Question Does anyone know how to make DCC safe for Z scale?

I have a micro-trains F7 locomotive equipped with a soundtraxx mobile decoder and a NCE power cab. In the instructions for the decoder it says I can set the start voltage and maximum voltage. Its CV2 and it says to use a number between 0 and 255 and has this funky math equation to find wich number is for the voltage you're wanting ?? Is anyone familiar with this? I am very very confused and afraid to do anything because I dont want to fry my train or decoder. Any help is greatly appreciated.

Thanks,

Jonathan

 

No No NO No NO ! (imaging reading in ryhme). Contrary to what you read (and smack the one that wrote it), the CV's do NOT change the actual voltage to the motor. The motor ALWAYS gets the track voltage, less a little for the decoder parts, about a volt less. The motor speed is controlled by Pulse Width Modulation (PWM). In a sense, the Speed steps you use (e.g. 1-28 or 1-128) essentially set the PWM ratio. For example, speed step 14 is half way so the PWM is set to 50%. What the motor feels is more like "1/2" the voltage as it is only on half the time. Following me here? If not, below image:

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On the left, you see Speed Step 1 and the near full track voltage is applied when the decoder is On. Doesn't look like 1/128 of the track voltage but the motor thinks it is. In the middle, 50% from Speed Step 64 and lastly, nearly constant track voltage from Speed Step 125 (if I went to Speed Step 128, it would be essentially full track voltage.

Now, what the CV's due is scale this range so that if the max Speed Step makes the loco run to fast, you can 'range' it down. If you change CV5 (Max, it really should be called just Max) from the default 255 to 128, then the effective max ''''''voltage''''' would seem like half the maximum. If you were to look at the above CV change on the oscilloscope, at Speed Step 28 (or 128 selection), you would only see '50'% duty cycle, like the middle pix ! CV6, Mid (yeah, I know they are 'backwards') would normally also be scaled midway or about 64 from what would be the default 128.

Please tell me you are still with me here.

CV2, Min or Start """"""Voltage""""" is really just what minimum duty cycle you want to begin with when you go from 0 (stop). This is commonly only used in Permenant magnet motors, like Märklin or MTL's F7, not Coreless motors like AZL or MTL's GP's or SD40s. The permenant magent motors need a minimum voltage before they will easily start to turn. The Coreless motors have essentially no inertia and will start at very low voltages. CV2 also helps overcome friction at the beginning.
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Okay so if im understanding what youre saying is that the CV only changes the maximum out-put of the motor? Like if i put the CV at 130 instead of 160 lets say, the train will run slower at 130 than 160? And if the out-put on my DCC system is 12 volts or so, no matter what, 12 volts is always going to the track correct? So i guess what im really asking is how do i regulate the voltage coming out of the DCC system to the track so that it does not excede 9 volts or so? Or is this even possible? I just want to make my layout safe to run my Z scale trains with no worries of frying it.

 

I would think that since decoders can run properly on DC (and start operating under 6 volts) that they could theoretically run full-time on regulated/reduced AC track voltage like 8 or 9 volts..... (yes DCC runs AC to the tracks, decoders rectify the AC into DC and send only DC to the lights and motor etc.)

Am I way off base?

 

EMD F7A

You're not off base.

I ran my NCE Powercab system off a 8VDC 0.5A power supply that I had laying around. The transformer that came with it was 12VDC 1.1A with a center positive plug. I haven't experienced any problems running at the lower voltage, but I'm going to get a supply with a higher amperage when I convert my Z to DCC. McMaster-Carr sells 9VDC 1.3A supplies. Its the easiest way to drop the voltage at the track.

-Curn

 

Sure, you could run at a lower voltage (figure no more than about -1v from the DC supply so 9V DC supply would be worst case about 8VDC to the motor). NMRA says decoders are supposed to work at a minimum of 7.5V but who wants to run at the edge? There are so many losses during operation that you are just asking for trouble. We first tried running at 10V DCC to the track and repeatedly had problems programming certain decoders so we went back up to 12V to the track (about 13V DC supply). My Power Cab came with a 13.5 V unit.

However. the 9V rule is not an explosive-the-world-is-going-to-end and all (or most of) your motors are going to fry ! TRUST US, THEY WON'T !!! Sooooo many locos (WAY into the thousands are doing it every day in all scales!). You would think that OMG, reports everyday that 'hey, I fried another motor' would be the regular on the forums. It is not. Think about it, train and motor manufactures (maxon, Faulhaber, etc.) would be posting repeated warnings on the forums.

