Date   
Re: Adding a Reverse Loop to the Bus

Mark Gurries
 

On 6/12/12 at 3:23 PM, the_crusher@... (D. L. Turnock) wrote:

Thanks for the explanation. I went to the web-site and read through and
have a couple of additional questions.



I'm still working track plans and such but I'd prefer to get things figured
out before I have to re-do it J


One of the comments at the web site was terminating the bus (if required) at
each end.
No DCC manufacture require termination but it is recommended by
some. Placement is at the far ends of the wire longest run
relative to the booster. The booster itself terminates its own end.


But, this is also after talking about sending the bus two
directions from the booster with the booster in the middle. So, first
question is: If using a terminator, is it required at the end of the bus
run away from the booster or is it also required at the booster?
Yes. Think of each bus run away from the booster as it own bus.

Second question is: Even if you keep the run down under 30', would a
terminator undermine performance?
No. In fact just the opposite if you check out the scope photos
of the results.


Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics: www.markgurries.com

Re: Adding a Reverse Loop to the Bus

Steve Haas
 

Dave T asks:

<<So, first question is: If using a terminator, is it required at the end
of the bus run away from the booster or is it also required at the
booster?>>

At the far end of the bus run is more than sufficient.


<<Even if you keep the run down under 30', would a terminator undermine
performance?>>

A terminator will not undermine performance in any way, shape or form.


Best regards,


Steve Haas
Snoqualmie, WA

Re: Adding a DCC bus terminator

Vollrath, Don <dvollrath@...>
 

Allan,
Indeed an interesting and somewhat controversial subject. As always the perfect engineering answer is... It depends ^_^
Anecdotal evidence sways both ways. Yes, the NCE boosters do have significantly faster switching times than many other brands and you probably have much more ringing on your layout wiring now than you think. As long as it continues to work I wouldn't spend much effort to fiddle with it. But adding bus terminators won't hurt. I have seen several older converted layouts with long haphazard wiring that seem to work OK... except that there were areas on the track where certain locos didn't seem to respond to DCC throttle commands well. (confused decoder signaling) Problem solved by adding terminators. There have been other stories of where there seemed to be a lack of track voltage or current available under load despite use of 12 ga wire. (too much wiring inductance) Problem solved by using twisted pair wiring. There are other stories of damaged decoders or decoders that have scrambled CV values supposedly due to short circuits happening nearby or elsewhere on the layout. (voltage spikes are the prime suspect)

Fact: Some boosters have faster voltage transitions than others. There was in article published in the NMRA bulletin several/many years ago showing the effects caused by several booster brands types. I don't recall the actual month or year, but it was an eye opener. NCE brand was and still is one of the fastest switching types that caused much more ringing than with other brands. Also... Yes some loco or accessory decoders are more susceptible to being confused than others.

Toned Down Techno-babble:
The DCC bus wire pairs essentially form a transmission line at high frequencies. A transmission line has a characteristic impedance, Ro, made up of distributed inductance, capacitance and resistance, all affected by the manner in which the conductors are placed. As each of these measurable physical features increase, the signal velocity or transmission speed of the signal from one end to the other at any particular frequency gets slowed down. Higher frequencies get slowed down more so. The equivalent L and C values of the wiring construction also determine a frequency of resonance where signals close to that frequency want to resonate or 'ring'. These characteristics plus the overall length essentially determine how long it takes for a signal to propagate from transmitter (our boosters) to the far end of the DCC bus run at any particular frequency. We know that the DCC frequencies of interest are only 5-8 kHz and should ordinarily not be affected by those minor matters. However DCC is not sinusoidal voltage waves but nearly square or rectangular waves of +/- 'track' voltage being rapidly switched in polarity to create the DCC signal frequencies. It is the speed at which the voltage transitions take place, as sent by the booster, that need to be considered when again looking at the transmission line effects. The booster switching speed (in terms of microseconds) defines equivalent frequencies of the 'edge' of the signals of interest. When the overall propagation delay time of the DCC bus wiring from booster to an open end starts to approach the booster switching time the resonant ringing of the L and C of the wiring (MegaHz) starts to become obvious with relatively large voltage overshoots. Multiple echoes or reflections of the signal transitions can flip back and forth with amplitudes far greater than the intended DCC signal. [See my scope photos at http://www.wiringfordcc.com/dcc_waveforms.htm] When/if the reflections cause multiple voltage polarity changes, some decoder receivers seeing such signals may not properly decode the intended DCC information. The ringing phenomenon usually begins as the bus runs get longer than 25-30 ft. Usually adding some kind of loading at the open wiring end will damping such oscillations. Placing a resistor of the ohm value that matches the characteristic impedance of the transmission line will do it. (typically 75-200 ohms or so) However, a simple resistor of that value will also absorb energy from the entire DCC signal and get hot. So we place a capacitor in series with the resistor to let it terminate the bus to dampen the high frequency ringing but block the remainder or 'flat top' steady voltage portion of the DCC signal. It turns out that the resistor value is not critical. 100-150 ohms works nicely as that is close enough to Ro of most wiring schemes. Ditto with the capacitor. 0.047 to 0.1 microfarad seems to work OK. The resistor of 100-150 ohms will absorb power related to the DCC frequency, track voltage and value of capacitance. [ 4transitions x 1/2 x C x V^2 x f = ~0.26 Watts with C=0.1uF, f=7 kHz average & Vdcc=14V. A minimum of a 1/2 watt resistor should be used.]

