Power bus terminators

Michael Beckemeier

Hi All,

I recently scoped the DCC signal at the far end of my booster districts and saw considerable ringing and excessive overshoot on the rising edges of our DCC signal.
After reading previous post on this subject and checking out the data on the below links, I decided to see what effect they had.
To see a learn more about blown decoders and long bus runs and a RC filter solution....
http://www.WiringForDCC.com/track_2.htm#c2
and
http://www.WiringForDCC.com/dcc_waveforms.htm
Well I must say I was stunned by the remarkable difference they made in cleaning up the overshoot and ringing.
I used a 330 ohm 1/2 Watt resistor in series with a 0.1uf 50V capacitor.
With this in place my Tony's DCC RR Volt ammeter no longer gave odd readings at the power district either, as it was now seeing a cleaner DCC wave form.

However......
I have found that the 130 ohm resistor is quite HOT.
Not warm, Hot!

In looking closer at the un-terminated DCC wave on the scope, I see that the nominal voltage is 14.5 peak, but the overshoot voltage is 35-40 volts peak for a duration of about 200 nanoseconds.
Lets see....
80v p-p / 130 ohm = 0.615 Amps
0.615 amps x 80v = 49 Watts !
However this should only be for the 200 nanosecond duration of the spike that the resistor would ever see this much power, so I never suspected that the temperature would rise so quickly for a small duty cycle.
I was surprised that the resistor got so hot.

I did an experiment to see what wattage resistor would not be hot to the touch.
Not having a 1 or 2 watt resistor handy, I put 4 of the 130 ohm 0.5 watt resistors in a series / parallel arrangement, which still measured 130 ohms.

With this I could feel some minor warmth from the individual four resistors, but nowhere near the heat off the 1/2 Watt resistor when it was the only one in series with the cap.

I suspect that I will have to search for a 2 watt resistor.

Has anyone else who has used this terminator noticed how much heat this circuit gives off?

Mike Beckemeier

Re: Power bus terminators

Vollrath, Don <dvollrath@...>

Mike,
sounds like you have a shorted cap! [ the R & C ARE in series...Right? :-) ]
e^2/R = 1.5 watts for 130 ohm resistor directly on a 14V DCC bus
For cap & series resistor: resistor Pw = 1/2 X C X V^2 X ~8kHz X 2 or 0.156 watts for 0.1 uFD
DonV

-----Original Message-----
From: WiringForDCC@...
[mailto:WiringForDCC@...]On Behalf Of Michael Beckemeier
Sent: Sunday, August 21, 2005 5:03 PM
To: NCE-DCC@...; WiringForDCC@...
Subject: [WiringForDCC] Power bus terminators

Hi All,

I recently scoped the DCC signal at the far end of my booster districts
and saw considerable ringing and excessive overshoot on the rising edges
of our DCC signal.
After reading previous post on this subject and checking out the data on
filter solution....
http://www.WiringForDCC.com/track_2.htm#c2
and
http://www.WiringForDCC.com/dcc_waveforms.htm
Well I must say I was stunned by the remarkable difference they made in
cleaning up the overshoot and ringing.
I used a 330 ohm 1/2 Watt resistor in series with a 0.1uf 50V capacitor.
With this in place my Tony's DCC RR Volt ammeter no longer gave odd
readings at the power district either, as it was now seeing a cleaner
DCC wave form.

However......
I have found that the 130 ohm resistor is quite HOT.
Not warm, Hot!

In looking closer at the un-terminated DCC wave on the scope, I see that
the nominal voltage is 14.5 peak, but the overshoot voltage is 35-40
volts peak for a duration of about 200 nanoseconds.
Lets see....
80v p-p / 130 ohm = 0.615 Amps
0.615 amps x 80v = 49 Watts !
However this should only be for the 200 nanosecond duration of the spike
that the resistor would ever see this much power, so I never suspected
that the temperature would rise so quickly for a small duty cycle.
I was surprised that the resistor got so hot.

