I do think the ringing shown depends as much on teh design of the booster circuit as it does with the wiring. You can do a lot to damping anying ringing on the transmission line by properly designing the driver circuit.
You did the right thing by testing with the scope. If there wasn't excessive ringing, adding a snubber would do nothing.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
The Problem - We did some testing over the past several days on our club layout. The layout has over 2200 of trackage, with three 8 amp boosters. The bus wiring has either #10 or #12 solid core wire, with either 3M ScotchLoc or Positap connecters to #18 feeders soldered to the code 100 rail. Measurements with a RRampMeter to each of the power districts shows a solid 13.8v(there are 62 power districts fed by Digitrax PM42's). The 'Quarter' test trips the PM42 in the power district every time. The layout is just over 10 years old. Several sections of the mainline started having issues - The main booster would shut down, even though there was a PM42 between the track and the booster. We checked the configuration settings on the affected PM42's and found that they were good. Further testing with a 2.2 amp load(1156 lamp) showed that the we were getting a shut down with that small of a load. We swapped out the affected PM42's with some in the staging area, and the problem went away. I ordered new PM42's and we installed them Wednesday.
In the meantime, the MR article came out and we had a lot of discussion about 'noise or spikes' in the system. So, a $25,000 digital scope was borrowed from an employeer and we set up a testing procedure. We scoped the affected areas on the main line. The noise/spike issue was just about non-existant. Adding a 'snubber' did nothing. We did this with no load, a static 2.2 amp load, and with 6 engines running in the tested power district.
We then decided to do some testing where the suspect PM42's had been moved to. A 2.2 amp static load tripped the old PM42's and the new PM42's! We again checked the rail voltage under no load - 13.8v. Wth a 1.3 amp load - The voltage dropped to 12.5v. And with a 2.2 amp load, the voltage to just under 12v. No wonder the PM42's were not working correctly! The original bus for the staging area was split in half, and had about a 40-50' runs to either side. At some point the booster was moved(to be closer to the main Command Station booster) and now there was something like a 110' run from the booster to the end of the power district. Too much line loss - and a check about half way back verified that the voltage loss when a 'load' was applied.
The 'Plan' is to move that booster back to the 'mid-point'(We will need to get another power supply for it as it is 'sharing' a 20 amp power supply with the Command Station currently). A further test verified that the PM42's will 'act up' if the input bus voltage gets down to the 12v range. 30' to 50' for a Power District run is about the max one should have(even with #10 bus wiring).
Conclussions - The 'ringing' displayed in the MR article(and lifted from 'Wiring for DCC') appears to be a rather severe example. Good wiring practices(heavy wire/short Power Districts) seem to keep this issue at bay. Our issue was because no one 'measured' the total run of the main DCC bus in the staging area when the 'move' of the booster was done. And our testing procedure did not call for testing with a large static load on the track(just a 'Quarter' test on an unloaded Power District).
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
The easiest way to reduce inductance is to place the wires close together, or even easier, a slight twist in the pair to keep them together.
An inductor, to direct current, is a short with only the resistance of the wire limiting the current. In AC, the changing current causes a magnetic field to be created which stores current, as the current decreases that stored energy si returned, thus the inductor resists changes in current. Adding a capacitor can create a resonant circuit, which can be quite dangerous.
The DCC protocol was designed to handle a lot of wiring issues from the start. Inductance in the wire isn't a problem until you start having really long runs of wire, because it is related to the length of the wire. Putting bus wires close together can reduce/manage that issue.
If the DCC signal gets really distorted by wiring related issues, yes, you can have a runaway. This is solved by disabling the analog mode of the decoder, so it will only work with a DCC signal. No valid signal, it stops.
The only true method to diagnose signal issues is with an oscilloscope. It will show the amplitude and the waveform so you can see what the problem is before installing all the voodoo methods that are immediately offered. Since the DCC signal has a lot of harmonics present, interesting things can happen with poor wiring.
I'm often disappointed by MR articles such as this which introduce fairly sophisticated topics but don't explain the topics very well.
Without a more complete explanation, many modelers may be worried about a problem that may not exist on their layout and may end up spending $$ unneccesarily.
most of us are familiar with inductance in the form of motors or relays. Inductance resists (not to be confused with resistance) the change in current. This is why a reverse biased diode is placed across an inductive load so that when the voltage across the load is removed, the current through the inductive load has a path to circulate through and diminish. Otherwise there is a potentially harmful voltage spike. This is how coils on cars generate the high voltage for spark plugs.
the article says "Inductance is sort of like resistance, but only affects alternating current ". But as I just explained, inductance is something to consider even in DC circuits that are simply switched on and off.
