# Does DCC run on AC or DC?



## DB Rails (Jun 9, 2020)

From what I've read elsewhere DCC is run from AC, yet I wanted to get this clarified. The only reason I ask is because of a ROCO model I found which has sound, yet says that it's DC.

This is the train i'm looking at: Roco Modelleisenbahn Products LOCOMOTIVES Diesel locomotives 73733 Diesel locomotive 218 497-6, DB AG


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## MichaelE (Mar 7, 2018)

As I said before, any foreign HO DC locomotive is compatible with US track, DCC, and accessories. The Br.218 already has the DCC sound decoder installed. NEM and NMRA standards are the same concerning supply power and DCC.

I don't know how to make that any clearer.

I've been modeling Germanic railroads for over two years now.

To answer your question, DCC can be used on AC or DC.


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## mesenteria (Oct 29, 2015)

Let me try my own answer:

All decoders marketed after about 2005/6 have been deemed 'dual mode', meaning they'll sense and operate on either DC or AC current, with the latter being DCC (digital) current with the signal embedded. Note that this is true only for decoders on factory default settings. Second hand decoders, or second hand engines you purchase with unknown decoders, or with decoders having the previous owner's custom sound levels and motion characteristics programmed into the appropriate CVs, _should be restored to factory defaults by performing a reset...FIRST THING YOU DO. _Why? Because many of us remove that dual mode capability by entering a value into CV29, and if you don't know this and expect your decoder to sense your DC-only layout, you will get nothing out of your locomotive. No lights, no motion, no sounds.

So, about this Roco locomotive. It is possible it has no decoder, and somehow makes sound. That would make it a DC locomotive. I've never heard of such a thing, except for the short-lived BlueLine experiment by Broadway Limited Imports about 13 years ago. But, maybe Roco is doing something similar. I suspect, however, that it merely has a 'dual mode' decoder and that it comes ready to operate on either type of track current, DC or DCC.

Your post headline asks if DCC is AC or DC. It is AC. The DECODER will respond in a rudimentary way if it senses DC current, and you'll be able to move your locomotive forwards and backwards, sound the horn/whistle, and the lights will work. But, without a DCC controller, you'll not be able to get all of the motion and sound qualities and adjustments that your DCC decoder is capable of. There's no sound level adjustment possible in DC, no motion adjustment such as inertia and momentum (CVs 3 and 4 respectively), and so on.


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## gregc (Apr 25, 2015)

the loco description says it uses a PluX16 DCC sound decoder. it's not clear what "Elect. system: DC" is attempting to say. 

i wouldn't say "DCC is run from AC" as in AC power, but that DCC is AC, carrying both power and signalling.


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## DB Rails (Jun 9, 2020)

MichaelE said:


> As I said before, any foreign HO DC locomotive is compatible with US track, DCC, and accessories. The Br.218 already has the DCC sound decoder installed. NEM and NMRA standards are the same concerning supply power and DCC.
> 
> I don't know how to make that any clearer.
> 
> ...


Thanks for the clarification. I'm just trying to be cautious and understand what I'm buying since I've never bought from Europe before. It certainly is a lot simpler than I originally thought though, so this'll probably be the last post I make regarding stupid questions like this (and hopefully I'll be getting my hands on a 218 sometime soon!)


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## DB Rails (Jun 9, 2020)

mesenteria said:


> Let me try my own answer:
> 
> All decoders marketed after about 2005/6 have been deemed 'dual mode', meaning they'll sense and operate on either DC or AC current, with the latter being DCC (digital) current with the signal embedded. Note that this is true only for decoders on factory default settings. Second hand decoders, or second hand engines you purchase with unknown decoders, or with decoders having the previous owner's custom sound levels and motion characteristics programmed into the appropriate CVs, _should be restored to factory defaults by performing a reset...FIRST THING YOU DO. _Why? Because many of us remove that dual mode capability by entering a value into CV29, and if you don't know this and expect your decoder to sense your DC-only layout, you will get nothing out of your locomotive. No lights, no motion, no sounds.
> 
> ...