I did an experiment to see what wattage resistor would not be hot to the
touch.
Not having a 1 or 2 watt resistor handy, I put 4 of the 130 ohm 0.5 watt
resistors in a series / parallel arrangement, which still measured 130 ohms.

With this I could feel some minor warmth from the individual four
resistors, but nowhere near the heat off the 1/2 Watt resistor when it
was the only one in series with the cap.

I suspect that I will have to search for a 2 watt resistor.

Has anyone else who has used this terminator noticed how much heat this
circuit gives off?

Mike Beckemeier

http://www.WiringForDCC.com

Re: Power bus terminators

Vollrath, Don <dvollrath@...>

Sorry folks I goofed
Resistor dissipation of series R-C across DCC track bus is Pw = 1/2 X C X V^2 X ~8kHz X 4 (not 2)
So for a 0.1 uFD cap and 14.25V DCC, the resistor heat will be ~0.32 watts. Select a resistor at least 2X that value for long life. So use a 1 watt resistor with a 0.1 uFD cap.
Note that if you use a different value cap be sure to re-calculate the resistor watts!

DonV

Re: Power bus terminators

Michael Beckemeier

Hi Don,
But I have 40 volts peak for 200ns.
It sounds like the power would be the .156 watts at nominal voltage, PLUS additional power when it has to pass the 40v peak.
I believe this is why the 0.5 watt resistor gets so hot.

No the cap is not shorted, (at least not all 40 of them).

Mike B

Message: 1 Date: Mon, 22 Aug 2005 10:15:20 -0500
From: "Vollrath, Don" <dvollrath@...>
Subject: RE: Power bus terminators

Mike,
sounds like you have a shorted cap! [ the R & C ARE in series...Right? :-) ]
e^2/R = 1.5 watts for 130 ohm resistor directly on a 14V DCC bus
For cap & series resistor: resistor Pw = 1/2 X C X V^2 X ~8kHz X 2 or 0.156 watts for 0.1 uFD
DonV

-----Original Message-----
From: WiringForDCC@...
[mailto:WiringForDCC@...]On Behalf Of Michael Beckemeier
Sent: Sunday, August 21, 2005 5:03 PM
To: NCE-DCC@...; WiringForDCC@...
Subject: [WiringForDCC] Power bus terminators

Hi All,

However......
I have found that the 130 ohm resistor is quite HOT.
Not warm, Hot!

In looking closer at the un-terminated DCC wave on the scope, I see that the nominal voltage is 14.5 peak, but the overshoot voltage is 35-40 volts peak for a duration of about 200 nanoseconds.
Lets see....
80v p-p / 130 ohm = 0.615 Amps
0.615 amps x 80v = 49 Watts !
However this should only be for the 200 nanosecond duration of the spike that the resistor would ever see this much power, so I never suspected that the temperature would rise so quickly for a small duty cycle.
I was surprised that the resistor got so hot.

I did an experiment to see what wattage resistor would not be hot to the touch.
Not having a 1 or 2 watt resistor handy, I put 4 of the 130 ohm 0.5 watt resistors in a series / parallel arrangement, which still measured 130 ohms.

With this I could feel some minor warmth from the individual four resistors, but nowhere near the heat off the 1/2 Watt resistor when it was the only one in series with the cap.

I suspect that I will have to search for a 2 watt resistor.

Has anyone else who has used this terminator noticed how much heat this circuit gives off?

Mike Beckemeier

Re: Power bus terminators

Vollrath, Don <dvollrath@...>

Mike, You can't fool Mother Nature. 40v for 200nSec across a 100 ohm resistor 8,000 times/sec is only 3.2 micro-watts. You have something wrong with your set-up. Wrong value of C or shorted C or a polarity sensitive C or the resistor is somehow wired directly in parallel with the track. With your R-C networks installed you should NOT see any more voltage ringing at the track. The ringing IS gone...Right?

