Best Of The TP56/TP70 Kitbash thread N scale

[dcutting]

Ok, I just ordered two of these critters from shapeways. I friend of mine wanted one too... 

Thomas

Thanks for your order! I just got that email. That means there are 5 or 6 in production or produced so far. More than the prototype lol 

[tehachapifan]

Looks like Kato is out of the mid-production trucks. Wonder if it was this thread that ended up cleaning them out.

[peteski]

Pete.
You may be right.

I don't know much about finer motor tinkering in DCC.

But I do know that when this loco is run under analog, there is a clear surging pattern.
Like some kind of electronic harmonic effect.

Do you have any suggestions of what CV to mess with?
This is a Digitrax DZ126.

No, I do not. I do recall experimenting with tuning the BEMF CVs on Digitrax decoder few years ago but I didn't record the values I used, or the results.

When I asked about DC test I meant without a decoder in the circuit.

[u18b]

Looks like Kato is out of the mid-production trucks. Wonder if it was this thread that ended up cleaning them out.

No.  They were already sold out.

I just placed an order with Kato and bought a pair of HTCR radial trucks for the SD70ACe.
The gearbox is the same so I hope this would be a good substitute.

[u18b]

No, I do not. I do recall experimenting with tuning the BEMF CVs on Digitrax decoder few years ago but I didn't record the values I used, or the results.

When I asked about DC test I meant without a decoder in the circuit.

Oh.

Well, since I made the components modular, I just need to build an analog socket for the plugs.  Then I could run it off of DC.

The only problem is that I lose voltage protection.
Under DCC, I can set Vmax so that the motor never gets over 6 volts.

[peteski]

The only problem is that I lose voltage protection.
Under DCC, I can set Vmax so that the motor never gets over 6 volts.

Um, not really. decoders use PWM circuit to drive the motor - it still gets 12V in pulses, just at 50% duty cycle.  It averages to 6V but to me it is not the same as feeding "real" 6V DC to the motor.  I doubt that the companies which design and rate their motors expect them to operate this way.

Why are you worried about over-voltage on DC? You testing it for slow speed smoothness, and you will be controlling the throttle knob and - you should be able to trust yourself not to crank it up too high. 

[u18b]

Um, not really. decoders use PWM circuit to drive the motor - it still gets 12V in pulses, just at 50% duty cycle.  It averages to 6V but to me it is not the same as feeding "real" 6V DC to the motor.  I doubt that the companies which design and rate their motors expect them to operate this way.

Why are you worried about over-voltage on DC? You testing it for slow speed smoothness, and you will be controlling the throttle knob and - you should be able to trust yourself not to crank it up too high. 

Testing?  Sure.
I was thinking long term, other operators, etc.

Had not thought about the decoder's PWM.
Is there a Digitrax setting to give pure DC?

[u18b]

Made a bit more progress.

I milled out the styrene from the fuel tank.
Made a lead weight- and my guess was correct.  1 gram.

That brings total locomotive weight to an even 40 grams.




Added the cab headlight.  You can't see it on this side, but I used a silver Sharpie to make a mark on other side of the plug/socket so orientation of the plug could be known.




Of course, had to cut a notch in the weight.
I glued the wire to the roof.  It will come down around the center post of the front window.




A little Kapton tape.

The headlight shines through the shell (the material is a bit translucent).
So I will eventually paint the inside of the shell and/or paint the tape black.
Or I was thinking that I might add a heat shield.
I only gets a little warm, but I'll probably err on the side of caution.




Wow.
I can't believe it.
I'm functionally done.

I can now move to detailing.
Ordered the A/C unit from Kato last night (thanks for the tip Jane).

[u18b]

Verbal performance report.  (video coming one day    )

Did a bit of yard switching last night.

Of course, forces are greater moving through turnouts vs straight track.

At 40 grams, this TP70 appears to do well with about six 50' cars.
It could pull more, but that seems to be a sweet spot.
One time backing with 8 the loco was slipping.

I added a little styrene support brace under the motor to gently lift it.
Performance actually went down.  So a motor tightly snug and glued to the worm cover is all it needs.

Except for the little jerkiness at starting (Randy's gearhead runs better), it really runs smoothly overall.

I'm pleased.

I toyed with placing MT Truescale couplers on it.
But this is a switch engine.  So stuck with 1015s.

 

[nkalanaga]

You could use 903s.  They're smaller than the 1015s, but still couple with standard N couplers.

[dcutting]

Could you possibly use a pair of 6V voltage regulators that are switched on and off by the PWM signal on the motor leads? One for forward and one for reverse plus some diodes.

David

[u18b]

You could use 903s.  They're smaller than the 1015s, but still couple with standard N couplers.

Yes, I may try that.
I think I have some laying around here somewhere.

[u18b]

Could you possibly use a pair of 6V voltage regulators that are switched on and off by the PWM signal on the motor leads? One for forward and one for reverse plus some diodes.

David

Whew.  That would take up some of the room of the open cab.

But I don't think it's needed.
DCC does help with some of the problem.

And further, except under total maxxed out strain, I have not seen (so far) any evidence of overheating.

[peteski]

Is there a Digitrax setting to give pure DC?

No. Also , PWM is not directly related to BEMF circuitry. You could have PWM motor speed control with no BEMF feedback. I have never encountered a DCC decoder which would use analog voltage control for any of its outputs (motor or functions). Why? The answer is the power dissipation problem. Analog motor driver circuity which varies its voltage needs to dissipate the unwanted portion of the voltage as heat. That is why for example analog audio power amplifiers need large power transistors and sizable heat sinks, which do get hot during operation.

However, in PWM circuit the power transistors will be in either of 2 states: either fully off or fully on (very low resistance) So, the circuit is very efficient and very little power is dissipated in the power transistors. So the output transistors can be much smaller and do not need heat sinks. Perfect for decoders where miniaturization is very important.

Here is some related info:
https://electronics.stackexchange.com/questions/87103/why-is-pwm-used-to-control-dc-motor-speed-instead-of-using-a-variable-resistance
https://robotics.stackexchange.com/questions/7778/electric-motor-speed-control-pwm-vs-analog-voltage

As an added bonus, since PWM driver pulses power to the motor, during the motor driver's  off-state the BEMF of the motor can be sampled and measured, creating the opportunity to utilize that voltage as a feedback for controlling the the motor's speed.  So, PWM and BEMF feedback are utilized in conjunction with each other.

[nkalanaga]

PWM was used in electronic throttles long before DCC came along.  One of the original reasons was claimed to be smoother running than regular DC.  As motors became more efficient, they also became more sensitive to the pulses.