That's what I was originally assuming, so thanks for the clarification.


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## mesenteria (Oct 29, 2015)

Gregc, DCC, DCC is AC. It is dual mode capable conventionally for those who can't operate the decoder using AC. It will take the DC current and pass it through to the outputs, but with limited range of function. In fact, all of a decoder's outputs are rectified to DC, as one would eventually realize due to its DC can motor, DC lights, and DC amplifier and speakers. But if you want the decoder's full range of functions, you must have the digital signal, and that can only be passed to the decoder via a square wave AC current. DC, to my knowledge (I'm not an electrical engineer, not even an electrician) is incapable of carrying a digital signal because it has no wave form ("direct").


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## gregc (Apr 25, 2015)

mesenteria said:


> Gregc, DCC, DCC is AC. It is dual mode capable conventionally for those who can't operate the decoder using AC. It will take the DC current and pass it through to the outputs, but with limited range of function.


not sure what I said that elicited this response. 

yes, some DCC decoders will operate on DC if the DC voltage is high enough. The processor must just pass the rectified (max) voltage to the motor switching the polarity of the h-bridge to match the track polarity.

similarly it is possible to operate one DC locomotive, one without a decoder, on a DCC system using loco address 0. the DCC signal typically has an average voltage level of 0, half the time it has one polarity and the other half the other. but the command station can stretch the idle pulse so that there is an average DC voltage, either + or -, to drive a DC locomotive in either direction this is not recommended because even an AC current is generating heat in a DC motor across the tracks, even when the average voltage is zero.


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## mesenteria (Oct 29, 2015)

Perhaps I read more into your pervious post to mine than I ought to have. You said, "i wouldn't say "DCC is run from AC" as in AC power..." You can't power a DCC system with DC. You must plug it into an AC source suitable for the power supply powering the system, and that power supply is not a DC power supply. A DCC power supply, like my Digitrax article, outputs 15 volts AC. 
I know that my Digitrax DB150 can run a DC locomotive, no decoder, using Add "00" simply by zero stretching (or rather, not) to that address.


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## gregc (Apr 25, 2015)

mesenteria said:


> You said, "i wouldn't say "DCC is run from AC" as in AC power..."


maybe I misinterpreted the OPs original statement. can't imagine why anyone would care how their DCC system is powered.




mesenteria said:


> You can't power a DCC system with DC.


you don't think a DCC system would work if I replaced the wall wart outputting ~13VDC with a car battery?


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## Lemonhawk (Sep 24, 2013)

There seems to be a fair amount of misunderstandings here. Limiting things to just the track power, there are DC systems (like non DCC HO), A/C systems (like Lionel) and DCC systems. The DCC output is not A/C it is pulse width modulated DC signal. The Pulse width modulated DDC signal contains packets of information that are sent to the decoders. And Mesenteria, your DB150 can be powered by either a 15 A/C or DC power supply (I have powered my DB150 with either), most DCC controllers now just use a DC power supply.


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## gregc (Apr 25, 2015)

Lemonhawk said:


> The DCC output is not A/C it is pulse width modulated DC signal.


doesn't the track polarity change?

from S9.1
The NMRA baseline digital command control signal consists of a stream of transitions between two equal voltage levels that have opposite polarity


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## Severn (May 13, 2016)

I think this is a confusing area. The DCC power input is DC. It turns that into a PWM pattern (pulse width modulated) onto the track which sometimes is described as AC.

Well maybe it is technically ... But to the just knowledgeable enough to be dangerous like me ...


When I think AC I think wall current, 60 hz 120 v.


