more DCC questions

[richg]

The Scope test probe was right on the motor leads for the DCC test. It should be very clear, the width of the pulse determines the motor speed. That is typical for DCC operation. I do not remember what the frequency is. It was not important to me. I do know original decoders had a lower freq. And could cause motor noise but the newer generation decoders changed all that.
My locos start moving at around 0.7 to 0.9 mph. Some can do better than that but the decoders need a lot of fine tuning, sometimes.

The DC power pack had a pulse as soon as I moved the pot a little but the DC level was very low. It is a combination of the pulse to overcome the cogging of the motor. All that I have read back in the 1970's pulse power was to overcome the cogging as was not needed after the loco started to me. The design I used just kept the pulse in operation.
I am not at hope until Sunday night to check my power pack.
I will put the schematic together that I built but I already posted a link to this which is quite similar as it is by the same author, Peter Thorne. Did you look at the links? You did not give any indication if you did.
I do not use this power pack anymore. I only refer to it when someone has a question about DC with pulse power. I and many others have found that DCC is much better.

The locos I modified where old worm to worm gear, about 33 to 1 ratio Tyco's.. I used NWSL Sagami parts, A worm, worm gear and two spur gears  My original intent was to experiment with modifying a loco for better DC operation in the 1970's.  I don't remember what the start speed of the locos was. I do remember being very satisfied with the results. I do remember having to play with the 555 pulse width/period to achieve no motor noise. Also, minimum gear lash while not being too tight for no gear noise.
I also modified the 10k pot with a resistor to be able to use more of the pot for the 0-4-0 that had 72 to 1 ratio.
This is old school stuff. I am not interested in arguing with anyone. It worked for me at the time. I like to experiment anyway.
Sometime, check your locos for gear reduction as an experiment. I do know the Spectrum 0-6-0T has excellent gear reduction.

Rich

[LDBennett]

richg(Rich):

I hope you don't think I am arguing with you. I am just seeking information. So far you have offered very good info indeed and the O-Scope pictures were excellent. Thank you!

OK. The DCC motor controller is getting 12 volt pulses from virtually zero pulse width to 60 us with a period of the wave form of 60us. Got it. Thanks for the info.

That means the frequency is about 16KHz if the O-Scope pictures are correct. The Thorne link schematic seems to be operating at about 150 Hz. It also mixes the pulse with DC. Unless that makes the engines run smoother I don't understand why they do it that way. It appears that is a common theme in these older pulse DC power paks (??). I'd be interested  in seeing your schematic if it was just pure pulses.

I got my layout wired last night but have to set up the train detectors for the Auto Reverse before I can run the trolleys to see if they will run smoothly at slow speeds after they break in. I'll be busy all weekend so it will have to wait until next week.

Again, thank you. I really appreciate your efforts.

LDBennett  (Lynn Bennett)

[Jim Banner]

Caution - long, boring technical comment follows.  READ AT YOUR OWN RISK of falling asleep.

Some of the better decoders implement CV 9 which is used to set the total pulse width modulation period, that is, the on time plus the off time.  This is the inverse of the pulse repetition rate, a.k.a. frequency.  With these decoders, you can set the frequency for the best compromise between low speed performance and noise.

The reason that decoders use pulse width modulation rather than pulse rate modulation is that pulse width modulation will work with all motors while pulse rate modulation, using fixed width pulses, must to tuned to the motor for best operation.  This is particularly true of older or cheaper motors with a lot of cogging.  It the pulses are too narrow, a slow repetition rate will not be able to kick the motor through a cog and it will just vibrate until the pulses are frequent enough to cause jack rabbit starts and poor or nonexistent low speed performance.  With pulses wider than necessary, the motor will advance the locomotive in large steps, or at least, steps larger than really necessary.  With pulse width modulation, you can turn up the speed control until the pulses are wide enough to kick the motor through a cog.  An ideal solution that was, to my knowledge, never implement commercially but was used by some home builders had both a repetition rate control. which served as a speed control, and a pulse width control.  The idea was to set the repetition rate to some low value, then advance the pulse width control until each pulse was just wide enough to advance the locomotive a tiny amount.  Thereafter, the pulse repetition rate control was used as the throttle.

With cheap motors, the sort used in the seventies to power train set locomotives, pulses in the 5 to 10 millisecond range were required.  For better motors with the armatures more tightly coupled to the fields and five instead of three poles, pulses of around a millisecond were long enough.  Really good can motors with five poles and skewed windings that appeared in the eighties needed as little as 100 microseconds pulses.  Having to tune the controller to each locomotive separately was a bit of a pain but it meant that cheap, train set locomotives that normally started at about 10 smph could be reliably run at one tie per second, or less than 1 smph.  This meant that just about any locomotive, including the brass clunkers of the day, could be used for realistic switching.

Nowadays, just about any decent locomotive can do the same, even on pure dc.  The use of pulse width modulation in DCC decoders is simply a way of reducing decoder heating.   With ideal transistors driving a motor,  the heating in those transistors is E (voltage across the transistor) times I (current through the transistor.  When the pulse is turned on, there is no voltage across the transistors so E * I is zero.  When the pulse is turned off, there is no current through the transistors so E * I is again zero.  If E * I is always, zero, then there is no heating in the transistors.  Of course, the transistors are NOT ideal and so there is some voltage across the transistors when they are on and there is both voltage across the transistors and current through them during the finite time it takes the transistors to switch from on to off and vise versa.  This means the transistors do have to dissipate some power, but only a very small fraction of what they would have to dissipate if used to control pure dc.

Jim

[LDBennett]

Hunt:

Review of the richg O-Scope photos reveals a frequency of about 16KHz and that is close enough to 22 Khz to be the same. The 150 Hz pulse power pak in the Thorne link is not even close and implementation of it might be problematical. If I attempt this pulse power scheme then it will be at the 16 to 22 KHz frequency, not 150 Hz.

Jim Banner:

Thank you for the good explanation. It kind of makes me believe that after I break-in my two trolleys I may be able to operate them smoothly without pulse power.

This exercise has been very educational for me and I appreciate all the good info provided by everyone. Knowing how DCC works takes away all the mystery of DCC. If I ever expand to a bigger layout (probably not in the future as I am space limited and age limited) I most certainly would implement DCC at the highest level as it is surely a neat system. For now DC control ((either pure DC or maybe (??) pulse control)) is all I need or want.

LDBennett

[LDBennett]

I finally got the little layout all wired with the Circuitron auto reverse controller and the intermediate train detectors that allow stops along the way with a delay. The Bachmann PCC Street Car runs smoothly but only moderately slowly while the Spectrum trolley will run slower than a tie per second (plenty slow enough). My concerns about slow smooth running are for naught as both satisfy me.

My initial test was with the Spectrum Birney trolley with the DCC controller still in place and the little trolley was balky for starts. After I removed the DCC board and installed the supplied dummy plug the trolley seemed to perform better at slow speeds. But I did spend a couple of hours on each trolley letting it run continuously to break them in. I think that might have had something to do with the increased smoothness too.

Anyway there is NO pulse power in my future. All is well as is. What is remarkable to me (apparently a product of current motors having more poles than my trains of 50+ years ago) is the smoothness at slow speeds of both these trolleys but the Spectrum Birney is superior to the Bachmann PCC Street Car in that respect.

My idea was to kind of create my remembered environment of the 1950's with the Los Angeles Yellow  and Red cars (it does not have to be exact and the Birney is wrong for LA but it is red). I got so stoked after running the two trolleys that I ordered the Spectrum LA Lines Peter Witt street car.

Thanks again all for the help.

LDBennett