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Author Topic: Cheap Momentum???  (Read 3833 times)
LDBennett

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« on: June 15, 2012, 09:34:03 AM »

I have a special case DC powered small trolley layout. It is an 8 feet long straight track that runs in front of a small town scene. My goal is to allow it to run for hours unattended. To that end I have installed a Circuitron auto point to point system with two intermediate stop points with variable delay for the stop length. It works with photo detectors in the track. The end ones stop the trolley, wait for the delay to time out then reverse the DC voltage to the track. The intermediate detectors stop the trolley, wait for the delay to time out, and then proceed.

Depending on the speed selected for the trolley the stops and a starts are abrupt. I was thinking of adding a large capacitor to the track wiring so that when the control circuit removes the DC from the track for the stop, the capacitor will act like a big battery and taper the speed off over the time period of part of a second. When the power is returned to the track the capacitor will have to charge up so the startup of the trolley will be a bit less abrupt.

But here's the problem: If there is NO delay then the capacitor will not have time to discharge before the polarity changes. That will mean the power supply (the throttle power source) will be looking in to a reverse charged capacitor and a large inrush of current will flow from the throttle power supply. As long as I make it so there is always a delay at the ends of the trolley run there should be no problem. But when the DC power comes back after the delay there will be a smaller inrush of current.

I fear for the life of the Bachmann small throttle power supply. Should I?

Anyone got any insight into this.

Can one of the momentum circuits I see advertised work in this scenerio?

Sorry for all the questions but while I am doing the scenery and setting up the buildings, my engineering mind keeps wandering. The correct approach would be to use the photo detectors in concert with a microprocessor  running a custom program to control everything in a random way. But I just don't think I want to make this a long project. I just want to "get it done". So I keep gravitating towards simple and easy fixes. I know, not good, but it is what it is. I have limited time (I have to help my 11 year old grandson get his N gage setup going since the cat ruined it and I need to do some motorcycling, some photograph, some camping, etc...). It's hell being retired as there is never enough time to do everything!

LDBennett
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jward


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« Reply #1 on: June 15, 2012, 04:45:28 PM »

have you thought of using resistors to accomplish the same thing? what you would do is put isolated sections (known as blocks) on either side of your stopping points. in each of these  you'd have a resistor in series with the rail you gapped for the block. the value would be whatever would run the trolley at half speed.

the advantage is that with resistors there is no outrush of reverse polarity current to possibly damage your transformer.

google block control wiring for examples of how to gap the rails.

a display railroad i worked for as a teen did something similar to this many years ago, and it worked well with full lengt h trains.

one thing you will want to do first, is to run your trolley on a normal track, at as low a speed as you can. that is going to be as smootth a start or stop as you'll get without modifying the trolley.


or......

you could just invest in a better pack which has momentum built into the circuit.
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Jeffery S Ward Sr
Pittsburgh, PA
richg
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« Reply #2 on: June 15, 2012, 09:41:27 PM »

I cannot visualize exactly what you want to do BUT, do not put any kind of electrolytic cap across a power pack that will reverse polarity. I would fear for the cap and power pack, depending on how much current the pack can deliver.

Rich
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LDBennett

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« Reply #3 on: June 16, 2012, 09:18:46 AM »

 jward:

The Circuitron auto reversing is after the throttle electrically and momentum in the throttle would get shut off early by the relay that does the auto reversing. The resistors might work. I'll think about it. I am aware of blocks as my 1950's DC layout had about five of them in the layout with two throttles (one block was a reversing track) that I had to manually manipulate to reverse train direction. At junior high school age and without help (my father was not into electrics) I learned a lot about electricity. That experience of wiring up that layout fostered my interest in electronics. Electronics became my living after college (BSEE) for over 30 years. That layout was the key to my future.  The layout was removed after I entered high school. I did resurrect the old rolling stock  on a very small layout for my son in the 1980's but the old stuff eventually died as did his interest and mine.

richg:

I was thinking the same thing but if the transformer and the circuitry could handle the inrush it might work. I doubt the little Bachmann power pak is heavy duty enough and may even have internal current limiting. The slowdown is not the problem if I have delay until the polarity changes but when the relay turns the power back on there definitely would be an inrush of current. This may be a bad idea. It was just food for thought.

