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For a railway museum layout we have an automated model train loop of track. When someone walks in to the display they trip the motion detector that turns on the power; this turns the lights on - on the layout and turns on a MRC power pack. The power pack is set with the momentum switch on, so when it turns on the DC volts slowly increase to the set speed. The Lionel small Switcher engine is wired direct to the track for DC volts (no reverse board)  the train starts running slowly. The problem is when the power cuts off this stops the engine abruptly. I was wondering if a large capacitor or BCR was installed in the engine if this would allow it to slow down and stop instead? What size capacitor would work?

Last edited by kj356
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@kj356 posted:

...When someone walks in to the display they trip the motion detector that turns on the power; this turns the lights on on the layout and turns on a MRC power pack.

...

The problem is when the power cuts off this stops the engine abruptly.

Out of curiosity, when the power cuts off don't the lights also turn off and hence the visitor doesn't see the abrupt braking?   OK, let's say the power pack is on a timer that turns ON with the lights, but turns OFF before the lights.

To size the capacitor, you'd need to tell us how much power the engine needs...Volts and Amps for example.

I think you'll find you need a ginormous capacitor - perhaps as large as the engine itself - to effect any meaningful momentum slow-down. That is, a typical momentum control on a DC power pack would be set to, say, 5 to 10 seconds?  It would take one huge (and expensive) capacitor to run the motor for a similar interval.

OTOH if the abrupt stoppage is a maintenance issue such as slamming couplers and derailing cars, then any slowdown is better than none.  So it could be a case of installing whatever fits and taking whatever you get.

The general issue of engine coasting distance upon power loss has been discussed in different OGR threads.  If there is any way to add flywheel weight to the motor you are much better off.  Of course the motor may not have a flywheel at all!  That is, cubic inch for cubic inch, storing momentum mechanically (in a spinning flywheel) is much more effective/compact than storing the momentum electrically (in a capacitor).

I appreciate the keep-it-simple logic of simply installing a capacitor and calling it done.  But even if you always run the engine in one direction if installing a gigantic capacitor, I'd want to protect against some summer intern (no disrespect to summer interns intended) re-railing the engine backwards...or during maintenance someone bumps the direction (polarity) switch on the power-pack.  That would be a train wreck without some protection to the capacitor...albeit potentially as "simple" as an inexpensive diode.

This being a discussion forum, if the conversation can be expanded to alternative methods, I'd lean toward something on the power-pack side.  In other words, some method to slowly change the DC track voltage up and down.  This could have the additional benefit of adding interest to the layout such as the train changing speed when passing points of interest or whatever.

Last edited by stan2004

Thanks for the info. good comments. Just to clarify the reasons that I want the train to slow down slowly rather then an abrupt stop is that it causes wear on the gears and can derail the following cars. Humm from the comments I am thinking that perhaps what I need to do is to cut one of the wires to the throttle on the power pack and have this wired into a relay. When someone walks into the display the motion detector turns the power on that turns  the relay on which closes the contact and completes the power to the throttle thus with the momentum switch turned on the power to the track slowly comes up and the train starts slowly. When the motion detector times out the power turns off to the relay that then cuts the power to the throttle with the power pack still on this will allow the power pack to slowly turn the power down to the track.

Using a larger flywheel in the engine is a good idea but I am using small simple Lionel engines. I am hoping to find an electronic solution.

Here is an article on using capacitors a friend sent me that was very informative!

http://www.trainelectronics.co.../capacitor/index.htm

Last edited by kj356
@kj356 posted:

...

I am thinking that perhaps what I need to do is to cut one of the wires to the throttle on the power pack and have this wired into a relay.

...

Agreed.  I think your idea is indeed the closest exit.

momentum

This is not an MRC power pack, but I'm inclined to think they are similar in concept.  So you flip the Momentum switch to "ON".  Then you have a slide/linear or rotary control that is really just a 3-wire potentiometer (variable resistor) when you look inside the box.

So to your point, you could have a relay that selects either the running speed voltage or 0 voltage...and the power pack's "momentum" circuit will smoothly accelerate/decelerate your engine.  Of course this means you need to open up the power pack and splice a wire or two but I believe this is a better solution than messing with huge capacitors and the "baggage" they bring.

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Last edited by stan2004

So I  have tech 3 9500 and Control Master 20 MRC transformers 2 of each. I have been using this set up for humm perhaps 20 years every Christmas. Normally for the shows I have 1 or 2 of these transformers set up with the momentum switch turned on and the speed set to where the engine runs OK. They run for a about 6 weeks all day reliably at the Christmas displays with the motion detector turning them on as people are moving in front of the display this keeps the motion detector on and the timer is set to about 1 minute so when they walk away it shuts off after a minute. So it would simple I think to on the Control master 20 to open the hand held remote control and clip one of the wires to the rheostat. Wire this to a relay, when the motion detector turns this on it is like having someone turn the throttle up quickly then when power to relay is cut it  is like turning the throttle off and the momentum control will lower the volts slowly. Then I need to find some way to keep the power on on the power pack longer then the relay. Humm perhaps two motion detector circuits???

