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Point solenoid activation - alternative(s) to switches?


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The points on my layout are all solenoid-operated, these being activated by a CDU via SPDT momentary mini toggle switches in the usual way.  All is good - until the time comes when a switch fails to activate a point, and the question then arises is it a dud solenoid or a dud switch.  That can sometimes be a bit of a pain to sort out, but I always manage to do it.

 

However, my Lady Wife is determined to keep playing trains after I've turned my toes up.  She's well handy with a screwdriver and a soldering iron so will be able to swop either a switch or a solenoid as required, but I fear she may well struggle with identifying which one's failed on her own.  I'd therefore like to reduce or preferably elimate the potential for switch failure, so she knows that if a point doesn't change, it's just about certain to be the solenoid.

 

Am I correct in assuming that my only real options are to replace the Ebay switches with probe and stud, or replace them with "professional" quality mini-toggles from the likes of RS and Farnell?

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How many solenoid v. switch failures do you see?

 

It's breaking the circuit that causes damage to the switch contacts, due to the arcing.

 

If you hold the switch until the CDU is discharged when changing points, the current through the switch will be much lower and less likely to cause damage.

 

Next time you have a failure try to dissect the switch and determine if it is the contacts that are damaged or something mechanical.

 

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1 hour ago, Junctionmad said:

Or use a CDU design that doesn’t pass the activation current through the switch ! 

 

Never heard of such a thing.  How's it work?  At present I use the CDU built into my Morley controller.

 

36 minutes ago, Crosland said:

How many solenoid v. switch failures do you see?

 

Off the top of my head, in the last couple of years two solenoids and three switches.  Judging by the feel of the offending switches, the failure has always been mechanical.  That's with three seemingly-different Chinese makes - two from Ebay and one from a model railway electronics supplier.

 

Logically the answer has to be probe and stud, but I'm perhaps unfairly biased against it.  I've never seen a picture of any setup that looks to me anything other than a bodge-up, and it's got to be slower in operation than switches (and yes, I do realise how slow the real thing could be).  Then there's the fact that my 27 points are fairly evenly split between two panels, which also incorporate isolating switches.  And those panels have to be A4 size ...

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49 minutes ago, spikey said:

Never heard of such a thing.  How's it work?  At present I use the CDU built into my Morley controller

You could do it with a relay. That would take all the power away from the switch.

Alternatively, use pushbutton switches instead. My friend uses solenoids, CDU and push button switches very successfully and the switches have lasted many years.

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17 minutes ago, ikcdab said:

You could do it with a relay. That would take all the power away from the switch.

Alternatively, use pushbutton switches instead. My friend uses solenoids, CDU and push button switches very successfully and the switches have lasted many years.

 

How could I do it with a relay?  Surely I'd need 27 of them? 

 

Ref push switches, I actually have 6 of them controlling 9 more solenoids via diode matrices.  The "ideal for point activation" ones I got from a recognised model shop started failing after well under 6 months: the industrial ones I got from Farnell are still fine after more than a year.  But so many of the points just need either-way switching rather than the route setting for which diode matrices are ideal, so I'd need far more push switches than I have room for on the panels.

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Hi spikey,

 

This is a really interesting query because it's not something that has come up much before. If it was a general problem you'd expect to see lots of people saying the same thing but that doesn't seem to be the case, AFAIK.

 

I can imagine that cheap switches might go wrong after a while but I think point motors ought to be more reliable than you suggest.

 

So, I'm wondering whether the CDU is in fact the real problem? Maybe it's just putting too much energy through the system?

 

Arcing in switches is a well known problem and it's worse in DC systems than AC because the conditions for the arc across the switch contacts can last longer and always transfers metal the same way. With AC the voltage reverses very quickly, of course, and any arc deposits metal randomly between the contacts. There are simple ways to reduce arcing but you'd need an electronics expert to suggest what might work in your case.

 

On my old layout (40 years ago) we used stud contacts to drive our point motors and it worked well except that the violent flashes were a bit scary and there was always a small chance that the probe tip would weld itself to the stud when you touched it! :wink_mini:

 

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Or use a CDU design that doesn’t pass the activation current through the switch ! 
 

There are several published designs where a transistor switches the output of the CDU and the switch simply switches the transistor. This removes the arcing and high current pulses from the switch 

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I’ve used cheap push buttons and they failed fairly quickly. Quality switches have been ok.  Probe and stud is easy and reliable - best if you have somewhere safe for the probe when its not in use - not a hole in the control panel where it can touch things on the other side.

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The standard arc reduction for this sort of situation is a diode wired in reverse across the solenoid coil, to catch the back emf induced by the collapsing field. The arc is when you take the probe off, or release the switch. 

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8 minutes ago, cliff park said:

The standard arc reduction for this sort of situation is a diode wired in reverse across the solenoid coil, to catch the back emf induced by the collapsing field. The arc is when you take the probe off, or release the switch. 

 

That's the solution to a different problem, for protecting semiconductors that may be driving the relay/solenoid.

 

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53 minutes ago, cliff park said:

The standard arc reduction for this sort of situation is a diode wired in reverse across the solenoid coil, to catch the back emf induced by the collapsing field. The arc is when you take the probe off, or release the switch. 

That’s what my brief research suggested too. A “snubber”.

 

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It works just as well with contacts. It is the suppression of the arc that matters. The diodes can, of course, be fitted at the switch if that is more convenient.

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Posted (edited)
11 hours ago, Junctionmad said:

Or use a CDU design that doesn’t pass the activation current through the switch ! 
 

