# Does PWM cause problems ?



## Del Tapparo (Jan 4, 2008)

Another thread was discussing potential problems with PIKO locos having potential problems when running from a control that uses PWM (Pulse Width Modulation), as opposed to Linear (constant DC), so I thought I would just start a new thread on the topic.

I would like to relate my personal experience with "PWM problems". 13 or so Years ago, I used the old Aristo-Craft Train Engineer system for remote control of my track power. It had a switch for "PWC or Linear" modes. I witnessed problems one time on my layout with a friends loco on PWC. On PWC the lights were going whacky (that's a technical term). I can't remember the exact loco. I think it was LGB.

I now produce my own control systems (G-Scale Graphics) which all use PWM motor control. In the past 10+ years, I have never seen any problems with any of my locos, and I have never had a customer complain of any problems associated with PWM.

It has always been my assumption, that the problems with the older PWM systems was the frequency. I think they operated at a fairly low frequency, 600 HZ or something like that. You can tell when the frequency is low, because you will actually here the motor hum. All of my controls, and most other modern controls run at or around 20KHZ. You can't hear any motor hum, and to my knowledge the electronics that used to be affected by the lower frequencies work OK.

I'm sure there are probably exceptions. But any "problems" will be associated with auxiliary electronics (usually of poor design) in the loco, controlling lights or sound, not the motors.

This subject applies to both track power and battery power.


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## TonyWalsham (Jan 2, 2008)

Hi Del.
I too have never had a problem using pwm to drive Large Scale motors.
The problems occur when the locos are running on track power with pwm. Some DCC decoder equipped locos and some LGB locos with sound systems see the pwm waveform as a type of DCC and can get "confused".
That is the reason why Aristo/Crest introduced their PWM - Linear converter which is no longer available.


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## Del Tapparo (Jan 4, 2008)

TonyWalsham said:


> Hi Del.
> I too have never had a problem using pwm to drive Large Scale motors.
> The problems occur when the locos are running on track power with pwm. Some DCC decoder equipped locos and some LGB locos with sound systems see the pwm waveform as a type of DCC and can get "confused".
> That is the reason why Aristo/Crest introduced their PWM - Linear converter which is no longer available.


And perhaps, since most PWM is now higher frequency, the converter is no longer needed. (I guess if it was needed, there would be more sales, and thus it would likely still be available.)


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## toddalin (Jan 4, 2008)

I find that with the PWM turned on, my AristoCraft Train Engineers cause my Sierra sound boards to respond more erratically than in linear mode (i.e., makes the warning whistle blow more often with no deliberate change of speed).

If you want to send me a unit for trial, I will gladly do a comparison.


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## Del Tapparo (Jan 4, 2008)

toddalin said:


> I find that with the PWM turned on, my AristoCraft Train Engineers cause my Sierra sound boards to respond more erratically than in linear mode (i.e., makes the warning whistle blow more often with no deliberate change of speed).
> 
> If you want to send me a unit for trial, I will gladly do a comparison.


 Well Todd, I have no doubt of the outcome, so that won't be necessary. But I'm sure you would be pleased with the results (especially a light weight transmitter that fits in the palm of your hand or pocket).


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## toddalin (Jan 4, 2008)

_Maybe_ not as erratic as with the TE in PWM due to a higher frequency rate, but I don't know that it wouldn't be more erratic than the TE in linear mode.


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## krs (Feb 29, 2008)

Interesting topic!

All depends how deep one wants to get into that technology.

As was already mentioned, when it comes to PWM for model railroads one has to distinguish the PWM drive used by DCC and other types of decoders and PWM on the track.

With PWM drive of a decoder (of any type), the PWM only drives the motor(s) in the loco. Most motors in the loco are robust enough to handle pretty much any PWM frequency - the exception being core-less (also referred to as Faulhaber) motors which tend to overheat if the PWM frequency is too low. A low frequency also causes a regular loco motor to hum.
So the PWM frequency used by decoders today is usally 16KHz (16000 cycles per second) or higher. That's because 16KHz is more or less the upper threshold of human hearing so there is no audible motor noise and core-less motors can handle that high frequency PWM signal without any issues as well.
Some DCC decoders go all the way up to a 32KHz and even to a 40Khz PWM frequency.

Now with PWM on the track the situation gets a bit more complex.
Still the same issues and solutions as above but if there are any electronics in the loco that are connected to the track one can run into a few other issues.
One possibility is that the front end of the extra electronics has some sort of smoothing capacitor which essentially acts as a peak detector and thus provides the peak PWM voltage to the circuit.
I ran into that problem with the LGB steam and diesel sound cars attached to a Stainz. The loco ran fine with PWM, nice speed control, but the steam sound was always at full throttle. That's because the electronics in the steam sound car just picked up the peak voltage of the PWM signal which was always at maximum. Switching the PWM controller to linear solved that problem.
The second issue with PWM to the track and electronics connected to the track are the back emf voltage spikes generated because of the inductance of the loco motor. If the spikes are high enough and there is no supression built into the electronics, the attached electronics can burn out in a fraction of a millisecond.
If that can happen dpends on a lot of factors - the inductance of the motor, the rise and fall time of the PWM signal, the overvoltage protection built into the electronics.

Regards,
Knut


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## Del Tapparo (Jan 4, 2008)

Good comments Knut.

The back emf from the motor isn't a problem, as long as the motor driver has a proper snubber diode in the circuit, which they all do (mine sure do).

I think the input caps in the other electronics acting as a peak detector would depend on the input impedance of the circuit. It is actually the diodes in the input circuit that cause the peak detection, but if there is any kind of reasonable load on the output of the cap, it will provide a discharge path and average the voltage. I think this is likely a case with older electronics, like LGB and Seirra stuff designed 20 years ago. PWM wouldn't even have been considered. Current sound cards, like Phoenix, certainly have no problems with PWM.


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## Cosmos (Apr 16, 2011)

I have been running a low frequency PWM with a cheap switching power supply almost daily since 2011. No issues to speak of.

http://forums.mylargescale.com/38-traditional-power/19181-my-beginner-track-power.html


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## Stan Cedarleaf (Jan 2, 2008)

The only problem I've had with PWM is with the old analog sound in the early LGB Moguls (Made them go crazy), the motors in a couple of early LGB 0-6-2's overheated and as Todd shared, caused trouble with the old Sierra Sound boards. The auxiliary board for the Sierra fixed that as did the Aristo brand.


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## Treeman (Jan 6, 2008)

I have found that the directional lighting in some USA locomotives does not work from the Revolution output. I have installed a board that converts PW to linear to fix the problem.


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## Stan Cedarleaf (Jan 2, 2008)

Treeman said:


> I have found that the directional lighting in some USA locomotives does not work from the Revolution output. I have installed a board that converts PW to linear in them to fix the problem.


Yes, Mike.. I should have added that to my list as well.... 

And the Aristo PWM to Linear module did fix that..


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## Treeman (Jan 6, 2008)

I had a similar board made, as the Aristo one is not available at this time.


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## Mike M (Jan 3, 2008)

I was just on Crest's site and they are marked as in stock
Mike


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