Note that the resonant ringing of the wiring can also be triggered by an intermittent connections or removal of a short circuit as that too causes sharp dv/dt voltage transitions. Hence, add bus terminator R-C networks to help prevent over voltage ringing in case of sparking short circuits. (Are there any other kind?)

Using larger diameter wire for the DCC bus to lower the electrical resistance makes no difference in this discussion. One still needs terminators to squash high frequency ringing effects of the transmission line. However, too much inductance of the wiring can also limit current at the fundamental DCC frequency enough to noticeably slow down locos at certain parts of a layout. Long bus runs using well separated wires create a loop with added inductance (as well as radiated electronic noise) which creates a voltage drop when DCC current flowing attempts to reverse in direction. Hence, use twisted pair wiring to avoid those possible effects.

Regards,
DonV

-----Original Message-----
From: WiringForDCC@... [mailto:WiringForDCC@...] On Behalf Of Allan
Sent: Tuesday, June 12, 2012 7:29 PM
To: WiringForDCC@...
Subject: RE: [WiringForDCC] Re: Adding a Reverse Loop to the Bus

Don,



Do you think I should admit that I have runs well over 30', no twisted
wires, and no snubbers??? J My railroad was all wired before we decided
all these things are a good idea. I still think they are good ideas to
ensure that people don't have problems.



When you and Mark first mentioned these things, I wasn't having trouble, but
I thought maybe since you both were using NCE, I thought their rise times
might be faster than Digitrax. Now I am a half-NCE user. I have replaced
two of my Digitrax boosters with NCE boosters. I just added the second
booster. I'm waiting to see if I develop problems.



My reason for using NCE is that the Digitrax boosters are overly finicky at
tripping. They are supposed to be 5A boosters, but they start tripping at
about 2A. I called Digitrax, but received a technically unsound reason for
the problem and no offer of a solution. So I thought I'd try NCE and so
far, things seem to be good.



Allan





From: WiringForDCC@... [mailto:WiringForDCC@...] On
Behalf Of Vollrath, Don
Sent: Tuesday, June 12, 2012 11:17 AM
To: WiringForDCC@...
Subject: RE: [WiringForDCC] Re: Adding a Reverse Loop to the Bus





Some people (me included) use words that almost fit but don't quite belong.
The function is to help prevent voltage spikes and signal reflections on the
bus wiring caused by abrupt voltage changes of the DCC signaling scheme. The
issue has comes up when the switching speed (how fast the transmitter
changes the voltage signal) starts to become less than about 1/10 the time
it takes that signal transition to sweep down the wire, be reflected and
return back to the source. A properly placed Resistor and capacitor wired in
series and placed across the far end of the bus absorbs the energy and
prevents it from reflecting. The actual propagation speed is controlled by
inductance and capacitance of the wiring... ie: distance between the wires.
This is why twisting them together works better than other random flow of
the wires. Long leads also enter to the formula for reflections. The R-C
network should be called a terminator when used in this context, as it
terminates or completes the transmission line like characteristics. In other
circles an R-C network is called a snubber as it 'snubs' or absorbs voltage
transition energy or even a filter as it filters out high frequency
components, like 'ringing' reflections. See
http://www.wiringfordcc.com/track_2.htm#c2

