Idiots guide to DCC.......

Fitz Orchard

Fitz Orchard

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I have a muppet question I cannot find the answer to in plain English despite research and thus I turn to the forum experts here in the hope of an answer or two.
Background - I converted my little old LGB set up to DCC about 15 years ago and then due to other commitments ( a growing family) it was all soon packed away for over a decade, during which my memory and faculties aged. Now in my dotage and after downsizing and getting the family off the payroll I returned to the trains and remembered just enough of the instruction session I had from GRS back then to operate my Massoth set up at a basic level but am struggling to understand the basics of how it actually works and my lack of knowledge leads me to seek advice.
I am an electrical and electronics idiot. So here goes -
Simplest solution would be if there is literally an idiots guide to DCC written in plain English for non scientific/engineering brains. I haven't found one yet but perhaps someone out there knows of such a thing.
My quandary and confusion can be summed up in one question as follows:
With old fashioned DC power the train gets current from the track from a supply regulated by a controller. As the current increases the train moves with the flow of current determining speed. Wind the control knob down and the train slows down and stops after which there is no current in the track.
As I understand it with DCC the current flows at full voltage constantly with the central digital controller 'sending' digital instruction signals (also through the track). These are received by the decoder in the loco which controls the speed. Other such instructions can be used to control lights/sound etc. The responses to the instructions are determined by CVs (control variables) which are unique to each loco identity and which have been pre-programmed into the device decoder. Without the decoder responding no current reaches the motor. That is about as far as my understanding goes.
If that is all true then the track is constantly fully powered under DCC. Why then is it that my only loco which has no decoder and still operates under DC does not race at full speed if I put it on the DCC track? I would have thought that it would but such is not the case - it does nothing.
If I can understand a little better I may then be able to approach the underlying reason for my enquiry - I want to find out if it is possible to run a DC loco on DCC track or do I need to have the DCC switched off and turn on my old DC transformer and speed controller?

Again my apologies for what to most of you may seem an incredibly naive question but G Scale Central has such great content and advice on so many areas I thought I just had to throw it out there.

Thanks
 
Mostly correct yes.

But when using an analogue loco on DCC, (address 0) the control signal will be pulses of dc at the maximum voltage (not current which is different to voltage).

These pulses will be wider for increase of voltage and narrower for decreased voltage, keeping it as simple as I can for a non technical person....
 
Fitz Orchard said:
Why then is it that my only loco which has no decoder and still operates under DC does not race at full speed if I put it on the DCC track?
Click to expand...

If you don't want a scientific/engineering answer, just accept it as magic and move on.

As far as operating a DC loco on a DCC equipped track, for many DCC systems selecting loco address 0 will allow a DC loco to be controlled by the DCC handset..... but it will most likely be quite "noisy" (higher pitched "humming" that varies with speed) due to the non-scientific/engineering magic going on.

"DCC on 0" operation works with LGB MTS, Massoth and Piko DCC systems but not all DCC system support that method of operation.
 
Brilliant. Thankyou. How simple. I just put it on the track assuming it go off like a shot. Carriage lighting works so I thought…..
Emboldened with new knowledge I will now embark on another quest - to understand the difference between current and voltage! I vaguely recall a plumbing analogy - current is the volume of water in the pipe and voltage the pressure that pushes it through. Probably not right but this was 55 years ago in 3rd year physics.
Really appreciate your thoughtful input - may the magic of science live on!
 
It looks to me like you have already started your own guide. As you ask questions answers you get can add to it. For my part I hope to explain what occured with the DCC voltage inside the chip.

Track voltage is rectified (converted) from AC to DC by the chip and there is a controller in the chip that responds to your DCC controllers commands making the loco accelerate, slow and if commanded stop.

That is a crude description but basically is what occurs.
 
I think whats missing here is that the DCC wave form is different from the DC waveform and the motors only operate on DC waveform. DCC is more like AC in structure, which is why your DC loco does not go racing off when DCC high static voltage is applied. The decoder converts the DCC waveform/signal to PWM DC to drive the motors.

This is why when using a PWM (pulse wave modulation) DC power pack and you put a DCC-decodered loco on the track it will shoot off, because the decoder thinks the PWM-DC waveform is a DCC signal (and doesnt know how to interpret with no data). DCC locos with analog capability need pure DC and not PWM DC to operate correctly. This is why G scale graphics makes a device that converts PWM DC to pure DC to be used with modern decoder-loaded locos. The 10amp MRC Power G and USA Trains DC power packs are good examples of PWM that will send decoder locos shooting off when power is applied.
 
