Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
daveb wrote:If the DCC signal is not AC then why ... Dave
The mind is like a parachute. It works better when it's open. www.stremy.net
Both are valid points and folks are free to modify this design as needed. My goal was to make it as simple as possible and yes, I may have left out some protections. A snubber diode or capacitor would be easy to add. The relay I am using is pretty small and I doubt the inductance of the coil is more than 1-2 mH but it isn't a bad idea. I considered the optoisolator option for controlling the variable voltage but Radio Shack no longer carries anything which would work easily and I wanted to use parts from there, since they are everywhere. Of course in quantities of 1-2, their prices are outrageous. Another thing I've noticed is that the value of C3 can be dropped quite a bit. The goal was to provide a smooth voltage to the Darlington so that the lights didn't flicker as track voltage changed under load. When I was powering lights directly from the decoder motor outputs, I could see flickering of the lights. At 220uf there is some delay between when you turn the speed down and when the lights dim. More of a delayed effect than a major issue. I also added 3 diodes in series of the common lead on Q3 to get the output voltage up to 14V instead of 12V. The Minatroics yard lights I am powering use 16V bulbs that look better at 14V instead of 12V.
Thanks for your feedback. I hope somebody finds this useful.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
jbinkley60 wrote:I completed my design for a decoder adapter that can be used to turn on/off layout lighting (or anything else which requires 12vDC), that also has a constant 12vDC output (for control panels, switch machines etc..) and has a 0-14v DC variable output to adjust layout lighting brightness. All are controlled via DCC and utilize a Digitrax mobile decoder to drive the adapter. Now power is used from the rails, expect to power the decoder itself. There are two versions. Here is the schematic for one version: If you want to read more about it and see the unit I assembled go to: http://www.thebinks.com/trains/decoder%20adapter.html
I completed my design for a decoder adapter that can be used to turn on/off layout lighting (or anything else which requires 12vDC), that also has a constant 12vDC output (for control panels, switch machines etc..) and has a 0-14v DC variable output to adjust layout lighting brightness. All are controlled via DCC and utilize a Digitrax mobile decoder to drive the adapter. Now power is used from the rails, expect to power the decoder itself. There are two versions. Here is the schematic for one version:
If you want to read more about it and see the unit I assembled go to:
http://www.thebinks.com/trains/decoder%20adapter.html
DALCruiser wrote:You might find the following link useful for constructing a simple circuit for using a DC meter to measure the DCC track voltage. http://jdb.psu.edu/nmra/dcc-voltmeter.html I have not tried it but it looks like it should do the job.Dave
You might find the following link useful for constructing a simple circuit for using a DC meter to measure the DCC track voltage.
http://jdb.psu.edu/nmra/dcc-voltmeter.html
I have not tried it but it looks like it should do the job.Dave
Joe Fugate Modeling the 1980s SP Siskiyou Line in southern Oregon
I completed my design for a decoder adapter that can be used to turn on/off layout lighting (or anything else which requires 12vDC), that also has a constant 12vDC output (for control panels, switch machines etc..) and has a 0-14v DC variable output to adjust layout lighting brightness. All are controlled via DCC and utilize a Digitrax mobile decoder to drive the adapter. No power is used from the rails, expect to power the decoder itself. There are two versions. Here is the schematic for one version:
Engineer Jeff,
You said it well. I was trying to be brief so as to not confuse some, but guess I should have gone into more detail. What you have said concerning this is 100% correct. One thing you didn't address is the streched bits some DCC systems use to operate non DCC equipped locomotives with DCC equipped engines. From what I understand you can even MU them with the Digitrax(and probably others,) which I have never tried. The streched bit should further confuse some who are trying to combine analog definitions and technology with digital. The analog part ends with the power input to the DCC command station. As long as we have fun with our model railroad that is all that matters.
Paul
NYC, Dayton to Springfield fan
It looks like you are measuring one of the funtion output voltages. Isn't there some voltage consumed by the decoder before getting to the F(0) output? I think it depends on the mfgr. of the decoder, but I seem to remember that the funtion output voltage can be 1.5V to 2.5V less than what is actually on the rails. To measure DCC track voltage, couldn't you measure one rail to ground and double it?
Scott Groff
Adirondack & Saratoga Rwy.
