more DCC questions

[LDBennett]

Thanks to the links given here I pretty much understand DCC. But I have another question.

When the motor controller part of the decoded follows the directions of the cab controller for speed and makes up a variable pulse width pulse train to run the motor to control the speed, at what frequency does it do it at?

Does this variable pulse width speed control really give and allow better slow speed performance?

As a mental exercise I wondered if I could get better and more realistic slow speeds with a variable pulse width voltage to the track of my non-DCC point to point trolley. I might consider coming up with an auxiliary circuit to do that if I knew at what frequency the action of the pulses on the HO locomotion would look smooth.

I also wondered if anyone made such a power pak/throttle that used a variable pulse width for speed control rather the variable  voltage. I do not need or want DCC.

Thanks:

LDBennett

[richg]

PWM gives very good slow speed performance, BUT, a poor running loco via DC will not be improved by PWM.
I made my own PWM power packs many years ago using the 555 IC.
Today, one or two companies might still make a power pack with pulse power but I have not looked for those in many years. Much of the time, the pulse power was to get the loco moving and really not needed after running.
At one time, the power pack put a pulse with a constant pulse width. I think one company had the option to vary the pulse width a little.

Rich

[richg]

PWM gives very good slow speed performance, BUT, a poor running loco via DC will not be improved by PWM.
I made my own PWM power packs many years ago using the 555 IC.
Today, one or two companies might still make a power pack with pulse power but I have not looked for those in many years. Much of the time, the pulse power was to get the loco moving and really not needed after running.
At one time, the power pack put a pulse with a constant pulse width. I think one company had the option to vary the pulse width a little.

Rich

I did a quick search. Look in the below link. Someone still sells what you would like.

http://tinyurl.com/7cduf36

Google should be your best friend. I find tons of answers on many, many different subjects and find a lot of good buys on the Internet with searches. I can even compare shipping charges.

Rich

[jward]

most of the mrc power packs contain some form of pulse power. according to the block diagrams i've seen, the pulse is a constant width and amplitude. its effect fades out as the base dc voltage of the pack rises, as you turn the speed control knob. at soome point, the dc voltage equals the amplitude of the pulse, and completely cancels it out.  pulse power as used in dc power packs is thus different than dc with an ac ripple riding the dc. it is more like square wave ac superimposed on the existing dc voltage.

as an example, it the amplitude of the pulse is 9 volts, and you set the dc control for 3 volts, you'd have 3 volts with the reamining 6 volts of amplitude as the pulse. at 6 volts, the pulse is 3 volts, at 9v or above the pulse is cancelled out completely.

[LDBennett]

OK. So the "pulse power" is not a variable pulse width at full voltage but a combination of DC and a pulse riding on the DC.

I would think just a variable pulse width at full DC voltage would do fine IF the frequency was high enough so the engine (or trolley in my case) did not surge with every pulses such that you could see the surging. What frequency would it take to achieve that? Anyone tried?
RICHG, is that what you made years ago with the 555 IC?

Some 50+ years ago I added a "slow switch" to my layout that gave a 50% duty cycle of DC (kind of) and it did indeed improve the slow running. I did it by half wave rectifying the 60 Hz AC then provided the resultant half wave rectified  variable DC to the track. I was thinking that full voltage variable pulse width DC might do a better job. The question is what frequency works to keep the action of the trolley smooth and slow? The variable full voltage pulse width  appears to be what DCC does(). What frequency to they use?

In my situation there is no load to speak of as it is just a single trolley. I want it to go slowly at scale trolley speed. Slow speed operation of a DC motor is always hard to achieve without surging. Can the system I described make the motion of the trolley surge free or virtually?

Maybe I just have to experiment?

LDBennett

[richg]

Pulse power started with low voltage AC connected to the DC via a resistor and capacitor to over come the cogging action of the motor. Not really a pulse though. It was a 120 Hz waveform.

Eventually electronics allowed a 12 volt pulse to be added to the filtered DC as many companies where making power packs with nearly pure DC. As the DC voltage was increased, the pulse would start to disappear. There where many variations on this.

I made the Pacematic power pack from a Model Railroader article and made quite a few modifications using the LM555. With a potentiometer , you can vary the pulse width a certain amount.

The link I showed before, the first one is a DC power pack with variable pulse width control.

http://www.awrr.com/throttle_Thorne.html

Rich

[Doneldon]

LD-

There's a 10-amp pulse pack here: http://www.hobbylinc.com/htm/crx/crx55401.htm

My MRC Power Command 9500 has switchable pulse power but its frequency doesn't change as far as I know.

                                                                                                   -- D

[richg]

Below is a little more DIY for DC throttles if you like to experiment.

http://home.cogeco.ca/~rpaisley4/CircuitIndex.html#Throttles

Rich

[LDBennett]

My thoughts were to emulate the DCC motor controller in the DCC trains.

According to my reading the DCC decoder drives the motor. The diagrams indicate a constant period of the wave form (frequency) with a pulse width controlled by the decoder to match the speed command from the train cab controller/throttle. At small throttle settings the period of the wave form appears to be the same as that at nearly full throttle. When at full throttle the pulses disappears and all that is left is pure DC. But the period of the pulse train appears to be constant at all other throttle settings. There appears to be NO pure DC mixed with the pulses. What is that period?

The stuff I read does not show the DCC decoder/controller mixing pure DC with the pulse as so many of the DC pulse power paks appear to do.

All I am after is smoother slow speed operation and perhaps slow speeds not attainable with a common variable voltage power pak/throttle. Does it take the mixed pure DC with a variable pulse to attain that? or will just a variable pulse width, constant voltage, no pure DC mixed in version do the job better or as well???

