Best Of The TP56/TP70 Kitbash thread N scale

[mmagliaro]

A pair of 1/2w zeners, back to back, in series with one motor lead, will also allow you to run the motor on a full 12v, and the
zeners will drop a fixed amount that won't vary with load.  The only drawback is that the engine won't move until you turn the throttle up past the zener voltage, so if you have a 6v motor, and you put some 6.2v zeners in there, the motor won't see anything until the throttle gets up to 6.2 volts.  Personally, I don't mind this and it's handy because things like LEDs will come well up to full brightness before the engine moves.
1/2w zeners are pretty small.

[narrowminded]

A pair of 1/2w zeners, back to back, in series with one motor lead, will also allow you to run the motor on a full 12v, and the
zeners will drop a fixed amount that won't vary with load.  The only drawback is that the engine won't move until you turn the throttle up past the zener voltage, so if you have a 6v motor, and you put some 6.2v zeners in there, the motor won't see anything until the throttle gets up to 6.2 volts.  Personally, I don't mind this and it's handy because things like LEDs will come well up to full brightness before the engine moves.
1/2w zeners are pretty small.

Hi Max.  That works well on straight voltage but for slow speed control I was recommending a low frequency PWM controller and in my tests the Zeners didn't work well with those.  They are still pulsing full voltage, in this case 12V, and as such still got the motor creeping even at a low throttle position because the voltage pulses were still above the Zener limiting value and as such, passed through those pulses.  They are limited but are still there and the motor started turning.  I quit using them because of this.

[peteski]

Hi Max.  That works well on straight voltage but for slow speed control I was recommending a low frequency PWM controller and in my tests the Zeners didn't work well with those.  They are still pulsing full voltage, in this case 12V, and as such still got the motor creeping even at a low throttle position because the voltage pulses were still above the Zener limiting value and as such, passed through those pulses.  They are limited but are still there and the motor started turning.  I quit using them because of this.

Let me see if I understand this. When PWM throttle is set to 0 speed (off), it produces no voltage. When it is set above zero speed it produces full-voltage square wave where the duty cycle of the pulses varies from very small percentage (like 1%) to straight 12V DC (at full throttle).

I would expect the Zener diodes series-connected with the motor to clip those pulses by whatever the Zener voltage is plus the Vf of the other Zener. So if the throttle produces a 1% duty cycle 12V the motor should see the same pulses but at about half the amplitude (or 6V). Are you saying that the 12V is not clipped at all by the Zener diode?

I looked at the data sheets for Zener diodes from about half a dozen different manufacturers and none list the diode's response time. But if the response time of the Zener operating mode was too slow then the diode would not conduct at all (since in the Zener mode it is reverse-biased). Interesting . . .

[narrowminded]

I will revisit this when the current tests are done if only for my own curiosity.  It's been an eternity (a couple of years?) since I messed with them and as I think about it, it really doesn't add up, as you're pointing out and as my recollection of it is.  And I've had such good results with simple dropper resistors, as I was suggesting might be tried, that I haven't revisited it.  Now I/ you have just made more work for me.

[randgust]

I've used dropping resistors to 3v motors (100-ohm, as Kato did for a while) with varying degrees of success.

On a high-RPM, low-torque motor, the results were that it worked - but as the load diminished (such as pulling a train up a hill and hitting the crest) the resulting decrease in amp draw literally created a 'runaway' that was difficult to control.   And then there was the heat.  I had 100-ohm resistors on Kato 3V get so hot that plastic melted and the resistors failed.    Ohm's law can be a bitch when the current decreases and the voltage shoots up faster than you can shut it down.

One of those things that works fine on the workbench and doesn't work for nothing when you're actually running a train on the layout.

Now, if you can control the amp load - it still works.   Do the same stunt to a high rpm reduction gearhead like the Solarbotics GM15A and it worked great, resistor didn't heat, and even at full slip it just chugs along.   Because the current, given the mechanical advantage of the gearhead, remains relatively constant.

I've never bothered to try using the 3V motor that comes on the Gizmoszone motor on the gearhead, mostly because I had a big surplus of the Kato 12V motors and those were definitely better performers anyway.

I've put dropping resistors on Faulhaber high-efficiency motors with the same problems - load runaways and overheating.

[u18b]

Sounds like DCC and adjusting the Vstart, Vmid and Vmax is the best way to go.

[Lemosteam]

I've done it twice and the screws didn't fit either time    I was wondering if the Kato holes were even threaded at all.

Even in the one photo I took of both motors it looks like the Kato holes aren't threaded.


Maybe they can self tab into there but after a few seconds of trying I just used the tape.

What I have done in the past, @Chris333 , is take a screw (stainless, or whatever I have extra of) of the thread I want to tap, and using a cutoff wheel edge, grind a groove into the threads with one face of the cutoff wheel parallel and even with the centerline of the screw.  This creates a sharp edge at each thread right at the center.  Looks like a self-tapping screw in the end.  Just be careful how far you run the screw into the housing.  Then I take that screw and save it in an Xacto blade flip cover box and mark it with the thread size for later use.  I have run into this with metric screws that I want to tap into something. I have done this with screws and bolts up to any size, so long as the material I am tapping into is soft enough.

[narrowminded]

Now, if you can control the amp load - it still works.   Do the same stunt to a high rpm reduction gearhead like the Solarbotics GM15A and it worked great, resistor didn't heat, and even at full slip it just chugs along.   Because the current, given the mechanical advantage of the gearhead, remains relatively constant.