However, there IS a difference between temperature (as in Hot) and average heating power (as in watts). [A burning match may be hotter than the electric stove element, but does not have the heat energy to boil a pot of water.] The fine print on many resistor data sheets shows that a lowly 1/2 watt resistor may have a temmperature rise of 70C...thats 90C and too hot to hold on to with bare hands when the room temp is 20C. "Rated" operating temp may indeed be 100C. If you want it to run cooler, you need to increase the case size to add more surface radiating area. i.e. - a bigger watt rated resistor. But yeah, it should not get hot quickly, or begin to smoke. That's a good sign that there is something wrong.

re - The R-C network
The voltage spikes you see at the track without the R-C are created by the DCC booster attempting to rapidly change the polarity of voltage stored on rail capacitance when fed by wires with a significant amount of inductance. Using your example, the remote rail to rail capacitance and layout wiring inductance of the DCC bus form a resonant L-C with a 1/2 period of ~200 nano-seconds (0.2 uSec). Current must flow in to (or out of) the rail capacitance in order to flip rail voltage polarity. The faster you attempt to flip it (power FET switching speed inside the booster) the more peak current must flow to the rail capacitance. This causes higher and higher current and subsequent voltage ringing in the wiring-Rail L-C. You can calculate the energy content of the actual ringing which represents energy flowing back and forth between the L of the wiring and C of the track...only if you know the actual circuit values...But it is rather small. However, there is little or no energy absorbing damping to keep the L-C 'tank' circuit from voltage 'peaking' and ringing. Adding an external resistor to the L-C circuit will provide damping to the point where it limits the voltage peaking effects and doesn't ring any more. But we don't want to add series resistance as that would reduce power available to our locos on the track. A parallel resistor can be used, but it will also see the rms value of all the DCC track voltage as well and heat up accordingly (14.25 volts continuous, squared, divided by R ohms). A capacitor of the right value placed in series with a damping Resistor in parallel with the track will the reduce resistor dissipation if sized so that the cap will charge up to the full track voltage on each voltage transition but much more slowly than the ringing we are trying to suppress. [100 ohms X 0.1 uFD = 10 uSec, maximum DCC transition time is ~2uSec (I'm sure someone will correct me), NCE/Sys-1 boosters switch in ~0.6 uSec. Digitraxx boosters switch much more slowly and don't generate the ringing problem.]

In brief, current flows through the resistor only during the DCC voltage polarity transitions, about 4 X 8,000 times per second. And the 40 volt spikes aren't there when you connect the R-C, becauses the ringing energy of the track L-C (a much smaller value of C) is being damped and absorbed by the resistor. But whatever energy flows in and out of the external added capacitor must also flow through the series resistor, causing it to heat up. The extreme and easy to understand case is when you fully discharge a capacitor into a resistor. The entire energy content is 1/2 X C X V^2. If you discharge it only once, the energy content is so many joules or watt-seconds, all absorbed by the resistor (regardless of the ohms value of the resistor). If you do it repeatedly, multiply the watt-seconds of each event by the number of events per second to get the average watts being dissipated as heat in the resistor. But a resistor will heat up with current flow in either direction...so charging the capacitor up to a certain voltage will cause a series resistor to heat up with the same energy content as discharging it. When you apply the DCC square wave of voltage to the series R-C network and assume that the capacitor becomes fully charged up to the the appropriate DCC level (the peak voltage is gone on your scope traces, right? That's why we put in the R-C) the cap charges and discharges 2 times each per DCC cycle [0 to + to 0 to - to 0] So the resistor sees 1/2 X C X V^2 X 4 X the number of times per second (~8,000) watts. Notice that you don't want to make the external capacitor too large, as it does determine resistor watts.

An R-C network used in this manner is referred to as a "snubber".
You need to locate it out where the ringing to be suppressed is occurring...At the end of the DCC bus wires. [i.e.- Where the inductance of the wiring meets the capacitance of the track.] It does no good if placed at the booster terminals.

DonV

-----Original Message-----
From: WiringForDCC@...
[mailto:WiringForDCC@...]On Behalf Of Michael Beckemeier
Sent: Monday, August 22, 2005 10:40 PM
To: WiringForDCC@...
Subject: [WiringForDCC] RE: Power bus terminators

Hi Don,
But I have 40 volts peak for 200ns.
It sounds like the power would be the .156 watts at nominal voltage,
PLUS additional power when it has to pass the 40v peak.
I believe this is why the 0.5 watt resistor gets so hot.