DonV

-----Original Message-----
From: WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
[mailto:WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
] On Behalf Of Eric
Sent: Tuesday, June 12, 2012 10:46 AM
To: WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
Subject: [WiringForDCC] Re: Adding a Reverse Loop to the Bus

Thanks, Don and Steve. I am unfamiliar with the term choke/snubber/filter.
Can one of you explain?
RicZ

--- In WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
, "Vollrath, Don" <dvollrath@...> wrote:

Eric,
You will need an auto-reverser for the reverse loop. Just locate it near
the reverse loop itself and connect it up to the existing DCC bus at that
end of the layout and to the reverse loop tracks. The 27-30ft length is not
an absolute limit, but merely a suggestion for optimum performance. Adding
an R/C snubber/terminator to the DCC bus wires at the far end from the
booster (near the input to the reverser) will also help prevent signal
degradation. Yes, twisting the DCC distribution bus run also helps to
minimize problems. Is it absolutely necessary?... No but it you are building
the layout, it is a good idea.

DonV


-----Original Message-----
From: WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
[mailto:WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
] On Behalf Of Eric
Sent: Monday, June 11, 2012 4:46 PM
To: WiringForDCC@... <mailto:WiringForDCC%40yahoogroups.com>
Subject: [WiringForDCC] Adding a Reverse Loop to the Bus

In Alan's web site the possible length of a bus element is 27-30' from the
booster. Does this length also include the reverse loop wiring? For instance
if my bus length to the reverse loop is 20' and the reverse loop is 12',
have I exceeded the 27' limit? If so I would have to move the booster closer
to the loop or add another curcuit breaker.

Also, should a long bus always be terminsted?
RicZ



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DCC Power Supplies - Phase and Polarity.

Carl
 

Hi Gang:

A while back I wrote about a DCC Booster problem. I have a DCS100 and
DB150 to power an HO layout. At the gaps between the two boosters I
would find voltage in all directions. There was no combination where I
could get zero voltage at the rail crossing a gap. So I ran everything
on just the DCS100.

At home on my O gauge layout I have 6 boosters and have never had this
problem. So I thought what is the difference? On the O layout I have one
DC power supply for all the boosters, each booster has its own breaker
for protection. On the HO layout I had two Digitrax AC adapters. On
Lionel layouts it is important to have the AC transformer in phase. So I
thought: Perhaps I need to phase the Digitrax power supplies?? I wasn't
sure why it would matter since the boosters rectify the supplied current
anyway. So with nothing to loose I swapped the leads from one Digitrax
power supply. It did the trick! One way I had full voltage and the other
way was Zero! Now I could correct polarity at all the gaps and all looks
well.

Good luck, Carl.

Re: DCC Power Supplies - Phase and Polarity.

Mark Gurries
 

If your measuring with a high impedance voltage meter, I am not
surprised for your measuring 60 hz noise not polarity.

The true test for polarity is to place a light bulb across the
gap your measuring to eliminate the noise.


On 6/14/12 at 7:20 AM, carl.blum@... (Carl) wrote:
Hi Gang:

A while back I wrote about a DCC Booster problem. I have a
DCS100 and DB150 to power an HO layout. At the gaps between the
two boosters I would find voltage in all directions. There was
no combination where I could get zero voltage at the rail
crossing a gap. So I ran everything on just the DCS100.

At home on my O gauge layout I have 6 boosters and have never
had this problem. So I thought what is the difference? On the O
layout I have one DC power supply for all the boosters, each
booster has its own breaker for protection. On the HO layout I
had two Digitrax AC adapters. On Lionel layouts it is important
to have the AC transformer in phase. So I thought: Perhaps I
need to phase the Digitrax power supplies?? I wasn't sure why
it would matter since the boosters rectify the supplied current
anyway. So with nothing to loose I swapped the leads from one
Digitrax power supply. It did the trick! One way I had full
voltage and the other way was Zero! Now I could correct
polarity at all the gaps and all looks well.

Good luck, Carl.


[Non-text portions of this message have been removed]



------------------------------------

http://www.WiringForDCC.comYahoo! Groups Links


Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics: www.markgurries.com

Re: DCC Power Supplies - Phase and Polarity.

Carl
 

Hello Mark:

I started with the trouble light, why should it light in all directions?
So I decided to "force" the booster auto-reverse function by putting a
solid jumper across one gap. This shut the DB150 down for the day. It is
OK now. The locomotives would stop at the gap no mater which way the
wires were connected.