Sumner, WA
daveb wrote:If the DCC signal is not AC then why are bridge rectifiers needed in order to measure current with a DC ammeter and as voltage dropping devices ? Dave
Because DC ammeters are based upon straight DC and the easy way to take a pulse signal and convert the partail duty cycles of the waveform into pure DC is via a bridge rectifier. The problem is that you don't end up getting a true RMS value of the current because the duty cycle of the pulses is not the same as the standard .707 RMS value for traditional AC sinewave, when going through a bridge. This is why standard multimeters only give an approximate value of the steady state current. The PDF above shows a better way to convert the DCC pulses into their steady state DC components and then they mention using a small resisitor for measuring the current component. This is a traditional method for measuring both voltage and current in a pulse technology. The thing to keep in mind that the current draw and actual voltage on the rails is an instantaneous value based upon exact spot on the pulse cycle you choose to look at. During a pulse it should remain steady state through the end of the rise time and to the beginning of the falling time. The current component will follow the same cycle. In AC the current and voltage are often out of phase with each other due to the types of loads (inductive or capactive in nature) the AC voltage must deal with. Voltage leads current with inductive loads but current leads voltage with capacitive loads. In pulse technology capacitive loads look like a rounding of the corners on the pulse. Inductive loads may look like an overshoot on the rising edge. With what we ae dealing with, I doubt we will see much of this.
I have not tried it but it looks like it should do the job.
Dave
grayfox1119 wrote:Thank you Jeff!!! I hope now we can put this baby to bed.....many of us have been saying this, but you have shown it graphically to clarify the confusion. Maybe you can use a differential amp and a ladder network to switch on/off bits to help your circuit design.
I am actually looking at something more simple than that. Basically start with a simple power supply. Transformer, fuse, bridge rect and cap. Then add a 12V regulator for the fixed voltage output for control panels, switch machines etc.. Then add a simple 12V minature relay off of the decoder function leads for an on/off function control. For the variable voltage portion I plan to simply use the motor leads with a cap, regular diode, resistors, fuse and a TIP120 darlington transistor to drive a variable voltage off of the decoder. The decoder will simply be a Digitrax DH123D or similar. It can be expanded in a number of ways. Add another relay and function control for multiple on/off zones. Add a bigger transformer and rectifier for more current.
Right now I am verifying the DC isolation capabilities of the decoder since with the variable supply I will mixing two different source supplies together. I blew up one decoder today already. They don't like either motor lead taken to a true earth ground potential. Fries the decoder almost instantly. It looks like a floating ground will be fine. I've got a little more testing to do.
Hookup should be simple. 115VAC for the power supply and two leads to the track. Outputs will be 12VDC fixed, 12VDC on/off decoder controlled and 0-14VDC variable decoder controlled.
There is some mangling of definitions going on here. DCC is a digital communications protocol made up of pulses that are coded in a format that decoders can read and write to/from. This is true digital technology. Thus digital logic circuits, processors and similar technology can be utilized to manipulate the pulses. If it were AC (i.e. analog) then there would be no definite 1 or 0 bit definitions because true AC has no point along the sinewave where two consecuvite time intervals on the waveform have the exact same amplitude. Thus it is continually changing. You may find a repeating set of events in subsequent wave intervales but not within the same 360 degree rotation of the sine wave. That would be a violation of the sinewave function.
What everyone is talking about is how does DCC get sent down the "wire" onto the tracks, be able to go over some distance without the waveform becoming "non-square" over distance and how does it also carry a current component to power the locomotives and such. The DCC spec calls it a baseline encoding method. What this is, is a differential voltage signal between the rails. So think of rail A as the positive rail and rail B as the negative rail. The voltage difference between the two rail equals the total amplitude of the DCC pulse. Here's an oscilloscope snapshot of rail A.
You can clearly see it is a 0 to 15V positive pulses. Rail B would be the other half of the wave form. In this case I used ground as the reference point for the measurement. Decoders look at the entire waveform differential between the rails but what they reallu care about is whether the pulse is a 1 or a 0. Now comes the question of bit polarity. If you look at the PDF posted above, you will notice that the voltage referemces between the rails doesn't change whether the bit is a 1 or a 0. In other words whether the bit is a 1 or a 0 the A rail will still have a positive polarity pulse with regards to ground and the B rail will have the opposite. This doesn't change. If it did then the power that decoders pull from the pulses would constantly be changing polarity and the decoders would need full wave bridge rectifiers in them to compensate for the changing voltage references, also the concept of forward and reverse would have no meaning. So if you look again at the PDF on page 1 it describes the bit encoding method. It also talks about keeping the DC component at zero. What they are saying is that the the two halves of the waveform (the Rail A half and the Rail B half) both be equal and opposite in duration and polarity. In other words if a 1 bit was encoded simply as a long positive voltage pulse on Rail A then the longer the pulse the more is starts to look like a pure DC voltage for the duration of the pulse cycle. This is bad for transmitting current down a transmission medium and will also cause more voltage drop because this DC component and the resistance of the rails. This technology and encoding method is very similar to what the telcos use to send T-1, DSL and similar signals long distances down twisted pair wires but also provide power for repeaters and similar devices where local power is not available.
