Suggestion for you

Hi Jeff,

A suggestion - Ask your students why does the turntable indexing work?? 

Hint - The Geneva movement 

More info, please

Transformer reduces AC volts from house power to lower AC for Lionel trains, simple doorbell, and other stuff that runs on AC.

You have a power pack, which is transformer plus rectifier to supply DC volts plus maybe AC as well.

Does the turntable run on DC or AC?? If the former I suggest you change the word you use..

Glad that the table worked right off. Can't say the same at our club.

DrJolS

Fascinating

Hi John,

I'm familiar with the Geneva mechanism but wasn't aware that it has been deployed in a railway turntable, until just now as I discover that it was used in an old Atlas model. I did a little more searching to see if they were used in prototype situations, but quickly became overwhelmed by the hydraulic and pneumatic considerations of heavy duty moving platforms! 

My students are acquainted with the kind of optical sensors that are used by the Walthers model, but you've inspired me to bring out some old models of classic mechanical linkages! 

Jeff Allen

power pack it is

Thanks for pointing that out DrJoIS! I'm exceedingly ignorant when it comes to electricity and do get careless with terminology.  Very bad for a molder of young minds I'm afraid. 

The Walthers turntable runs on either AC or DC. I understand that it has been through a few redesigns over the years with some versions rather poorly regarded, but this one seems quite well done. We'll see how it holds up to tough service in my classroom!

Jeff Allen

Middle School Model Railroad: turntable power

Here’s a learning opportunity for some electrical fun fundamentals.  As seen on an oscilloscope, AC is a sine wave produced by a generator’s rotation.  DC is much better for model motors. Below is how a simple diode makes a form of DC probably produced by that Aurora pack.

 
below it’s half wave DC, not quite as smooth as battery or filtered DC ,  but good enough for models 

Recent movie “The  Current War” dramatized the dispute between Edison and Tesla.  Edison advocated DC and Tesla AC.  Relevancy ?  Westinghouse financed AC and was wealthy thanks to his railroad air brake business!

would I notice the difference with a battery?

That's a superb, crisp presentation Chris, thank you! 

When you say "...good enough for models..." you get me wondering: would I notice the difference in how smoothly the turntable turns if I powered it from a battery?

The turntable turns very smoothly, yet not quite buttery smooth, if you know what I mean. At the very limits of my vision and sense of motion I seem to detect a fine stutter, almost like a visual buzz, in the motion. I might be imagining it, that's how subtle it is. Could that be a 30 beats per second "texture" in the voltage to the motor? 

Would that change if I used a battery instead of the old power pack? Just curious.

Thanks!

Jeff Allen

Not pure DC

Older power packs especially can put out a sort of pulsed DC, and 30 beats per second could be a reflection of the 60 cycles in the current.

Battery power would not have this pulsed component, and neither would filtered DC. See the diagram.

The bottom line is typical pre-transitorized era power packs using an old style rectifier. It basically takes the AC sine wave and forces the half below the line to now be above the line like the other half.

Old style "pulse power" is the middle picture, with one half of the sine wave just cut off. This distinct pulse kicks a motor and helps to keep it moving at lower voltage, which made for a great slow speed performance.

And finally at the top is the battery output -- or what a fully filtered DC output looks like.

Update: it appears power supply is not the issue

Great feedback, and it was clear that if I wanted to know if the turntable motion was affected by the type of power I supplied I needed to try an experiment. So today I did. 

These two lads watched me hook up the old power pack and agreed that the motion was detectably "grainy." So they replaced the power pack with two 9-volt batteries in series, then three 6 volt-batteries in series, and found that each time the motion was "grainy." Not silky smooth. No change. 

If we want smoother motion, we're going to have to investigate the turntable mechanism. But I think we're going to live with what we've got, we're very eager to start operating this branch of the railroad!

Thanks to everyone for your help!

Jeff Allen

pulsed DC for incremental control

Incremental control of voltage can be accomplished by pulsating the power.  This results in a square wave as seen on an oscilloscope. The width of the pulses determines how much power is delivered.  Light dimmers likely use this technique, and it’s also used for precise motor control.  Maybe the pulses are far enough apart that turntable motion is not quite silky smooth.   I don’t know how the Walthers turntable indexes.  However it achieves the precision to align the rails at the correct location, slow motor control is important. 

Audiophiles’ turntables must be smooth to get that nice sound from their vinyl records !
 

source:   https://www.electronics-tutorials.ws/blog/pulse-width-modulation.html

oscilloscope

Oscilloscopes have been reduced to a hand-held size, something I hadn't realized.  This illustrates what the device looks like.  At $200+, I don't need one, but it's interesting how inexpensive they've become.

https://www.circuitspecialists.com/hantek2d72_70mhz_oscilloscope.html