Again, the BIGGEST and MOST PROBABLE is that you short in an incorrectly thrown 'turnout' or derail there. This will force noticible current, more that your DC power pack and statistically 8765% more issues with your wheels or wheel wipers, NOT YOUR MOTORS.


We just got through running close to a hundred locos at the National Train Show over 4 days and again dozens the past weekend with NO issues. Please, just plug it in and enjoy running your trains !

Oh, by the way, did I show you the waveform of the DCC power to the track????? There are transients WELL into the dozens of volts, FAR more than the 9-12 Volts on the track. Please, just plug it in and enjoy running your trains !

And, MANY out there use the GuageMaster or other 'electronic' track cleaners. These produce HUNDREDS of volts when there is no motor load due to dirty track. As soon as the wheels or wipers finally connect everything to the motor, it instantaneously sees this hundreds of volts. Again. THOUSANDS of users, no complaints.

Please, just plug it in and enjoy running your trains !
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Sorry to bring up an older thread, was too lazy to start a new one. Anyways I have an NCE Powercab which am already using to control my AZL GP38 with TCS decoder. The motor is rated to 12V which is fine because the stock power supply is 13.5VDC@1.8A and outputs no more than 12V to the motor (I think) when its all said and done. So, I thought I was all good.

But then I am planning on retrofitting my Marklins with Velmo decoders which recommend a track voltage of 12V. So, I am looking into new power supplies for the NCE Powercab and found a 12VDC version at both 1A and 2A ratings ($6 on ebay). Is the 1 Amp version going to be ok? I noticed after running my GP38 for a while it barely exceeds 0.08A.

 

Yes, the 1 Amp should be fine - even with a bunch of Marklin motors running around. I can't recall the exact power draw of the Markin 3/5 pole motor, but I seem to remember it is in the 200 mA zone under full load.

If you plan on more than 5 locos moving simultaneously, then maybe the 2 amp version is better.

It all depends on the size of your empire.

 

Perfect, thank you! Yes I usually run no more than 3 locos at a time. 1 clockwise and 2 consisting CCW.

 

I'll 2nd that. Don't worry.

The MOST important point is to: ALWAYS WATCH YOUR TRAINS ! With DCC, there is more 'Power' (Volts, Amps) to the rails. If a train derails (mostly IN/ON Turnouts and, usually the Locomotive, under many conditions the DCC system will not trip the 1 or 2 Amps, leaving 25-60 watts of energy (relates to heat quite well).

So run your trains with what you have, its fine. Just keep and ear and eye on it.

 

10-4 to that. Currently on my layout I've had a few derailments on the turnout situation where I have a locomotive entering the turnout expecting to turn left, but for some reason (due to tolerances, etc.) the frog was not at the furthest extent so the locomotive jumped the track going straight and caused a short. Thankfully the Rokuhan controller instantly shuts off in such a scenario. I've been purposely avoiding this turnout ever since I got DCC.

 

I don't have any, I've read about people using the PSX1 and the NCE EB1 but seems pretty complicated? For simplicity does anyone have any experience with the NCE CP6? The fuse will break at 1 Amp. It is pretty rare that I short. I don't run any boosters.

 

I'm totally in ignorance of DCC but I consider switching to it sooner or later..

When you speak of "short protection" (I understand it like a fuse or a light bulb), do you mean set on locomotives, or set on track's feeder line ?

Dom

 

It would be on the tracks feeder line.

 

Can someone explain how the lightbulb way works exactly? If I send voltage to the tracks don't they act like a resistor to drop my voltage to the track instead of actually providing short protection?

Or are the bulbs oversized with a very low resistance and low voltage drop that only lights up when there is continuity (a short) on the track line?

 

The tungsten filament light bulb works as a current limiter. As a tungsten wire heats up it's resistance increases and increased current flow causes the filament to heat up. So a 12 Watt light bulb at running at 12V will limit current flow to 1 Ampere ( I = P/V) . When hooked up in series with the tracks, the current flow through the bulb is normally just the current to run the locomotive (typically < 0.1 Ampere) and the bulb doesn't ligh up or get warm.

I question the value of using a tail light bulb for Z because the limiting current is so high. I would suggest using a dashboard pilot bulb or a #52 pilot bulb, which these are really only available as a Lionel part.

Remember this trick only works with Tungsten filament bulbs.