Again this is just an mental exercise at this point as my layout is not yet far along enough to be able to run the trolleys for any length of time to even be able to break them in. I have run them to be assured the track has no shorts and the minimum speed was not to my liking but both trolleys are brand new and not broken in. This exercise is Plan B if I can not get the smooth low speed operation I desire after the trolleys motor are thoroughly broken in. I am days from finishing the Auto Reverse wiring and being able to run the trolleys point to point continuously to allow their motors to break-in. But I want to consider Plan B now, just in case.

Thanks to all those that offered help so far.

LDBennett

[Doneldon]

LD-

Smooth slow-speed operation is the product of both the motor and the decoder.

The decoder does vary the pulse width of the "power stroke." I believe you are correct that no pure, so-called pure DC accompanies the power pulses. It is not my understanding that the power wave is the same width at different throttle levels. On the contrary, the pulse width varies specifically to control speed/power. I do not believe there would be a way to control speed with equal length power pulses unless the waves' amplitudes vary, which, to the best of my understanding, they do not. That's the whole point of DCC: Power (speed) is controlled by varying the width of full-power pulses. Longer pulses, greater speed/power; shorter pulses, lower speed/power.

Modern electric motors, especially can motors, run at comparatively high RPMs so their operation smoothes way out when a gear box reduces the high revs to a reasonable rate. Combined with the consistently high power surges from the DCC decoder (albeit intermittently available) overall slow speed operation improves dramatically. I have even large locos which will smoothly creep as slowly as .3 smph.

I hope this clarifies things for you or, in the not unlikely event that I am talking through my hat, someone else will pick up the reins and make things right.
                                                                                                                 -- D

[richg]

Some years ago way before DCC, I modified a Tyco 4-6-0 and 0-4-0 with a Sagami 5 pole can motor, flywheel and 45 to 1 gears for the 4-6-0 and 72 to 1 for the 0-4-0. Those two locos could crawl at an unbelievable slow speed using the Pulse power pack I made. I did this as an experiment.
Diesels with a good can motor and flywheels, good gearing can do the same.
As soon as I turned up the throttle, I had a 12 volt pulse and the DC would increase as I increased the throttle.
With DCC, there is a narrow 12 volt pulse which gets wider as you increase the throttle.

Below is from my NCE Power Cab.

FWD, minimum throttle.



FWD, half throttle.



Rev. Minimum throttle.



Voltage stays the same.

Rich

[richg]

My thoughts were to emulate the DCC motor controller in the DCC trains.

. . .

Change the focus to a different technology.

Since you are leaning to using DC, not DCC, for model train control suggest  MRC, Model Rectifier Corporation , TECH 4 DC model train control products as an option. 

That is another option I have heard being used. I think one or two other companies sell a similar device.

Rich

[Doneldon]

LD-

I've been thinking about your project and I just have to ask, aren't you trying to reinvent the wheel? DCC is a mature control system which will do what you seem to want to do. Why not just use it? I appreciate the challenge of finding a new way to do something but I don't think you'll find a better way and I'm pretty sure you won't save any real money making a one-off prototype. So ... why are you doing this?
                                                                                                                                            -- D

[LDBennett]

Doneldon:

There is a difference between pulse width and waveform period. Pulse width is what is varied in DCC and apparently the repeat or period for nearly full throttle and small throttle is the same. Look at the O-scope pictures by richg. Note that indeed the pulse width changes from 10us to 30us to 40us while the period of the waveform is every 60 us. That is, the pulse repeats every 60 us regardless of the pulse width. The pulse voltage is always about 12+ volts DC. This is as I expected. The frequency is a bit higher than I would have guessed.

I'm a retired electronics (digital design) engineer and figuring how things work is part of engineering. I decided to figure out how DCC worked. After studying it some I realize that the decoder probably makes equal period pulse trains of varing pulse width. Was that the key to smooth slow speed operation? That is why I visited here to see if anyone here knew. A key factor is the waveforms period or frequency. All of this is doable with a minimum of parts  and I have the expertise.

My little layout is a straight section of single rail. It is a point to point city scene. I most certainly don't need DCC with all its computerized complexity. All I want is smooth slow speed operation of one trolley at a time. I have two trolleys, one with DCC and one without. The cost to use DCC would be pretty high to only get smooth slow speed operation. I know that using a pulse power will smooth out the operation from my own experience over 50 years ago. I am just trying to see if anyone else has experience using just pulse power without the mixing of DC with it as apparently several DC power paks do and determine what the period/ frequency of the waveform is.

This is not plan A (straight DC operation from a little Bachmann Power Pak) but plan B if the running of the trolleys is not smooth enough for me. I am not at the point where it is clear plan A will fail. But it does not hurt to research alternatives just in case.

LDBennett

[LDBennett]

richg:

Help me out here as I am not familiar with power paks by name.

Are you monitoring the DCC decoder controller for the motor as controlled by the NCE Power Cab? Is this the power form the engine motor sees?

If so then the motor sees variable pulse widths from virtually zero to 60us where the period is 60 us (??).

Does this result in very slow smooth engine operation?

The power pak you made years ago was straight pulse power with no DC mixed in and it gave smooth slow speed operation? Do you remember the period of the wave form? Was it about the same as the o-scope pictures? Do you happen to still have the schematic of the power pak or a link to where it might be found? I understand you added flywheels and low gearing but did you happen to test the setup without the mechanical changes? Was it smooth at low speeds?

Thanks,

LDBennett