Thanks, Carl.

Re: DCC Power Supplies - Phase and Polarity.

Mark Gurries
 

On 6/14/12 at 11:27 AM, carl.blum@... (Carl) wrote:

Hello Mark:

I started with the trouble light, why should it light in all directions?
So I decided to "force" the booster auto-reverse function by
putting a solid jumper across one gap. This shut the DB150 down
for the day. It is OK now. The locomotives would stop at the
gap no mater which way the wires were connected.
This sound like you have a sneak AC path going through your
layout power and finding its way it the DCC system. Something
is really wrong here. There is an AC ground loop here.

Do you have earth ground connected to your boosters or layout?


Both the DCS100 and DB150 are complete DCC system which means
they are a combo command station + booster in a single box.
With respect to the booster portion:

The first thing that happens with the AC from your transformer
output to the booster input is it gets rectified to DC which
takes the AC phase 100% out of the picture. (This is also why
you can hook up a DC power supply and not worry about the
polarity of the DC.)

The rectified DC is feed into a capacitor which convertes it to
filtered DC. Hence the phase and ripple effect of the AC is
completely removed. All that is left is some 120 Hz ripple
voltage which vary with the load on the boosters. At no load
this is very very low.

The filtered DC then feeds into what is called an H-Bridge which
generates DCC's special form of AC from the DC. This AC is in
the 5000 to 8000 Hz range and in no way is tied to the 60Hz AC.
The frequency is generated and control by the command station
portion of the system. This is exact same signal as found on
the RailSync which goes to all the boosters. RailSync is a
master signal that forces all booster to be in 0 degree or 180
degree in phase of each other depending on how you hook up the
track leads. As you know, you need to test the phase at each
booster power district to make sure it the same as the next booster.

The fact that shorting the gap causes an overload suggest to me
there is current flowing that is NOT related to DCC itself but
flowing through it and is tied to the AC line somehow.


Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics: www.markgurries.com

Piezo tester

Bernie Halloran
 

Guys,
Somewhere I read about building a simple piezo tester to use when laying track to avoid shorts. Where is the diagram? What are the instructions for use, to avoid false buzzes.
Bernie Halloran

Piezo tester

Bernie Halloran
 

Guys,
Somewhere I read about building a simple piezo tester to use when laying track to avoid shorts. Where is the diagram? What are the instructions for use, to avoid false buzzes.
Bernie Halloran

Re: Piezo tester

rg <richg_1998@...>
 

Does your multimeter have an audible alarm for the resistance
scale? Many do have an alarm.
Rich

Re: Piezo tester

rg <richg_1998@...>
 

I just remembered where I saw a simple
circuit. Store the link in Favorites as he has a lot of info. Check
all the links.

http://www.wiringfordcc.com/track.htm#a1

Rich

Re: Piezo tester

Carl
 

Hello Bernie:

I've made continuity testers from simple buzzers and a 9V battery. Two
leads with alligator clips and a sturdy case, ( I used a sports tooth
guard case ). If you are laying track you would connect it to the two
rails. Anytime you hear it buzz stop and undo what ever started it
buzzing. Also nice for tracing wires. One is a radio shack buzzer, the
loud one is a Sona-lert.

When I cut my layout into fully insulated blocks I used it a lot to make
sure I didn't reconnect to the other blocks. When I was starting I set a
locomotive with sound on the track and cut wires until it stopped. Once
I had each block fully insulated I would use the buzzer to connect just
the correct wires back to the source. Once you have a process that works
it isn't too bad, but the gaps in the tunnels were bears!

Good luck, Carl.

Re: DCC Power Supplies - Phase and Polarity.

Carl
 

Hello Mark:

The HO layout is 35 years old with a ton of legacy wiring. I thought I
had all the track cut free from other circuits, but it is hard to tell.
I think I have it now where nothing will buzz from the tracks to any
other metal or circuit.

Thank you, Carl.

Re: Piezo tester

rg <richg_1998@...>
 

In case you missed my first reply.
Below is a link to the circuit mentioned by Carl.
Many times a link or photo solves a
lot.

http://www.wiringfordcc.com/track.htm#a1

Rich

 
Inside every older person is a younger person wondering,
what happened?


