magnetic uncouplers, was manual turnout operation

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grovenor-2685
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magnetic uncouplers, was manual turnout operation

Postby grovenor-2685 » Fri Nov 16, 2012 11:53 am

I have found to my cost that where switches control electro-magnets for uncoupling they need to be rated at 3 amps
.
The problem here is the inductive spark generated when you disconnect the coil. You have effectively created a magneto and your switch is acting as the spark plug. The answer is not a higher rated switch, you are just postponing the inevitable! The answer is to install a diode across the uncoupling magnet coil to divert the current away from the switch so that the spark is never generated.
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Keith
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Martin Wynne
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Re: manual turnout operation

Postby Martin Wynne » Fri Nov 16, 2012 2:21 pm

grovenor-2685 wrote:The answer is to install a diode across the uncoupling magnet coil to divert the current away from the switch so that the spark is never generated.

That will work only on DC, and dissipating all the inductive energy in a diode is bad news for the diode -- a series resistor is advisable too.

A better option is to wire a 12volt lamp bulb in parallel with the uncoupler coil. That will work equally well in quenching the inductive kick to protect the switch, for AC or DC operation, and will also usefully indicate when the uncoupler magnet is on or has been left on inadvertently.

regards,

Martin.
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grovenor-2685
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Re: manual turnout operation

Postby grovenor-2685 » Fri Nov 16, 2012 6:21 pm

That will work only on DC, and dissipating all the inductive energy in a diode is bad news for the diode -- a series resistor is advisable too.

In my experience uncoupling magnets do not work well on AC, tend to be noisy and uncertain in operation.
Putting a resistor or lamp across the coil increases the voltage across the switch and leaves a significant proportion of the current to try and pass through, I have never had a problem with diodes used in this application, the diode has to be suitably rated depending on the coil resistance and supply voltage.
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Keith
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Paul Townsend
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Re: manual turnout operation

Postby Paul Townsend » Sat Nov 17, 2012 10:33 am

Martin Wynne wrote:
grovenor-2685 wrote:The answer is to install a diode across the uncoupling magnet coil to divert the current away from the switch so that the spark is never generated.

That will work only on DC, and dissipating all the inductive energy in a diode is bad news for the diode -- a series resistor is advisable too.

Martin.


Sorry Martin, adding a resistor defeats the object of the diode......some diodes are specifically designed for this job and will not suffer.

Such suppression is also desirable to reduce anti-social radiated signals that disrupt radio and TV etc as well as suppressing obscenely high voltages that can destroy delicate electronics.

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LesGros
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Re: manual turnout operation

Postby LesGros » Sat Nov 17, 2012 11:28 am

grovenor-2685 wrote:
...In my experience uncoupling magnets do not work well on AC, tend to be noisy and uncertain in operation...

Keith,
Presumably, a simple working solution would be to use a dedicated 12v dc supply for magnetic uncouplers?

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LesG

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grovenor-2685
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Re: manual turnout operation

Postby grovenor-2685 » Sat Nov 17, 2012 1:47 pm

the supply can be dedicated or shared with other functions, but it needs to have a high enough rating for the uncouplers, measure the coil resistance and work out V/R to see how many amps you need, both power supply and protection diode need to be rated for that many amps.
Keith
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martin goodall
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Re: manual turnout operation

Postby martin goodall » Sat Nov 17, 2012 2:18 pm

This must come as news to a lot of people (including me). Most of us have been innocently using push button switches for our 12v DC uncoupler electro-magents for years with no idea what horrors were going on inside the switches!

I didn't actually check the switches I replaced when they failed (very soon after they were installed) - so I don't know if the contacts really did burn out or whether they were just defective switches, but I noticed that those which had not failed were rated at 3A, and so I used switches of the same rating for the replacements, since when I have had no problems (well, not yet anyway). I think someone once told me that electro-magnets can draw 3A, and so this seemed logical.

As to fancy circuitry to overcome the perceived spark erosion of switch contacts [and any electronic component, such as a diode or a resistor is 'fancy' to my electronically illiterate mind] , how necessary is this (bearing in mind that many modellers have never realised they needed it) ?

