controllers ( H & M ) etc.

 

Just be aware of the fault issues.

 

Personally i would run everything of a good switched mode supply etc. common rail gets very confusing and troublesome once you start to power accessories etc, track occupancy etc.