I hope this helps shed some light on the topic. It is timely because I had the scope out today as I am working on designing a decoder add-on that can utilize an auxiliary power supply but use DCC to turn off/on lights with a decoder function command, vary the brightness of lights using the decoder speed function and provide straight 12VDC for powering control panels, switch machines etc. I needed to do some testing. I'll post the circuit and parts list when I am done. It looks like for around $50 (including the decoder) I can provide up to 3A of output for these functions all under DCC conrol and without using power from the rails (except a few milliamps to power the decoder).
Wow, what a discussion!!! ELI the ICE man is good especially when making adult beverages. Just thought I would lighten up the conversation. My answer remains the same as my earlier post, call the DCC signal what you will. I would call it a Bi-polar, pulse modulated signal,but what I would call it has no bearing on anything. What the manufactures call it is usually their lingo and may mean nothing as well. It does go positive and negative, usually by the same amount but not when you have stretched pulses. The reading of the inexpensive voltmeter will vary depending on what the signal is doing (pulsewidth.) I believe when adjusting the voltage you are adjusting the peek voltage. I do use an inexpensive Radio Shack $9.99 voltmeter to measure the track voltage. I know it is not accurate. I have True RMS voltmeters and an O Scope, and lots of other expensive working tools, but know what? I don't usually use them for measuring DCC signals. I use the Cheepo RS meter. I use it to take comparitive readings, and to see if a signal is present. I do not use it for a reference. If all your decoders are working and none are overheating your voltages are fine.
Now here is what I will really get flamed for. When you reduce the track voltage output of many DCC systems you are loosing that voltage by electronically converting it into heat in the DCC command station (or whatever you want to call it.) That is not necessarily a good thing, but it is another discussion and does not answer the original question. Someone could start a new thread on what to call the DCC signal and/or where does the voltage go when adjusting your command stations output.
Dayton and Mad River RR.
Paul3:
IF you read further in that NMRA bulletin : "To keep the DC component of the total signal at zero as with the "1" bits, the first and last part of the "0" bit are normally equal to one another."
They don't mention AC anywhere in that bulletin.
And it's not internet rumor, if all the manufacturers say it's half-wave DC (you'd think they would know what they're building) and you're the only one that disagrees, who are we to believe.
Jay
C-415 Build: https://imageshack.com/a/tShC/1
Other builds: https://imageshack.com/my/albums
Well, just when we thought we were figuring this out....
http://www.yodermfg.com/Tube_Mills/Main_TM_SqWave.htm
Hey, selector! "SMACK!" There, feel better? LOL
While DC is "polarity sensitive", so is AC. While DC current can vary from 0v to +12v (or whatever) or from 0v to -12v, it doesn't flip back and forth between -12v and +12 many times per second...like AC does.
According to the NMRA, DCC is an AC signal.
According to internet rumor, DCC is half wave DC.
Now, who are you going to believe?
Paul A. Cutler III*************Weather Or No Go New Haven*************
Paul3 wrote: grayfox1119,Have you ever looked at a DCC wave pattern? How do you know that it doesn't go below 0 volts? In fact, it really does, but for whatever reason, you won't take my word for it. But would you take the NMRA's word? Click on the following link (you need a .pdf reader):http://www.nmra.org/standards/DCC/standards_rps/S-91-2004-07.pdf Note that this is NMRA Standard S-9.1, approved in 1994, 2002, and 2004, latest rev. adopted in July 2004. Just in case you don't have Adobe, here's the important part: A: Technique For Encoding Bits The NMRA baseline digital command control signal consists of a stream of transitions between two equalvoltage levels that have opposite polarity. Now, doesn't that sound just like AC? Paul A. Cutler III*************Weather Or No Go New Haven*************
grayfox1119,Have you ever looked at a DCC wave pattern? How do you know that it doesn't go below 0 volts? In fact, it really does, but for whatever reason, you won't take my word for it.
But would you take the NMRA's word?
Click on the following link (you need a .pdf reader):http://www.nmra.org/standards/DCC/standards_rps/S-91-2004-07.pdf
Note that this is NMRA Standard S-9.1, approved in 1994, 2002, and 2004, latest rev. adopted in July 2004. Just in case you don't have Adobe, here's the important part:
A: Technique For Encoding Bits
The NMRA baseline digital command control signal consists of a stream of transitions between two equalvoltage levels that have opposite polarity.
Now, doesn't that sound just like AC?
I'm going to get smacked for this, but...Paul, it sounds to me like DC!! DC is polarity sensitive.