No the cap is not shorted, (at least not all 40 of them).

Mike B

modifying Walthers / Shin DCC double crossover?

I'm busy enhancing some DCC-friendly HO turnouts, following Allan Gartner's excellent
website. Has anyone had experience performing similar solder connections on the double
crossover? Some locos are hesitating on it but the darn thing looks pretty complicated, so
I'm hesitating too.

Re: modifying Walthers / Shin DCC double crossover?

The double cross is not as bad as it looks. If you study the color
diagrams, you will see that it is simply four turnouts connected
together. It is nothing more complicated than that. If you treat it
as four individual turnouts, all will go well.

Allan

Re: modifying Walthers / Shin DCC double crossover?

Dale Gloer

I have one and I found that the most important thing operationally is
to throw all 4 switches at the same time. That is all 4 crossed or
all 4 straight. This prevents any opportunities for funny thngs to
happen if a derailment should occur. There is no reason not to do
this either, you can only occupy one route at a time when crossing
over so crossing both ways makes no difference and you can't have one
route straight and one crossed.

Dale.

--- In WiringForDCC@..., "wirefordcc"
The double cross is not as bad as it looks. If you study the color
diagrams, you will see that it is simply four turnouts connected
together. It is nothing more complicated than that. If you treat it
as four individual turnouts, all will go well.

Allan

Re: DIGITRAX TRANSPONDING/DETECTION

mmogen2004

--- In WiringForDCC@..., "bobgrosh" <bobgrosh@h...>
wrote:
I knew I had worked on a page to explain transpondiing, and it had
to
be somewhere. Well I finally found it. Still needs to be cleaned
up,
but here it is.

http://1stclass.mylargescale.com/ally/04technology/0402Transponding/H
ard
ware/default.htm

Maybe there is something on these two drawing that will help.

B0B
Bob,

OOPS. After including the rest of the link, I got the pages to show
up and thanks, I'll look them over when I can devote more time to it.

Thanks,
Mike

Wiring Ditch lights with Digitrax in Kato SD80MAC

rswinnerton <rswinnerton@...>

Hi, I'm new to this, but I need to install ditch lights in Kato's
SD80MAC. I'd like to use LEDs, but will use GOW bulbs if need be. How
do I do that and how do I configure the decoder? I'm using the 163
decoder.

Re: modifying Walthers / Shin DCC double crossover?

Giles,

You definitely don't need to solder any wires to the guard rails
around the frog.

I'm not sure how Walthers makes their frogs. I get good electrical
continuity when I measure a new turnout. If you want to be safe, you
can solder a wire in a "U" shape and attach it to both frog rails -
the rails leaving the frog. That's what I have to do on my
Pilz/Tilleg turnouts.

Allan

Re: Wiring Ditch lights with Digitrax in Kato SD80MAC

I suggest you use grain of rice bulbs instead of LEDs. The reason
is because GOR bulbs have insulated wire leads and LEDs don't. GOR
bulbs are simly easier to wire in confined spaces.

How to use GOR bulbs is covered in my web page at:

http://www.wiringfordcc.com/gorhlite.htm#a4

Digitrax has some installation notes for a decoder into your
locomotive at:

Digitrax has wiring of ditch lights at:

http://www.digitrax.com/ftp/fxsetup.pdf

Note that they show using a 12V GOW bulb. You can definitely do
that. I'm just thinking you will want the smaller GOR bulb. Just
add the resistor specified on my web page or as they specify on
their web page. Also, the Digitrax web page tells you how to set up
the CVs.

Allan

Simple Minds

karenboots@sbcglobal.net <karenboots@...>

Will be purchasing radio DCC. NEC or Digitrax Super Cheif 8amp. Of
the two, which would be the simplist for overall set-up and
understanding system? Take this into consideration. My skills are
limited to a drop-in, PLUG AND PLAY brain. Thanks for any suggestions.