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Diesel2000 said:
This is why when using a PWM (pulse wave modulation) DC power pack and you put a DCC-decodered loco on the track it will shoot off, because the decoder thinks the PWM-DC waveform is a DCC signal. DCC locos with analog capability need pure DC and not PWM DC
Click to expand...
?

PhilP.
 
Diesel2000 said:
Whats your question here?
Click to expand...
I don't think that comment is true?

A DCC loco with DC enabled and a power-buffer, will take off when track-power is switched off.

A DCC loco on a DC powered track will behave as a DC loco, so if there is power on the track, yes it will move, just as a non-fitted loco would.

It used to be that some decoders were confused by a PWM DC supply, but most will now cope with this.

PhilP.
 
PhilP said:
I don't think that comment is true?

A DCC loco with DC enabled and a power-buffer, will take off when track-power is switched off. Yes

A DCC loco on a DC powered track will behave as a DC loco, so if there is power on the track, yes it will move, just as a non-fitted loco would. Only if analog is enabled in CV29, and the DC Power source is not PWM otherwise it will not respond properly until the voltage is way up.

It used to be that some decoders were confused by a PWM DC supply, but most will now cope with this. I know MLGB MSD3 decoders suffer from this with non-LGB power packs as described.

PhilP.
Click to expand...
answers above
 
Appreciate all the replies - although the last few on the effects of PWM were a little over my head!
I am going to use the input with some results of my own research to draft a few paragraphs - as an aide memoire for myself but it may even help other beginners. I will post a draft here and I hope if it contains mistakes one of the experts will help.
Thought - ‘Idiots Guide’ describes me but I think ‘Beginners Guide’
might be more respectful to others.
Thanks again
Fitz
 
Fitz Orchard said:
Appreciate all the replies - although the last few on the effects of PWM were a little over my head!
I am going to use the input with some results of my own research to draft a few paragraphs - as an aide memoire for myself but it may even help other beginners. I will post a draft here and I hope if it contains mistakes one of the experts will help.
Thought - ‘Idiots Guide’ describes me but I think ‘Beginners Guide’
might be more respectful to others.
Thanks again
Fitz
Click to expand...
Nice idea, we are always getting beginners queries on DCC/MTS etc so this will be a great addition to the Forum. Who better to write it than a beginner learning from all the sage advice passed on.

Yes PWM over my head as well, perhaps leave that sort of thing for anyone who wants to write the ‘Experts Guide’ to DCC.
 
Here is my attempt to draft a simple beginners guide to DCC. Not happy with it yet - I think I am guilty of repetition as I try to ensure I have captured all the details but before I go any further I though I should check that I have at least got most of the facts right as it is more than possible that I have misinterpreted the various inputs I have tried to synthesise here. Anyway for what it's worth -

Beginners Simple Guide to DCC - first draft for review

First step -a basic understanding of a couple of electricity terms and concepts.

Current and Voltage

Electric current is the rate of flow of electric charge (typically electrons) through a conductor, measured in Amperes.

Voltage is the pressure or "force" that drives this charge to move between two points, measured in Volts.

Current represents movement, while voltage represents the energy available to cause that movement.

A useful analogy is in plumbing - current would be the amount of water in the pipe and water pressure is what causes the liquid volume to flow along it. The higher the water pressure (voltage) the faster and with more potential power the water flows (Current). Without water pressure no water flows even though the pipe is full.

The higher the voltage the faster the current flows with more potential power to release when it reaches a destination. If the voltage stops the current remains but does not flow to provide power at the destination.

AC and DC

Electric power travels in a series of waves or a waveform. Different waveforms have differing characteristics. A waveform is just a graphical representation of voltage or current changes over time. The nature of the waveform differs significantly depending on whether the electricity is Alternating Current (AC) or Direct Current (DC).
  • Alternating Current (AC): AC, which powers homes and industries, constantly changes amplitude and reverses direction. When plotted on a graph, this produces a, usually, smooth, repeating oscillation pattern known as a sine wave.
  • Direct Current (DC): DC, such as from batteries, flows in one direction and maintains a constant voltage. On an oscilloscope, ideal DC appears as a straight, horizontal line. While it does not "wave" up and down, it still technically has a waveform (a flat line).
  • Between these 2 extremes lie many variations one of which is DCC
  • Waveforms are essential for understanding how voltage and current change, enabling analysis and control of electrical systems. While not all electricity fluctuates in a wave-like manner, all electrical signals can be described by a, specifically defined, waveform.
What is DC and how does it work?