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Re: manual turnout operation

Postby grovenor-2685 » Sat Nov 17, 2012 4:50 pm

For a DC user such as yourself, Martin, just good practice to increase the life of your switches, getting higher rated switches obviously helps.
For a DC user or anyone with other electronic gadgets in the vicinity, such as Servos, pretty well essential as the spikes at switch off if not using a diode, are excellent at causing interference to electronic signals.
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Lindsay G
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Re: manual turnout operation

Postby Lindsay G » Sat Nov 17, 2012 9:16 pm

Electromagnetic uncouplers seem to introduce more and more considerations. Might non-electro uncouplers simplify matters? There's a couple of neodymium magnets epoxied to the upper end of the brass piston in the image below. There's then no worries about keeping your finger on the button for too long. They're also a lot cheaper, and by leaving a crank elsewhere on the truncated servo arm several more magnet sleeves could be operated from the same servo, reducing costs even further and providing other benefits.

Of course we might then introduce other considerations and get back to such other issues as twitching servos again. It's still to be tested in anger, but seems to work fine on the work bench.

Lindsay

servo uncoupler.jpg
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Re: manual turnout operation

Postby Armchair Modeller » Sat Nov 17, 2012 10:21 pm

That looks to be a very interesting idea. Please let us know how successful it turns out to be.

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Paul Townsend
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Re: manual turnout operation

Postby Paul Townsend » Sun Nov 18, 2012 7:25 am

Lindsay G wrote:Of course we might then introduce other considerations <snip> It's still to be tested in anger, but seems to work fine on the work bench.


Its good to see a compact, metal & engineered servo mount arrangement.

Never mind the application for moving a magnet, this is a much better way to deploy a servo for turnout drive than the overlarge plastic mounts that are around...smaller, cheaper and quicker to make from stock materials. The nice plunger for the magnet could be simplified down to "wire in tube" for a turnout too, with or without the "sawn-off". :)

and get back to such other issues as twitching servos again.


Merg experts have done some work on this in recent weeks and seem to be nearly ready to publish the "Final Solution" recommendations. :ugeek:

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Re: manual turnout operation

Postby martin goodall » Mon Nov 19, 2012 10:32 am

I see that we also now have another sub-topic which might merit its own thread heading - operation of uncoupling magnets.

Pending someone finding the right home for this topic, I liked Lindsay's idea. This is a servo-operated version of an idea that Iain Rice put forward a year or two ago - a permanent magnet that is lifted into position to operate the coupling and then lowered. What I believe Iain had in mind is a purely mechanically operated mechanism, which could be worked by under-baseboard rodding, such as that illustrated by Keith.

I now have a complete set of electro-magnets installed on the Burford Branch, but I would certainly try out mechanically operated permanent magnets on any future layout. I have reservations about servo motors (for various reasons that have been discussed elsewhere), and I have a natural preference for mechanically operated equipment. As Lyndsay points out, mechanically operated permanent magnets would be cheaper than electro-magnets.

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Re: manual turnout operation

Postby nigelcliffe » Mon Nov 19, 2012 12:24 pm

Lindsay G wrote:Electromagnetic uncouplers seem to introduce more and more considerations. Might non-electro uncouplers simplify matters? There's a couple of neodymium magnets epoxied to the upper end of the brass piston in the image below. There's then no worries about keeping your finger on the button for too long. They're also a lot cheaper, and by leaving a crank elsewhere on the truncated servo arm several more magnet sleeves could be operated from the same servo, reducing costs even further and providing other benefits.

Of course we might then introduce other considerations and get back to such other issues as twitching servos again. It's still to be tested in anger, but seems to work fine on the work bench.


I've used something very similar before. It works fine for AJ's and other couplers requiring vertical pull.

BUT, there is a gotcha, which affects me (and perhaps others..). If you fit couplings on locos with my DCC controlled mechanism, it is very sensitive to the pull from magnets in the general vacinity. So, for my locos, permenant magnets which retract need to move large distances to remove the impact of the field, or be rotated, or otherwise magnetically shielded when not in use.


- Nigel


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