Re: Simple Minds

Vollrath, Don <dvollrath@...>

NCE is by far the easiest to use. Very intuitive operation. Push the labeled buttons & follow directions on the display screen when you need to.
Radio in Digitraxx works better but cab/throttle controls are much more difficult and somewhat confusing to operate than NCE.
If it is a small-medium home layout a single NCE 5 amp unit w/ radio unit will work well. (1-4 trains running at the same time.) If it is a club or basement empire also get the NCE radio repeater, and consider the need for additioal booster.

-----Original Message-----
From: WiringForDCC@...
[mailto:WiringForDCC@...]On Behalf Of
karenboots@...
Sent: Sunday, September 11, 2005 7:15 AM
To: WiringForDCC@...
Subject: [WiringForDCC] Simple Minds

Will be purchasing radio DCC. NEC or Digitrax Super Cheif 8amp. Of
the two, which would be the simplist for overall set-up and
understanding system? Take this into consideration. My skills are
limited to a drop-in, PLUG AND PLAY brain. Thanks for any suggestions.

SP Bachmann 4-8-4 w/smoke

zekda99 <zekda99@...>

I think i have figured out how to convert this one to a dcc controlled
engine....only thing I haven't figured out is how to control smoke,
however it is probably better without having control...Do have a
question though, I have a kenwood dual trace occiloscope but u
mentioned something about an isolation transformer...is there a cheap
one out there in Canada....by the way I also figured out how to revert
some lifelike engines over to dcc.

mt

Double Slip

allenzeesman <allen.zeesman@...>

I have a digitrax system and a shinohara double slip turnout without
insulated frogs. I cannot figure out how to wire it. Suggestions and
references are most welcome. Thanks.

Allen

Re: Double Slip

Allen,

Bob Clegg has provided some information regarding the Shinohara double
slip on my website at: http://www.wiringfordcc.com/switches.htm#a12

Allan

--- In WiringForDCC@..., "allenzeesman"
<allen.zeesman@r...> wrote:
I have a digitrax system and a shinohara double slip turnout without
insulated frogs. I cannot figure out how to wire it. Suggestions and
references are most welcome. Thanks.

Allen

bus runs

zach_sugar <zach_sugar@...>

You have indicated that the bus runs should be twisted. I have several
questions.
1. Should all three wires (when creating sub blocks) be twisted
togeather? I assume so.
2. Can the twists be in different directions clockwise in one
section, counterclockwise in the next section?
3. I have a seperate booster for my stationary decoders, should the
run for this be twisted also. Can this be next to the bus for the track.
4. Will other buses such as AC power, DC power for other electronics
affect the main bus.

Re: bus runs

Vollrath, Don <dvollrath@...>

1. DCC bus wiring should resemble a tall tree...a trunk with a few short branches. but each trunk and branch should be two wires. Yes, all sub-block (branch) wiring should also be twisted.
2. twisting direction makes no difference. No need to go overboard. You only need a couple of turns per foot to rotate the wires and hold them fairly tightly together.
3. Yes, twist the DCC accessory bus also. Track and accessory bus can be routed together.
4. DCC can interfere with other AC or DC power and signal wiring. Keep those separated from the DCC buses. Also keep the cab bus wiring away from the DCC power bus as best you can.
DonV

-----Original Message-----
From: WiringForDCC@...
[mailto:WiringForDCC@...]On Behalf Of zach_sugar
Sent: Monday, September 19, 2005 9:06 AM
To: WiringForDCC@...
Subject: [WiringForDCC] bus runs

You have indicated that the bus runs should be twisted. I have several
questions.
1. Should all three wires (when creating sub blocks) be twisted
togeather? I assume so.
2. Can the twists be in different directions clockwise in one
section, counterclockwise in the next section?
3. I have a seperate booster for my stationary decoders, should the
run for this be twisted also. Can this be next to the bus for the track.
4. Will other buses such as AC power, DC power for other electronics
affect the main bus.

http://www.WiringForDCC.com

Re: bus runs

blklng202 <blklng202@...>

...side question, what guage wire do you use for buss?
Thanks, Jack