Pre-DCC model trains were analog - DC motors using current direct from the track to provide motive power. Operating control was a simple controller device called a rheostat or variable resistor which varied the voltage. As the rheostat knob turns the voltage increases and the loco moves. Vary the voltage and the loco would accelerate or slow down.

This method only allowed one loco to be used at anyone time on a circuit. Multiple locos require electrical separation on circuits with a controller for each.

The solution to allow multiple devices to use single layouts was either to abandon track power and use on board battery power with radio control or adopt DCC.

What is DCC and how does it work?

Digital Command Control involves constant track power at operating voltage in a waveform that is more like AC in structure. Hence traditional locos set up for analog use will not ‘recognise’ or be able to use the power in a DCC track.

It requires an intermediate device (a decoder) which ‘adapts’ or alters the AC like DCC to the DC signals a motor needs. This not pure DC but behaves very much like it - enough to ‘fool’ the motor

The chip in a decoder sits between the power pick up and the motor. It prevents power getting to the motor unless it receives an ‘instruction’ via the track power.

The decoder in each loco receives digital signals from the track. It rectifies (converts) these from the AC like DCC waveform/signal to DC allowing it to uniquely recognise them from its programming. There is a controller in the chip which responds to the DCC controller commands making the loco accelerate, slow and, if commanded, stop.

These instructions allow independent control of speed lights and sound and can also be used for other devices linked to the track such as trackside points/switches and lighting.

Finally (so far) the question that started this journey of discovery off

How to operate a DC loco on a DCC track - and vice versa!

A DC only loco will not operate on DCC powered track as the current flows in waveform similar to AC which the loco does not recognise. To over come this most DCC systems will enable it to operate by selecting loco address 0. By doing this the control signals will be pulses of DC at the maximum voltage which ‘fool’ the loco that it is receiving the genuine DC current form it requires.

These pulses will be wider for increase of voltage enabling acceleration and narrower for decreased voltage (slowing down) and ceasing (to stop).

This method may be quite noisy with a higher pitched buzzing that varies with speed and this noise will occur even when the loco is not receiving any power.

End Draft
 
Brilliant.
As a brother idiot….

Now, how to change cvs…”
And, if one ventures into changing cvs, willy nilly, or Donny Balmy, as we say over here in the colonial reichstag, can one inadvertently render a locomotive inoperable? The key question being, can one simply dive in and experiment without risk of making a mess?
 
Fitz Orchard said:
End Draft
Click to expand...

So many issues there...... almost all the electric description is pretty much misleading or just plain wrong. There are plenty of simple and accurate description of what electricity "is" available on the web.

Short form..... DCC places a power controller inside each locomotive (or device) which receives digital instructions from a central control device (usually hand held). The control instructions are encoded within the power supplied to the locomotive (or device). Each locomotive (or device) has a selectable address so each can be controlled individually, and concurrently if necessary.
 
Well! Clearly that didn't go down well and it's pretty much a waste of time trying to do something positive to help. So instead of seeking to build on an idea I will retire hurt and assume cutting up rough from a position of knowledged privilege is preferable on here.
Have a nice day all!
 
Fitz Orchard said:
Well! Clearly that didn't go down well and it's pretty much a waste of time trying to do something positive to help. So instead of seeking to build on an idea I will retire hurt and assume cutting up rough from a position of knowledged privilege is preferable on here.
Have a nice day all!
Click to expand...
Don't take it to heart..

A lot of what you have written, is the 'lies to children' that is taught to give you a rough idea of how elek-trickery works..

Current (whatever that is) flows.. Electrons don't, they are lazy, and just drift. They take the easiest path to earth, and are colour-blind, so don't care what colour the wire is.

It takes a-g-e-s for an electron to cross the Atlantic, but if you stuff en extra one in at this end, one pops out at the other at nearly the speed of light.

All the 'simple' analogues are wrong, and break-down somewhere. It is a bit like rivets.. Some people get really uptight about them..

PhilP.
 
Fitz Orchard said:
Well! Clearly that didn't go down well and it's pretty much a waste of time trying to do something positive to help. So instead of seeking to build on an idea I will retire hurt and assume cutting up rough from a position of knowledged privilege is preferable on here.
Have a nice day all!
Click to expand...
You did a good job of explaining the problems and the consequences to people who might not have known, which is what you set out to do. Yes, it may not be quite as easy as that, but as long as it works, it's fine.
 
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