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## gregc (Apr 25, 2015)

i'm sure a DCC system can be designed to accept either an AC or DC output from a wall wart plugged into a ~120V AC wall socket.

the DC voltage used within a DCC system are not simply output as PWM. the ~14VDC voltage of a DCC system is applied to the track using an h-bridge, common motor controllers and used in a decoder, allowing the polarity of the output to the track to be easily reversed.

in the waveform from NMRA S9.1 below, the horizontal line represents 0V and the waveform is both positive and negative. in other words the waveform is AC (but not like your Lionel set).


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## Lemonhawk (Sep 24, 2013)

Yes, this is a PWM signal. It's also why you can make a neat DCC tester using a 2 lead bi-color LED.


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## gregc (Apr 25, 2015)

it is width modulated, but it is not a pulse in the sense that it goes from 0 to some voltage and back.

the NMRA standard clearly said "levels that have opposite polarity"

i would expect a red green bi-color LED driven by DCC to appear yellow. The same bi-color LED across a PWM DC signal would appear one color and when connected oppositely, the other color.


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## MichaelE (Mar 7, 2018)

All the OP wanted to know is if this locomotive will run on DC track and be able to use DCC.

Every mole hill turns into a mountain around here.


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## Severn (May 13, 2016)

Yes but I almost understood it. Getting closer. And I have a digital scope. It's mostly a paper weight. But I could put out it in the track and look at pwm signal. What I mean is I could connect the probe to the track. If I put it down n the track it would be something else.


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## gregc (Apr 25, 2015)

a pulse width modulated signal is typically used to create an analog voltage from a digital output by controlling the duty-cycle of the digital output to achieve a desired average voltage. DCC decoders use PWM to control the motor voltage.

but while the time between polarity changes on track voltage is used to convey the 0 or 1 digital signal value (see S9.1), i wouldn't describe the DCC signal as PWM any more than I would ethernet or I2C signals. And I'd hate to think anyone thinks that DCC drives tracks voltages using PWM to control locomotives.


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## Severn (May 13, 2016)

I thought it was an encoding scheme to embed cmd codes. I understand DCC engine side uses pid control and bemf for motor sensing.


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## mesenteria (Oct 29, 2015)

I agree that it is a confusing topic. I wish I had more physics and engineering, and a better memory than I have at present, but....

From the DCC Wiki:

*...The voltage to the track is a bipolar alternating current (AC) signal*. The DCC signal does not follow a sine wave. Instead, the command station quickly switches the direction of the DC voltage, resulting in a modulated pulse wave. *The length of time the voltage is applied in each direction provides the method for encoding data*.{ Me- 'zero stretching'} To represent a binary one, the time is short (nominally 58 µs for a half cycle), while a zero is represented by a longer period (nominally at least 100 µs for a half cycle).
Each locomotive is equipped with a mobile *DCC decoder** that takes the signals from the track and, after rectification, routes power to the electric motor as requested*. Each decoder is given a unique running number (address) for the layout, and will not act on commands intended for a different decoder, thus providing independent control of locomotives anywhere on the layout, without special wiring requirements. Power can also be routed to lights, smoke generators, and sound generators. These extra functions can be operated remotely from the DCC controller. Stationary decoders can also receive commands from the controller in a similar way to allow control of turnouts, uncouplers, other operating accessories (such as station announcements) and lights...

A link to another site that says the same things:






DCC Voltages







www.sumidacrossing.org


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## gregc (Apr 25, 2015)

some decoders use BEMF to measure the motor speed to adjust the motor voltage to maintain a speed corresponding to the speed step communicated to it

i see no need for PID. PID is used to control a parameter that is not directly controllable such as motor position vs motor speed. i've used PID to control the position of a turntable and the phase of a clock signal


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## Severn (May 13, 2016)

i believe some form of pid control is used in many dcc engine/sound decoders. but then again, i don't actually know -- it's just what i read. you know, stick your hand in front of the loco to slow it down -- take it away, and it smoothly come back to the original speed, etc...