There are several published designs where a transistor switches the output of the CDU and the switch simply switches the transistor. This removes the arcing and high current pulses from the switch 

 

And where, pray, might a chap find such circuit designs?

 

11 hours ago, Gordon A said:

Push button switches with two buttons per set of point blades.

Have you tried operating your points without a CDU in the circuit?

 

No space on the panels, which as I stated have to be A4, and yes.  It was a waste of time.

Edited by spikey
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1 hour ago, spikey said:

 

And where, pray, might a chap find such circuit designs?

 

 

No space on the panels, which as I stated have to be A4, and yes.  It was a waste of time.

All you ever need to know about CDUs. Near the end  there’s  a nice circuit that uses transistors to switch the CDU rather then the switches directly ( choice number 9) 

 

http://www.talkingelectronics.com/projects/75 Model Railway Projects/75 Model Railway Projects.html#Points PartB

 

 

quite a nice circuit as it uses standard toggles and provides mimic panel leds as well 

Edited by Junctionmad
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5 hours ago, Junctionmad said:

All you ever need to know about CDUs. Near the end  there’s  a nice circuit that uses transistors to switch the CDU rather then the switches directly ( choice number 9) 

 

http://www.talkingelectronics.com/projects/75 Model Railway Projects/75 Model Railway Projects.html#Points PartB

 

 

quite a nice circuit as it uses standard toggles and provides mimic panel leds as well 

Useful, but appears to be quite profligate with components for (as drawn) one turnout. I suspect that it wouldn't be too hard to adapt so that the big power capacitors could feed multiple switching circuits or, alternatively, the whole shebang could be used, via a multi-position rotary switch to control multiple point motors. The former would probably be preferable, otherwise the advantage of the switch position as an indicator of point position is lost.

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16 hours ago, cliff park said:

The standard arc reduction for this sort of situation is a diode wired in reverse across the solenoid coil, to catch the back emf induced by the collapsing field. The arc is when you take the probe off, or release the switch. 

 

I suppose it depends on how long you keep the switch depressed but with a CDU the current decays as the cap discharges. If the switch is closed longer than the time it takes for the cap to discharge there won't be much current flowing through the solenoid and the field will have already collapsed by the time the switch opens. In that situation there won't be any back EMF to suppress.

 

It seems more likely any arcing at the contacts happens when the switch is closed. At that time a fairly high voltage low impedance source (the charged cap) is connected to a low impedance load (the solenoid coil).

 

Perhaps someone who uses stud contacts can tell us what happens. Is there an arc when the wand is brought in contact with the stud and/or is there an arc when the wand is removed from the stud? (Easy to tell if you turn off the lights :)  )

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Posted (edited)

Gosh, this is all good stuff :)

 

Ref that points controller circuit, thank you but as I understand it I'd need one per switch, which with 27 switches is hardly feasible.

 

ETA - It's just occurred to me that some of my solenoids are connected to Gaugemaster GM500 relays for frog switching and signal activation.  These are fitted adjacent to the solenoid rather than the switch.  Could this have any bearing on switch failure?

Edited by spikey
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18 hours ago, cliff park said:

It works just as well with contacts. It is the suppression of the arc that matters. The diodes can, of course, be fitted at the switch if that is more convenient.

 

Now that seems to be both feasible and affordable, so I need to ask - which diodes would be suitable, and which way round across the switch contacts?

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Based on my experience I'm surprised this is an issue. My experience of push to make switches used with a CDU is a good one. I can't recall having a failure despite using a pretty hefty CDU changing up to 5 points at a time in recent years. I've used those push to make switches since the 1980s and many have been re-used in different layouts. For my current project I purchased a batch from Squires and these are looking good at the moment. My solenoid point motors have been rather mixed. These are very small sample sizes so may not be meaningful but this is my experience over the last eight years. These solenoids have been used on exhibitions layouts and so have regularly had two days of very heavy use. The way I run my layouts means that there is a lot of point changing happening all the time.

SEEP original solenoids - installed 12 and three failed. Failures replaced with Peco.

Peco original - installed more than 20 and one failure.  

Peco Twistlock - just one installed on new project but that failed after a few months and not much use. This of course cannot be seen as representative in any way and was probably just bad luck. I haven't returned it as I want to see if I can take it apart and fix it. Although expensive and even after my bad experience I do like the Twistlock design. I'm pretty sure mine was just a glitch.

 

At exhibitions I take a spare pre-wired point motor with me just in case. 

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In answer to the original question. Just take a feed from the CDU output that goes to the switches and touch the contacts on the point motor solenoid with this wire. If the solenoid flicks its the switch (or maybe a broken wire), if not its the solenoid.

 

 

Edited by Chris M
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37 minutes ago, Chris M said:

 ... My experience of push to make switches used with a CDU is a good one ... At exhibitions I take a spare pre-wired point motor with me just in case. 

 

My experience of push to make switches is also a good one, provided that they're of decent quality such as from Farnell or RS.  But as I said in my OP, most of the switches on my panels are SPDT momentary.  It's those that are the issue.

 

32 minutes ago, Chris M said:

In answer to the original question. Just take a feed from the CDU output that goes to the switches and touch the contacts on the point motor solenoid with this wire. If the solenoid flicks its the switch (or maybe a broken wire), if not its the solenoid.

 

But that wasn't the question.  As I said in my OP, my concern is to arrange things so that after I'm dead, my wife can continue to play trains with the minimum risk of point failure.   Given her aptitude with things mechanical, point failure is about the only issue which would be a major problem for her. 

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