# DCC speed



## vette-kid (May 2, 2020)

I installed a decoder on my E7 and notice it seems to have a lower top speed. With a different decoder I notice the same thing (one XL systems, one soundtrax). Is this just perception, or will modifying speed steps get more speed. Or maybe the controller is putting out less voltage than the DC unit did?

I know most don't really run top speed, but with some weight behind it it will slow down some, and my son likes to watch em go fast sometimes!

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## flyboy2610 (Jan 20, 2010)

CV 5 controls the max voltage. Set it to 255 and see if that helps.


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## MichaelE (Mar 7, 2018)

Is your son old enough to understand the relationship of speed for real trains compared to a model railroad?

There are high speed rail areas on real railroads, particularly in Europe where it is more frequent than in the States, but on a small railroad high speed just looks like a toy. Much like O and S scale on an oval of track with the throttle wide open.

I rarely run trains more than 45 scale MPH and my longest straight section is around 12 feet or so. Now, if I had some straight runs of 20+ feet I might bump up the scale speed to 60 or 70 MPH if the straight sections after a wide curve were that long or nearly so.

My largest radius though is 24" and the other longest two straight sections are only 7' and 9'. Not nearly long enough to open up the throttle past 45 MPH and maintain some sense of realism.


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## mesenteria (Oct 29, 2015)

Decoders use some of the voltage to operate themselves, and of course sound decoders will need that much more juice to work before the decoder can begin to meter out DC/rectified voltage to the can motor. And yes, the DC power supply may be capable of putting another volt or more to the rails over and above what the DCC base station is set to offer the rails in the way of full-time voltage. So, the chances are rather good that the DCC system is incapable of supplying the same robust voltage to the rails, via the decoder and its outputs, that a DC power supply can.


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## vette-kid (May 2, 2020)

Yes and no. But he is 7, so sometimes he wants to go fast... Dad can't blame him! It's all about having fun, if that means the occasional race around the track... So be it!

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## DavefromMD (Jul 25, 2013)

When you put a decoder in a locomotive that you used to run on DC, it's top speed will be slower. DCC is 12 volts where DC transformers can be 15 to 16 volts dc. I discovered the same thing.

Walthers F40PH in DC ran at top speed of about 90 scale MPH. When I switched to DCC and installed a decoder, top speed is 60 scale MPH.


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## ncrc5315 (Jan 25, 2014)

DCC should be ~15 volts.


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## mesenteria (Oct 29, 2015)

NMRA Track Voltages - Mark Gurries


Education about Digital Command Control (DCC) used in model railroads to run trains. This site is also a distribution point of my DCC presentations in PDF format. My Model Railroad Website




sites.google.com





If this site is correct, DCC engineers and NMRA gurus agreed among themselves that DC should not be below 12 volt capacity to the rails, and DCC should be about 2 volts more than that, so about 14 volts...minimum. Due to industrial and material exigencies, household current available at wall outlets, and other factors, the true voltages at DCC output terminals will vary by about 1.5 volts+/-. It should be clear that one is likely to see different performance from the same locomotive on different layouts being run by different DCC systems set to HO range voltage.

As I stated earlier, DCC takes some of the voltage for initial startup of the decoder, and that accounts for 2 volts-ish, right off the top. From there, you need to run your on-board amp and speaker, plus lights if they are set to be on, and only then can voltage from what's left be metered out to the drivetrain.


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## DavefromMD (Jul 25, 2013)

DCC is really square wave AC voltage to the rails. My voltmeter shows 12V AC.


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## vette-kid (May 2, 2020)

I'll have to get my meter out and check. I wander if there would be a difference between the same voltage ac vs dc

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## Shdwdrgn (Dec 23, 2014)

All this arguing over the "correct" voltage on the DCC rails... and nobody has even mentioned what scale they are speaking in relation to. I know OP is running HO, so 15V is typical. However N scale is strictly 12V, and other scales use different voltages.

And when it comes down to it, what really matters is how much voltage your particular setup is applying to the rails. My power supply puts out 16.4V, and all of my HO equipment is rated at 24v or more, so I tend to get some pretty good speed from my setup along with reliable low-speed operations. Everybody has a different setup, so it really matters to quantify your answers.

Vette-kid -- there IS a difference between AC and DC, but I'm not qualified to give an official answer. My unofficial answer (at least as I understand it) is that the given AC voltage is the _average_ of half the sine wave (either the positive half or the negative half), so the actual peak voltage on an AC line would be quite a bit higher. For example, for household power of 120VAC, I believe the peak voltage is somewhere around 180VA. However all of that probably has nothing to do with DCC, because these systems use a square wave. The difference is that there is no variation in the voltage -- it starts at a positive voltage, then immediately switches to a negative voltage of the same value. There's no slope like you see in a sine wave, so the peak voltage should match the given value. If your power supply puts out 15V then the DCC output should be +/- 15V.