Thanks for the inputs.

LDBennett
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richg
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« Reply #4 on: June 19, 2012, 02:49:58 PM »

Switching polarity with an electrolytic is the equivalent of a dead short. I am sure you know that. Enough current and the cap explodes. Some have done this.  Maybe burns up the diodes in the pack if no fuse or circuit breaker.
Bipolar caps exist but not for this purpose. Those are mostly for crossover networks in speaker systems.
SoundTraxx older sound decoders use a bipolar to isolate DC from the speaker.

Rich
« Last Edit: June 19, 2012, 02:51:47 PM by richg » Logged
BandO GLENNWOOD

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« Reply #5 on: July 27, 2012, 05:30:15 PM »

I would consider 2 capacitors that are diode protected this would allow 1 to see forward voltage and the other to see reverse The diode would prevent voltage from getting to the one that was not being used at the time Paul
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LDBennett

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« Reply #6 on: July 28, 2012, 08:06:21 AM »

I think a common solution is to use two capacitors in series with opposite polarity connections but the diode added would not hurt one bit. The connection would then be the diode in series with a cap and then the two cap/diode assemblies in parallel with reversed polarities.

My biggest fear is that the outrush from the power pak may tax the power pak too much over time but I intend to try it none the less. i'm working on a new control system other than the Circuitron auto reversing module using  an microcontroller  and a desecrete relay card. I'll be able to put the caps between the on/off relay and the reversing relay. The effect is the track always sees the capacitor (except when the reversing relay transition which is milliseconds) and always is powered by the same polarity. Just to be safe I'll protect the capacitors by using two with the diodes in case the reversing switch on the power pak is changed. This was not possible with the Circuitron module because the connection between the on/off relay and the reversing relay was inside the module.

I'll let you know how well this all works out but it will be awhile as I still have to implement the hardware and write the program and debug the results. But I'm working on it.

LDBennett
« Last Edit: July 28, 2012, 08:08:58 AM by LDBennett » Logged
Desertdweller

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« Reply #7 on: July 29, 2012, 11:16:59 AM »

I don't know what space situation you are facing inside your trolley car, but it would seem to me the simplest solution would be to fit a flywheel to your drive.

This was the purpose flywheels in railroad models came about in the first place: to propel a train over short dead spots, and to eliminate sudden, jerky movement when starting and stopping.

It is a simple solution that will require no wiring.

Les
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LDBennett

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« Reply #8 on: August 01, 2012, 09:41:23 AM »

There are six different trolleys that I have and most of them have an interior that probably limits adding an effective flywheel.

But one is a Bachmann PCC with a Bowser motor conversion (no interior) that I added a A-Line flywheel to. I was not impressed. I think the flywheel rim mass is not large enough to make a significant difference. It might help with carrying the trolley through rough spots on the track but it does not give a realistic slowdown for stopping or gradual speed increase for startups. There is just not enough room in these little trolleys to get a big enough flywheel in there. The science behind an effective flywheel says that the mass has to be on the rim and far from the shaft. There is not room in any of my trolleys to do that.

I am currently working on a microcomputer control system for my little point to point trolley layout. I hope to include the large capacitors to give me the effect of gradual slowing down and gradual speeding up. The point of the microcontroller for me is to randomize the automatic stops in duration and to randomly skip stations along the way. Hopefully it will trigger an external sound module to get a double bell ring at startup, too. Unfortunately I have to learn some C++ programming for the Arduino UNO which so far is an uphill battle even though I know some rudimentary programming, but I'm getting there. I have pretty much finished the original layout and only adding a few small details at this point so my efforts are now aimed at this microcontroller and thinking on how to add more running track (I have 8 feet and can add 12 feet more but that is for later).

LDBennett
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