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## J.Albert1949 (Feb 3, 2018)

*OP --*

Tell us:
Is your railroad dcc?
Or... is it dc?

Looking at the product page you gave in post #1, it looks like the Roco engine is dcc/sound equipped.

The product page description shows as having a decoder installed (listed as PluX16).

So... it should run as dcc when your railroad is set up for dcc, and...
... it should probably also run on a "straight dc" (i.e., non-dcc) railroad.

I'm not certain, but there may be SOME dcc/sound equipped locos that may not run _out-of-the-box_ on straight dc railroads. Be aware that there's a CV setting in some decoders that will "turn off" this capability (although it usually can be "turned back on").

Also be aware that there are also dcc/sound equipped locos that will also run and provide limited access to sounds on straight dc railroads.


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## MichaelE (Mar 7, 2018)

He has an NCE Powercab.


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## mcleantmw (Dec 28, 2018)

DB Rails said:


> From what I've read elsewhere DCC is run from AC, yet I wanted to get this clarified. The only reason I ask is because of a ROCO model I found which has sound, yet says that it's DC.
> 
> This is the train i'm looking at: Roco Modelleisenbahn Products LOCOMOTIVES Diesel locomotives 73733 Diesel locomotive 218 497-6, DB AG


DB Rails - Just to clarify, DCC decoders operate on track that's powered by DCC. Your DCC command station produces the DCC current, which is a unique form of current........DCC is not AC nor DC. Some command stations' power input require DC and then converts it to DCC; some other command stations require AC or either DC or AC. For example, the Massoth G Scale Command Station I use requires a DC Power Supply input. The Piko command Station that I also use for my Programming Track, originally allowed either DC or AC input, but their upgraded mode now only accepts DC power input. You can read about the unique nature of DCC current at:


https://dccwiki.com/DCC_Tutorial_(Power)


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## cid (Jul 3, 2014)

> AC on Wiki<


*"Alternating current* (*AC*) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. "
"Periodically reverses direction", like in DCC, which uses a data-carrying modulated AC square wave. 
Just to clarify...


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## gregc (Apr 25, 2015)

so many clarifications. thanks cid for correctly describing AC

i once had a supervisor who had a meeting to "clarify" a problem for everyone. He was the only person who was confused


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## Aard D'Vaark (Aug 1, 2019)

depends on where you put the ground, if it's on the end then you have current in one direction only , square wave direct current.. [in DCC]


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## gregc (Apr 25, 2015)

Aard D'Vaark said:


> depends on where you put the ground, if it's on the end then you have current in one direction only , square wave direct current.. [in DCC]


can you show a circuit connected to the track where that is possible?


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## cid (Jul 3, 2014)

Aardvark, all residential 120VAC in the US has one side of the line (neutral) grounded. That does not turn it into DC. 
Current still flows in opposite directions during the AC cycle. That's why it's called AC. The VOLTAGE on the hot side
alternates 85V above ground potential to 85V below ground potential, for an RMS 120VAC reading from line to line.
Give or take. The voltage regulation is less critical than the frequency.


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## Aard D'Vaark (Aug 1, 2019)

gregc said:


> can you show a circuit connected to the track where that is possible?


uh .. same as normal, one wire connected to power, one to ground.. ??
I do not understand your question ??


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## gregc (Apr 25, 2015)

Aard D'Vaark said:


> uh .. same as normal, one wire connected to power, one to ground.. ??
> I do not understand your question ??


we are talking about what the DCC signal is and not ow to connect a DC power supply to the track?

You can easily demonstrate that DCC current can flow thru a path in both directions between the rails by connecting a pair of LEDs across the rails, one opposite polarity of the other.

if the DCC signal were Not AC, but simply an on or off pulse as you (and others) suggest, only one LED would be on, albeit half the time.

because DCC is alternating polarity, power is constantly available on the rails which can easily be rectified by the decoder to provide power to the motor.


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