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## AmFlyer (Mar 16, 2012)

AC voltages are usually expressed as the RMS value. For pure, single frequency sine waves such as 60hz housepower RMS voltage = .707x peak voltage. As additional higher order harmonics are added to a pure sine wave the relationship between peak and RMS voltage changes. For a perfect square wave RMS voltage equals peak voltage, they are the same. The problem is many volt meters do not measure non sinusoidal voltage waveforms accurately. Some meters will display about 12V if connected to a 15V square wave source, some will measure it accurately. The easiest way to accurately measure a DCC source is with an oscilloscope unless you have a voltmeter designed to measure non sinusoidal sources.
By the way, AC voltage can have a DC offset where the percentage of the sine wave above and below the zero axis is not equal. Some train manufacturers used a DC offset to actuate tender whistles.


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## vette-kid (May 2, 2020)

Now we're getting somewhere! Thanks for all the information, it's way down in the weeds, but I enjoy reading through it. I'm using a home made DCC system and pushing it with a 15v power supply, I'll check teach voltage and read through my hardware documentation to see if I can find a higher voltage supply for it. My old DC pack was putting out 17.5v so if think going to 16.5 or even 18v power supply would be safe. This is just for grinds at this point

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## Shdwdrgn (Dec 23, 2014)

Which system is your controller based off of? For example, several of us here are familiar with DCC++ on arduino. The one I use works from an ESP32 but is based off the original DCC++ code.


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## vette-kid (May 2, 2020)

It's built around an stm32 board, I don't know about the code he uses. I just built it per his instructions and download his app. Information can be found on this link. Looks like I can use an 18v power supply with it. I'm curious if it makes a difference, so I may try it out as long as it won't damage the locomotives. I can't imagine a 3v difference frying any electronics.

Low Cost DCC Controller 

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## Lemonhawk (Sep 24, 2013)

If you check track voltage with the meter in DC mode, you should get about zero in DC mode and 15v in A/C mode. I have a 15 v DC power supply (first photo) that power my Digitrax DCS240 (not the most accurate voltage display). The next photos show the track voltage on DC mode and AC mode and a final graphical display of the track wave form.








PS input









DC mode voltage









A/C mode voltage









Crude waveform


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## AmFlyer (Mar 16, 2012)

That is a nice meter you have with the built in scope display. I need to get one like that so I can quit using the conversion tables when I measure the track voltage from my ZW-L's.


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## Lemonhawk (Sep 24, 2013)

The graphical "oscilloscope" is quite crude, but this is by far the easiest to read display in my assortment of meters. I bought this from Banggood several years ago and now they have an improved version that has a color display and vastly improved sampling and display resolution. I can only assume that the color display is as easy to read as this. Its the 
MUSTOOL MT8208 HD, I think you can get it for around $35 with a new user $20 discount at Banggood.


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## AmFlyer (Mar 16, 2012)

Thanks


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## Shdwdrgn (Dec 23, 2014)

@AmFlyer -- if you just need a simple scope, check this one out (it's a kit so you need to solder on the through-hole components). Lots of features for viewing and freezing the waveform or sending data to a computer through the USB port. I use mine for troubleshooting the DCC output (I've been working with the developer of the ESP32 DCC++ code to fix some bugs). You certainly won't find any better option for that price!


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## Lemonhawk (Sep 24, 2013)

Shdwdrgn, I have one of those also, and I think I may wire it in permanently just for looks. I don't use it so it's just in the junk box for now. Might as well put it to use. The Mustool does have an I2c Arduino bus decode function also, but its the display that's makes it the easiest to read meter. I would not buy the Mustool for its Oscilloscope function only.


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## Shdwdrgn (Dec 23, 2014)

Yeah I hope nobody would by a multimeter for its oscilloscope functions, they would be sadly disappointed!


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## Lemonhawk (Sep 24, 2013)

Well, if you never used a real 'scope you would never know the difference! Every once and a while I would wander down to the lab at work and use the 'scopes their, kinda spoils you.


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## Jscullans (Jul 8, 2019)

Dcc is a square wave form so it’s set up like a can bus in a car. Think of it as a series of 1s and 0s 1 is on 0 is off. Ac is a sine wave that goes from positive to negative in an alternating current. It’s easier to send a digital signal through an ac circuit than it is for dc. So the decoder will pick up the digital signal through the ac sine wave. If dc was used in dcc instead of ac it would create an excessive amount of “background noise” for the decoders to pick up on and possibly confuse the decoder. I have that issue with mine every once in a while if I have a metal wheel car derail and get a short in the track it will cause a random locomotive to go full blast in one of the two directions until it is stopped, loses connection, or derails. My digitrax db150 puts out around 15vac my bachmann ex command puts out around 13vac. I hope this isn’t confusing you but may help you understand your system a little better


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## gregc (Apr 25, 2015)

not sure why DC would be more noisy than AC. possibly because some other method (e.g. AC carrier) would be need instead of simply modulating polarity.

the decoder needs to rectify the track voltage, reducing it by 2 diode drops (~1.4V) and the there's yet another smaller drop through the transistors of an h-bridge. so motor voltage is ~2V less than track voltage.


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