And while I would be concerned that such voltage spikes may damage a decoder, I don't understand how such spikes can be misinterpreted as DCC commands that "result in loss of control, runaways and changes in settings". DCC commands are based on the time between edges not the amplitude of the edges.
I'm not a fan of voodoo and mysticism (to accept something as true without explanation).
greg - Philadelphia & Reading / Reading
MisterBeasleyDCC seems to be a very robust protocol. As long as you follow the basic rules, you'll be fine.
DCC is a very rebust protocol. The data rate is slow enough that it doesn't really need high quality wiring, and there's checksum bits (the last bit of byte 3 in the data packet) to ensure the integrity of the packet. And DCC only broadcast function commands once, but speed commands over and over again and again. Rebroadcasting speed information keeps the train moving if it loses power, instead of just waiting for a new speed command from the operator.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
I glanced at this article a couple of nights ago, and my personal reaction was "overkill." I've been using DCC for 9 years, and reading this forum for even longer. In all that time, I don't think I've ever read where this was a problem for anyone.
DCC seems to be a very robust protocol. As long as you follow the basic rules, you'll be fine.
It takes an iron man to play with a toy iron horse.
The NMRA standard s 9.1 line 17 dictates that 52 milliseconds should be interpreted as a valid "1" bit. So any line noise greater that 52 nanoseconds on the scope should be eliminated. And this makes sense because the DCC bit rate is 19.2kbps. (1/.000052) A simple inductor in parallel, or cap in series just past the output side of the rectifying (Schottky) diodes inside the booster will create a high pass filter to eliminate noisy line sources. Any quality transformer/booster should be doing this already.
http://www.allaboutcircuits.com/vol_2/chpt_8/3.html
S9.1 line 56 goes on to state peak should never exceed 22 Volts. But spikes of 27Volts should be handled by the decoder. This is risky however as caps on decoders rarely exceed 25V. You could have a spectacular fireworks display.
That said, to get a serious 12 Volt bump above 14.25 VDCC (HO standard) on a DCC track requires a serious circuit failure (ie: Lightning strike or some sort of short to another circuit) In that case your entire system is likely going up in smoke.
Twisting wires is an age old practice. In theory if some outside influence causes a voltage change, it will affect both wires in the same way. So a .1mV positive bump will occur on the positive and ground leg and the relative difference will be unaltered. Also twisting the wires help minimize noise. (Less inductively induced noise)
The spikes on the bus, and the use of a UPS, are two entirely different and mostly unrelated issues.
The spikes on the bus are a function of the bus length, the DCC signal frequency, and assorted other electrical factors. They usually manifest themselves as unexplained loss of control or runaways (especially near the end of the bus), decoders losing their programming for no apparent reason, etc. If bad enough, yes, they can damage a decoder in a manner similar to ESD.
A UPS, on the other hand, may filter or otherwise clean up the input power to the DCC equipment and protect it from noise or surges in, or the failure of, the AC power. That would be stuff like an old vacuum cleaner or similar that still uses a motor with brushes, far-away lightning strikes (if the strike is close enough, there is no protection), and so forth.
As an aside, anecdotal evidence seems to indicate that the spikes on the bus tend to be more of an issue on some DCC systems that others. For example, on the NCE list they are a regular topic of discussion that pops up every few months or so. On the other hand, on the Digitrax list, with nearly half again as many users, those conversations are less frequent and usually started with something along the lines of, "Do I need snubbers? I saw on the (insert other DCC list here) that..."
There is a new discussion related to this in the MRH site but cannot put the link here.
Yet to hear of any issues related to the boosters so far by ringing or spiking. Don't get MR anymore so not aware of the details in the article.
I would be curious to see what the buss lines look like on the huge layouts at a local train show I go to. Think I will ask at the next show.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
I belong to a number of Yahoo DCC Groups and see this this discussed quite a lot. Opinions and facts.
I have seen ringing and spiking on our club layout with a Scope but no issues from it.
I have seen references to programming on decoders that seem to happen. Lots of discussion on twisting wires.
I have seen a discussion in a forum about using a UPS for power line protection when a thunderstorm is in the area.
Below are a couple of people who are there. I doubt they would ever come here but you never know.
http://www.wiringfordcc.com/dcc_waveforms.htm
https://sites.google.com/site/markgurries/
Knowing how smart you are Jim about things electrical, I'm sure you have an opinion. (As do I)
I have read the article about signal distortion in the current issue of MR. It seems to be a re-hsh of what has been on multiple resources over the years. What is not addressed is how much of that 'spike' is damaging to our DCC equipment. There is going to electrical 'noise' on the DCC Bus - We have model trains with motors spinning(brushes making/breaking) and contact issues at the rail/wheel interface.
o Are those voltage spikes in the 'scope' view serious?
o Is a UPS for the boosters a good idea?