... and that's the whole situation I've been talking about in a nutshell.  When the gearing is right it runs nicely.  When it's not it runs for crap.  And that's with or without resistors because the motor is overloaded.  I've got literally thousands of running hours on these rigs and I have the motors running comfortably under their rated milliamp draw so they are not overloaded, they are pretty stable in speed up or down a hill all of the way up to wheel slip, and I've never had a resistor hotter than about 96 degrees. They are typically down around 85 degrees or within about 10 or less of ambient.  And the motors like it too evidenced in very good life, not being cooked and abused.

Those motors when hooked up to a 20:1 or 35:1 gear are like trying to pull out from the red light in your standard shift vehicle in high gear.  In fact, it's exactly like that and why it doesn't work well.  Get the gearing right and all of that goes away, resistors or not.  BTW, the gearings I've successfully tested are 150:1 down to about 50:1 for a 60+ MPH speed demon and the wheels were 33" down to 24" which is another huge difference from one with 40" or so.  Much more than that and the motor starts to be overloaded and depending on how far you go all of the characterisics that I've been talking about and that Randy independently confirmed several times in some of these recent conversations start to creep in.  It doesn't work well.  Also, I have had that in a complete running chassis that is smaller than just the motor you folks are working with.  The one in the video I posted a few months ago was one and it's running on twelve volts with resistors, 24" wheels, and 125:1 gear.  I now have a 150:1 in that same unit on the test loop. 

And that's the last I'll say it, much to folks relief, I'll bet. 

[u18b]

Made some progress last night.

I Beardenized it.

And then made a crude mock up from junk parts.

Now I have a running mock up that you might call a proof of design.

Still in analog.

Positives:
--It actually runs well at slow and medium speed.

--That motor is so low that there is tons of room for weight.

--smooth Kato drive.

--at scale speeds and pure DC (no pulses) is BARELY runs warm.  Very good.

Cons:
--Won't creep (sort of don't expect it to).

--Throttle range pretty narrow for scale speeds.   From moment it starts moving to good max scale speed (say about 60 smph) is only about 10% of the throttle range.   Decoder should give me better control.

--Kato has no trucks at present.  (this is going to be like the situation with the Kato Mike trucks that always sold out when kitbashers found out how useful they were.)



Hope to have pics and a video tonight.

[u18b]

For the electron heads....

The plain motor.....
6.0 volts draws about 36 ma.

weighted TP56 Loco chassis...

Forward,   6.0 volts  55ma
Reverse,   6.0 volts   60 ma

[mmagliaro]

Here's another data point on using Zeners.

I am currently using two 6.8v back-to-back zeners in my 0-6-0 engine, which has a 3v Faulhaber with a 16:1 gearhead + 20:1 ratio at the axle (so I have a 320:1 total ratio).  The 3v Faulhaber can easily be driven to 6 volts without harm.  I am not using a PWM throttle on it.  I am using a conventional transistor DC throttle.  However, it does ride half-wave sine pulses on top of the smooth DC that have an amplitude of about 2 volts.  Therefore, I would expect to see some point in the operation where those pulses are causing the voltage spikes to periodically jump over the Zener's breakdown voltage.

And that's exactly what happens.
On completely smooth DC, the engine will start moving at about 8.5v (which means the motor is seeing 8.5 - 6.8 = 1.7v ).
On the throttle, where the 'nominal' voltage on a meter is 7.5, the motor moves and the engine creeps amazingly well.  That's only 0.7v to the motor (7.5 - 6.8 ).  While a Faulhaber can turn at that voltage, it won't do that under load.  And yet on the throttle, it does, because it is actually getting more than that in the form of those halfwave pulses.

I don't see why a PWM throttle would do any different.  The zeners just have to be a high enough voltage to chop down the full 12v PWM pulses so they aren't too much for the motor.

[peteski]

Even when using a decoder I would still install some sort of voltage dropping circuit in series with the motor. I just wouldn't feel comfortable knowing that the motor sees full voltage pulses from the decoder. Either use Zener diodes or even just 3 or 4 bridge rectifiers (conventional silicon diodes) to eat up some of the voltage.

[u18b]

OK.. Here is the first very rough mock up.

I raided the junk box just to throw something together.

You can see that the motor should just clear the cab.

You can also see how low the motor sits.





And here is a rough video of it running.
Using my iPhone, so not terribly smooth.

The occasional hesitation is more a function of the limited throttle range.
I think DCC should work better.

I think it runs pretty smooth for such a small switcher.
Did pretty well over electrified frogs.
Can switch 5  50" cars.

[tehachapifan]

Very cool! Might have to build one someday (after all, I built an operating Hi-Rail MOW truck based on that article you did many moons ago! ).

[dcutting]

Very cool Ron. Another thought for consideration... if you guys wanted to get some slower operation you'll need to use a gear head. Why not use the motors that you bought on ebay (or a shorter version of one of them) with the gizmoszone gearhead and one modification...

Disassemble the gear head (probably a 1:5.14) and remove the drive shaft. From this photo, it looks like a shaft with a plate on the end that has six little holes in it:



One of you guys with a mill/lathe might be able to replicate that part in metal, but instead of the fat shaft you could make a 1 shaft that is the right length to fit through the worm on the truck. It would be quite a feat of micromachining but you might be able to pull it off somehow. Either that or remachine the shaft on the 1.5mm extended shaft gear motors.