What motion detectors are you using?  How is it powered?  Is the only motion detector "output" a line-voltage (120V AC) socket that presently powers both room lighting and the MRC power pack line-cord?

mrc control master 20

And when the motion detector no longer detects activity, 1 minute later it simultaneously removes AC line-voltage from the lights and the MRC power pack?

If you open up the handheld controller it looks like there MAY be a circuit board the size of the controller itself.  In other words the throttle "rheostat" and the other controls and connectors might be soldered directly to a circuit board.  This would make it a bit more difficult to just snip/splice a wire to insert a relay.  Obviously you'll know as soon as you open it up - a photo would be nice just to satisfy my curiosity too.

Let me understand where you're going with two motion detectors.  So if the "original" detector time-out is 1 minute, you'd have the 2nd detector time-out in say 50 seconds.  The 2nd detector output would control a TBD relay that sets the throttle to 0 speed (while the power pack is still powered).  The engine slows to a stop and then the power pack itself turns OFF 10 seconds later.

What is your thinking about what can be powered in the standby mode when the room is dark and train is stopped?  That is clearly there is some minimal (a few Watts) power required by the motion detector(s).  Is it acceptable to have the MRC power pack always powered but at zero-throttle when in standby mode?  I can imagine alternative methods that use only 1 motion detector but in general these require electronics to be powered in the standby mode.

Finally, are you averse to changing the motion detector?  Depending on how your present detector works (my questions above) there might be motion detectors that make the required timing/sequencing a bit easier.

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Just thought I would do a follow up that the automatic train circuit has been working very well at our railway museum. As we have a large model railway mounted in a full size passenger car with a HO model railway set up where we have staff drop in to check on it but otherwise it is unattended and automated to start when guests walk in.

We have mounted 2 motion detectors that an electrician wired to power plugs. Motion detector circuit #1 is set to a shorter period of 1 minute before it shuts off, so when folks walk in to the display this turns on the power plug which powers a transformer that runs some of the layout lights, a Menards Space ship stealing a cow from a farmer (very popular) and sound modules located through the model railway display. (https://www.ngineering.com/little_sounds.htm) This also powers a relay triggering a circuit to close (Normally open relay) We have cut the throttle wire on a MRC DC transformer with a momentum button. We have the throttle set for the speed we want the train to run, and the cut wire to the throttle closes the circuit when the relay closes. Thus it is like turning the throttle up. The 2nd motion detector right beside the first one is set for a longer period of 5 minutes. So when folks walk in both motion detectors trip the transformer turns on and relay closes the throttle the train starts very slowly with the momentum switch on and runs around the layout. When folks walk away the 1st motion detector shuts off, the power is cut and the sounds and lights stop and relay opens (like turning off the throttle) and the train with momentum slowly slows to a stop. After a few more minutes the 2nd motion detector shuts off and the transformer then shuts off. We only have one large loop of track running through the display with long straights and very gradual curves, no S curves and no turnouts on the loop. There are lots of separated sidings and lines not connected to the main track where Thomas theme trains and other trains are parked for display and full scenery throughout. The display is protected by high plexiglass. The engines we run - have a few; have the RDC's wired together for lots of wheels picking up power and some diesels with box cars wired to them with power pickup on the box car wheels. So far the display has been running very reliably  (only run 1 train at a time). We occasionally run a track cleaning car. There are access hatches throughout the display should we need access you can crawl under and access a problem on the layout. The transformer has a fuse between the the track and transformer. The motion detectors run through a bright red emergencyshut off switch located easily for guests and signs to ask them to turn it off if a problem that would cut all power to the trains. We have a motion detector mounted above the transformer. This set up allows things to run only when folks are around and saves the equipment from sudden starts and stops much easier on the gears.

The circuit with just one transformer I have used for 25 years + every Christmas O Gauge Christmas train displays so train would start slowly but always stop abruptly. This new improved version works much better very reliable and easy to set up.

Years ago, a friend put a standard 120 volt light bulb in series with the track, switched into the circuit when he wanted to stop the train. (I seem to recall a 40 Watt, but time has erased the exact memory, and the wattage will vary with other factors, including the size of the motor, the size of the train, etc.)  The effect is to slow the train and finally stop it gently. It can't hurt to try that simple solution.

Last edited by Arthur P. Bloom

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