Best Of The TP56/TP70 Kitbash thread N scale

[JanesCustomTrain]

The production details and all the specs are at https://www.shapeways.com/materials/brass
That's where I got my info.

And now that you have the info you HAVE to share it with us if we have questions. 

Jane

[coosvalley]

The production details and all the specs are at https://www.shapeways.com/materials/brass
That's where I got my info.

I think it actually could work for my needs, so thanks!

[nkalanaga]

"The SCR circuit would have most likely produced  60 or 120 Hz pulses, while today's PWM circuits in DCC decoders use much higher frequency pulses."

That's exactly what the result was, square-wave pulse power, instead of simply half of the AC cycle, like you'd get from using a single diode for a rectifier.  If I remember right, the pulse width was varied by adding a DC bias voltage to the AC control voltage, so the SCR would be on more or less than the usual 50%.  Rather primitive, which is probably why it didn't play well with newer motors, and seems to have quickly lost favor.

[u18b]

Making progress on detailing.

Pilots are finished.

Lift rings

Sunshades

Windshield wipers

Handrails are almost finished.  Saving the small ones at cab rear for last since they have no extra support.  Will wait until I'm not taking the cab on and off so much.

Still have sand hoses and cab interior.

[mmagliaro]

Zener diodes are a good idea.

As far as 50% of speed steps being wasted is not accurate. The motor sees an average voltage derived from the width of the full-voltage pulses.  So using PWM at speed step 1, a 12 volt motor will see lets say 0.5V.  If a 6V motor is used with Zener diodes, the pulses will be the same width but voltage will be clamped to 6V.  So the average voltage the 6V motor sees will be about 0.25V (proportionally the same as the 12V motor with no Zener diodes.   So the full speed steps range will be available with either 12V or 6V motor (with Zener diodes).
...

You're right in that it probably won't waste 50% of the range, but it will waste some of it.
Consider what happens at a higher step - say, step 8.  Let's say the averagevoltage would ordinarly be 2 volts, but is now 1 volt with the zeners in there.  A 6 volt motor that certainly would have started moving (on 2 volts), probably won't move at 1 volt.  My point is that without the zeners, the 6v motor would be moving in those lower speed steps, but it won't with the zeners in the circuit.  You will have to get to a higher step before the average voltage is high enough to move the motor, even with a 6v motor.  It would be better to reprogram the steps so that they start out close to where the motor can actually move.

Say there are 128 steps, and without zeners, the 6v motor starts running at step 10. You might use steps 10 ... 64 (and no higher, because you don't want to burn out the motor.  Why not reprogram the steps so you can use the whole range 1 ... 128, producing an average voltage from 0 ... 6, but with a sharper "curve" at the low end?

[peteski]

You're right in that it probably won't waste 50% of the range, but it will waste some of it.
Consider what happens at a higher step - say, step 8.  Let's say the averagevoltage would ordinarly be 2 volts, but is now 1 volt with the zeners in there.  A 6 volt motor that certainly would have started moving (on 2 volts), probably won't move at 1 volt.  My point is that without the zeners, the 6v motor would be moving in those lower speed steps, but it won't with the zeners in the circuit.  You will have to get to a higher step before the average voltage is high enough to move the motor, even with a 6v motor.  It would be better to reprogram the steps so that they start out close to where the motor can actually move.

Say there are 128 steps, and without zeners, the 6v motor starts running at step 10. You might use steps 10 ... 64 (and no higher, because you don't want to burn out the motor.  Why not reprogram the steps so you can use the whole range 1 ... 128, producing an average voltage from 0 ... 6, but with a sharper "curve" at the low end?

Max, you are missing the point. the PWM motor driving circuit always generates full voltage pulses (12V).  Even at speed step 1.
These pulses have a  very short "on" time period (duty cycle). But they are still full amplitude 12V pulses. The Zener will deal with those pulses the same as it does at higher speed steps pulses they are longer duration.  So Zener will start conducting current after the voltage of the pulse exceeds the Zener voltage. That is regardless of the pulse width.

Sure, there might be nanoseconds of time lag for the Zener diode to start conducting but that is negligible relative to the width of even the shortest PWM pulses at speed step 1.

It seems that the problem you are describing has nothing to do with how Zener diodes are acting but more about DCC and DC motors in general. In standard 12V motor installations if a motor does not start at speed step 1 then you change the Vmin (CV2) to some value which will produce a voltage high enough for the motor to start running. But you would want to do that regardless whether the loco uses a straight-wired 12V motor or a 6V motor with Zener diodes. That way you still have the full throttle range for running the motor.

