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Atlas N Scale Silver Series - High Speed on PWM?

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  • Member since
    June 2009
  • From: QLD, Australia
  • 1,111 posts
Atlas N Scale Silver Series - High Speed on PWM?
Posted by tbdanny on Sunday, March 5, 2023 4:21 AM

Hi all,

I'm building a coffee table layout in N scale, and I've got a PWM throttle for it and a few DC locomotives.  Most of the locomotives (Bachmann and Tomix) work okay.  However, it's a different story for the Atlas silver series Alco S-4.

When testing, I found that this locomotive would just sit on the track until about halfway through the throttle, then it would take off at close to full speed.  Further experimentation found that there was a very narrow area of the throttle knob rotation where it would run at a reasonable speed. But after this, it would just run off at close to full speed.

Another thing I noticed is that the other locomotives made a high-pitched whine before moving, and this dropped off as the speed increased.  The S-4 didn't make any noise at all.

Given the above, I suspect that something on the circuit board of the S-4 is interfering with the pulse width modulation performance.  Is there anything I can do to fix this?

The Location: Forests of the Pacific Northwest, Oregon
The Year: 1948
The Scale: On30
The Blog: http://bvlcorr.tumblr.com

  • Member since
    April 2021
  • From: saskabush
  • 127 posts
Posted by wvgca on Sunday, March 5, 2023 7:07 AM

PWM is basically only noticably effective at low loads / speeds .. as the throttle gets turned up the PWM is less of a factor, until at wide open throttle the PWM component is not functional at all ...

  • Member since
    January 2017
  • From: Southern Florida Gulf Coast
  • 18,255 posts
Posted by SeeYou190 on Sunday, March 5, 2023 8:39 AM

tbdanny
When testing, I found that this locomotive would just sit on the track until about halfway through the throttle, then it would take off at close to full speed.  Further experimentation found that there was a very narrow area of the throttle knob rotation where it would run at a reasonable speed. But after this, it would just run off at close to full speed.

It sounds like it might have a dual-mode DCC decoder installed.

These will work (poorly) on DC, but PWM DC can really mess with them.

I think this is what the standard DC board looks like.

That is my best idea.

wvgca
As the throttle gets turned up the PWM is less of a factor, until at wide open throttle the PWM component is not functional at all.

I do not miss the days of trying to teach people that 100% duty cycle PWM is the same as filtered DC.

My life is much easier now.

-Kevin

Living the dream.

  • Member since
    January 2009
  • From: Maryland
  • 12,758 posts
Posted by ATLANTIC CENTRAL on Sunday, March 5, 2023 9:20 AM

SeeYou190

 

 
tbdanny
When testing, I found that this locomotive would just sit on the track until about halfway through the throttle, then it would take off at close to full speed.  Further experimentation found that there was a very narrow area of the throttle knob rotation where it would run at a reasonable speed. But after this, it would just run off at close to full speed.

 

It sounds like it might have a dual-mode DCC decoder installed.

These will work (poorly) on DC, but PWM DC can really mess with them.

I think this is what the standard DC board looks like.

That is my best idea.

 

 
wvgca
As the throttle gets turned up the PWM is less of a factor, until at wide open throttle the PWM component is not functional at all.

 

I do not miss the days of trying to teach people that 100% duty cycle PWM is the same as filtered DC.

My life is much easier now.

-Kevin

 

Agreed, sure sounds like a decoder. They hate PWM DC.

I have removed lots of pesky decoders in the last 20 years or so.

I have not found a dual mode decoder yet that will run on the high frequency PWM of my Aristo Train Engineer throttles.

Sheldon

    

  • Member since
    June 2009
  • From: QLD, Australia
  • 1,111 posts
Posted by tbdanny on Sunday, March 5, 2023 1:35 PM

ATLANTIC CENTRAL
Agreed, sure sounds like a decoder. They hate PWM DC.

I've checked, and the model is listed as 'DC, DCC ready'.  It was also priced like a non-DCC locomotive.

The Location: Forests of the Pacific Northwest, Oregon
The Year: 1948
The Scale: On30
The Blog: http://bvlcorr.tumblr.com

  • Member since
    January 2009
  • From: Maryland
  • 12,758 posts
Posted by ATLANTIC CENTRAL on Sunday, March 5, 2023 2:07 PM

tbdanny

 

 
ATLANTIC CENTRAL
Agreed, sure sounds like a decoder. They hate PWM DC.

 

I've checked, and the model is listed as 'DC, DCC ready'.  It was also priced like a non-DCC locomotive.

 

OK, the other thing to check for is capacitors on the motor leads - they can mess with PWM throttles as well.

Sheldon

    

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