________________________________
From: Carl <carl.blum@...>
To: WiringForDCC@...
Sent: Thursday, June 14, 2012 5:50 PM
Subject: Re: [WiringForDCC] Piezo tester

Hello Bernie:

I've made continuity testers from simple buzzers and a 9V battery. Two
leads with alligator clips and a sturdy case, ( I used a sports tooth
guard case ). If you are laying track you would connect it to the two
rails. Anytime you hear it buzz stop and undo what ever started it
buzzing. Also nice for tracing wires. One is a radio shack buzzer, the
loud one is a Sona-lert.

When I cut my layout into fully insulated blocks I used it a lot to make
sure I didn't reconnect to the other blocks. When I was starting I set a
locomotive with sound on the track and cut wires until it stopped. Once
I had each block fully insulated I would use the buzzer to connect just
the correct wires back to the source. Once you have a process that works
it isn't too bad, but the gaps in the tunnels were bears!

Good luck, Carl.

Re: Adding a DCC bus terminator

dvollrath@...
 

For those that need more than gentle techno-babble on the subject check out http://www.ultracad.com/mentor/transmission%20line%20critical%20length.pdf (Note the two choices for better communication in the first paragraph) and http://en.wikipedia.org/wiki/Wave_propagation_speed (so you can calculate this characteristic in your wiring ^_^ )and http://en.wikipedia.org/wiki/Twin-lead to scroll 1/2 way down the page to where it shows formulas for calculating the characteristic impedance. Then you must take the leap from continous RF frequency babble back to where the ratio of propigation delay in the wiring vs the switching voltage rise time determines whether or not the signal will cause an unwanted amount of ringing at the far end.

This subject is also a big issue with variable speed drives for industrial sized AC and DC motors where long lead lengths (>50 ft) between the motor and a PWM controller using fast switching IGBTs to create variable voltage and frequency can cause voltage peaking large enough to cause insulation failure. Guess what... adding an R-C terminator at the motor end of the wiring is one of the easy fixes to use, and a lot less expensive than other alternatives.

DonV

Re: Piezo tester

Blair & Rasa
 

The next time you retire a smoke alarm, remove the buzzer and the 9V
clip before you throw it out. two small alligator clips and a few
inches of wire make a perfectly sufficient buzzer; for most small
layouts, it directly takes the place of the DCC booster whenever wiring
or debugging. For larger layouts, it needs to connect at the output
from district management blocks (like the PM42).

Just stay out of the ionization source module.
Blair

Re: Piezo tester

Bernie Halloran
 

Guys,
Thanks. I could not remember where I read about the piezo and I absolutely forgot about disconnecting the booster. Of course! Bonehead. I was thinking just not turning it on was enough. No. Not enough. It means simply pulling the plug leading from the booster to the track. Ah.
Bernie Halloran+

Re: Piezo tester

rg <richg_1998@...>
 

Again, look st the below link. Sigh.

http://www.wiringfordcc.com/track.htm#a1

Rich

 
Inside every older person is a younger person wondering,
what happened?


________________________________
From: Bernie Halloran <bfhalloran@...>
To: WiringForDCC@...
Sent: Thursday, June 14, 2012 4:21 PM
Subject: [WiringForDCC] Piezo tester

Guys,
Somewhere I read about building a simple piezo tester to use when laying track to avoid shorts. Where is the diagram? What are the instructions for use, to avoid false buzzes.
Bernie Halloran

[Non-text portions of this message have been removed]

wiring a reverse loop off a wye

coopernga
 

All,

I have uploaded two PDFs of my track electrical plan in the folder "Cooper" in the file section. I have a 4 track staging yard on a lower level under a peninsula that comes off of a wye. I have a PSX-AR from Tony's but I cannot figure out how to properly wire it. The legs of the wye coming down to the staging yard and and staging yard are all together on a separate DB150 booster. The mainline is divided up into 2 booster sections with a DSC200 master and DB150 booster.

All the particulars of the wiring and DCC system are on the diagrams.

My two main heartaches are:

Where is the proper place for the reversing loop?
How do I wire the feeders to that section and the yard ladders?

Thanks for your help.
Cooper
Cary, NC
coopernga@...

Re: wiring a reverse loop off a wye

Carl
 

Hello Cooper:

I would make the yard throat the reversing section. Put gaps on both
sides of the Wye close to the main line connections. Then put gaps in
all the yard sidings at least a train length from the mainline gaps.
Also where the yard siding loop back should be a full train length too.
The only thing to avoid is letting two trains cross the gaps at once.

Good luck, Carl.