[orionfield]

This was an amazing build to follow. I love critters, and its definitely inspiring me to try my hand at building one, as well as some other ones that I've had my eye on for so long.

[mmagliaro]

Max, you are missing the point. the PWM motor driving circuit always generates full voltage pulses (12V).  Even at speed step 1.
These pulses have a  very short "on" time period (duty cycle). But they are still full amplitude 12V pulses. The Zener will deal with those pulses the same as it does at higher speed steps pulses they are longer duration.  So Zener will start conducting current after the voltage of the pulse exceeds the Zener voltage. That is regardless of the pulse width.

Sure, there might be nanoseconds of time lag for the Zener diode to start conducting but that is negligible relative to the width of even the shortest PWM pulses at speed step 1.

It seems that the problem you are describing has nothing to do with how Zener diodes are acting but more about DCC and DC motors in general. In standard 12V motor installations if a motor does not start at speed step 1 then you change the Vmin (CV2) to some value which will produce a voltage high enough for the motor to start running. But you would want to do that regardless whether the loco uses a straight-wired 12V motor or a 6V motor with Zener diodes. That way you still have the full throttle range for running the motor.

Yes, the PWM pulses are always 12v, and they are just narrower or wider to control the speed.
Now, forget the 12v motor.  I think that's where the point of confusion is here. 

What I am comparing is how the 6v motor will behave with or without zeners in the circuit.   With the zeners,  that motor is going to start turning at a higher speed step than it will without them, because the motor is going to be seeing 6v pulses instead of 12v pulses, so the average voltage the motor sees will be lower at any given speed step.   
A given pulse width of 12v pulses will move the motor sooner than that same width of 6v pulses.

Or put another way:

Without zeners: 
6v motor will start turing at a lower speed step, but you risk destroying the motor at the higher steps as the average voltage gets well over 6v

With zeners:
You can never damage the motor, because the pulses reaching the motor are clipped to 6v, so the average voltage can never exceed 6, even at the widest pulse width.  But at lower speed steps, the motor won't move and it might be annoying to give up the lower end of that control range.

[u18b]

This was an amazing build to follow. I love critters, and its definitely inspiring me to try my hand at building one, as well as some other ones that I've had my eye on for so long.

You said "was."

I'm not finished yet!   

Come on and build one and join the fun!
 

[u18b]

Time to move to the interior.

I built this critter and did NOT put extra weight all in the cab, so the cab could be open.

So the first thing to do is hide the silver gray and orange of the motor and wires.



I built the cab weights so that their tops were about equal with the floor of the cab.
As such, I can now add a simple control stand and some chairs.

I scratchbuilt all of it.













Now I need an engineer!

[Philip H]

ok, that's it - if you people don't stop doing such fine work, I'm throwing @chicken45 in a vat of Nutella and taking up basket weaving.

Seriously @u18b - thats really fabulous work.  @Ed Kapuscinski - did we mark this Best Of?

[dcutting]

Making progress on detailing.

Pilots are finished.

Lift rings

Sunshades

Windshield wipers

Handrails are almost finished.  Saving the small ones at cab rear for last since they have no extra support.  Will wait until I'm not taking the cab on and off so much.

Still have sand hoses and cab interior.

 

Mine (I ordered two) should be here later this week!!!

[mu26aeh]

There is just so much wow in this thread 

So much more then I could ever think about accomplishing

[orionfield]

You said "was."

I'm not finished yet!   

Come on and build one and join the fun!
 

I just might, once things calm down at Out of the Box, lots of craziness trying to get models released.

[peteski]

Yes, the PWM pulses are always 12v, and they are just narrower or wider to control the speed.
Now, forget the 12v motor.  I think that's where the point of confusion is here. 

What I am comparing is how the 6v motor will behave with or without zeners in the circuit.   With the zeners,  that motor is going to start turning at a higher speed step than it will without them, because the motor is going to be seeing 6v pulses instead of 12v pulses, so the average voltage the motor sees will be lower at any given speed step.   
A given pulse width of 12v pulses will move the motor sooner than that same width of 6v pulses.

Or put another way:

Without zeners: 
6v motor will start turing at a lower speed step, but you risk destroying the motor at the higher steps as the average voltage gets well over 6v

With zeners:
You can never damage the motor, because the pulses reaching the motor are clipped to 6v, so the average voltage can never exceed 6, even at the widest pulse width.  But at lower speed steps, the motor won't move and it might be annoying to give up the lower end of that control range.

Ok I see what you are saying. But in that case then the decoder can easily be configured to have the motor running at speed step 1 (with Zener diodes).