# Maximum Peak to Peak DCC Voltage - NMRA 22V?



## rreiffer (Jan 14, 2009)

Hello all, 
This is a newbee question about the actual voltage level used on a large scale (NMRA?) track. Does the (maximum) voltage fluctuate between -22V and +22V with respect to ground (peak to peak would be 44V) or is it actually +/- 11V for a peak of 22V. Finally, why did the NMRA pick 22V instead of 24v as it's standard or did I miss something here? How many 22V supplies can you find out there!


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## Cougar Rock Rail (Jan 2, 2008)

Hi Rich, 

I can't answer your first question, but I recall reading in the NMRA standards that large scale decoders should be designed to handle up to 27V. I know that Massoth are designed for that, and probably Zimo, but I'm pretty sure there are many others that don't meet that requirement. Most track voltages though should be 22-24V I believe using a 24V supply. My MTSIII puts out about 22.5V at the track. 

Keith


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## rreiffer (Jan 14, 2009)

Keith, 
Here is the NMRA standard for boosters and it talks about a +/-22V maximum. http://www.nmra.org/standards/DCC/standards_rps/rp912.html 

Now here is the NMRA standard for electrical specifications: http://www.nmra.org/standards/DCC/standards_rps/S-91-2004-07.pdf 
When the NMRA talks about maximum voltage is that peak to peak or peak to ground? 

So I guess I am going to break out my 100MHz HP Scope tonight, put it on the track and see what in the world is happening! (I guess it's about time).


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## Trains West (Oct 4, 2008)

ther power is peak to peak ac 20khz square wave digail data 

so if you see 22 volts it would be 11v pos to 11 volts neg but most systems put out like 18 to 19 volts peak to peak


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## Greg Elmassian (Jan 3, 2008)

Yes, 22 volts is the spec, although all decoders are supposed to handle 24 volts, and scales over N should handle 27.

In europe, most systems allow you to go to 24 volts.

My personal testing has shown some locomotives cannot reach prototype speeds even with 24 volts DCC, like the Aristo E8.

Greg 


(oops my edit was during the next post... sorry for duplicate info)


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## Cougar Rock Rail (Jan 2, 2008)

Hi Rich, here is what I was talking about: 

Digital Decoders intended for "N" and 
70 smaller scales shall be designed to withstand a DC voltage of at least 24 volts as measured at the track. 
Digital Decoders intended for scales larger than "N" shall be designed to withstand a DC voltage of at 
least 27 volts as measured at the track.


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## rreiffer (Jan 14, 2009)

Greg,
I believe this is the link you are refering t http://www.nmra.org/standards/DCC/s...004-07.pdf
From what I read here this covers the communications from the Command Station to the Track. From what I can see there really isn't anything defined persay from a "booster" perspective but that a booster should conform to these specifications or have I got it all wrong? 
Now I think the light bulb may have finally gotten through the brick in my head







and I see the peak voltage can not exceed + and - 22V which would give a peak value of 44V. Now the RMS value of that is actually about 31v (.7071 X Peak = RMS), however that would exceed the decoder maximum of 24V, so based upon what I am seeing, a pure 24V DC signal (switched at DCC Frequencies) would make a very good large scale signal, am I correct?


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## Greg Elmassian (Jan 3, 2008)

The standard is in S 9.1 ... a much more fundamental document than an RP... RP stands for "Recommended Practice", a standard is just that, a standard which must be obeyed to say you are DCC. 

Since it is a square wave you need to calculate differently, the .707 is RMS for a sine wave ac, not square wave. most people will tell you that RMS of a symmetrical square wave is the same as the + and - ...

Put it on a scope, the peak to peak is 44 too... 


Greg


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## krs (Feb 29, 2008)

Posted By rreiffer on 07 Mar 2011 01:43 PM 
Now I think the light bulb may have finally gotten through the brick in my head







and I see the peak voltage can not exceed + and - 22V which would give a peak value of 44V. Now the RMS value of that is actually about 31v (.7071 X Peak = RMS), however that would exceed the decoder maximum of 24V, so based upon what I am seeing, a pure 24V DC signal (switched at DCC Frequencies) would make a very good large scale signal, am I correct?









You're right with your first comment,
The 22 volt maximum DCC voltage specified by NMRA swings in both directions, positive by 22 volts and then negative by 22 volts, so the peak-to-peak value is 44 volts. 

The picture of the scope trace below shows a DCC voltage of 14.4 volts (28.8 volts peak-to-peak) which is the maximum central station or booster voltage for H0.


However, your comment about the RMS value is not correct. The 0.7071 factor you quote only applies if the waveform is a sine wave. 













One consideration when it comes to waveforms with fast rise and fall times is the overshoot which you can see in these waveforms.
DCC Central Station and booster design should be such as to minimize that overshoot to avoid the DCC decoders being damaged.
You can see that this momentary peak voltage is now more than double that which is expected with a normal waveform. 











As to the other question - why 22 volts?

I would love to know that as well.

Just looking at the current NMRA documentation, they considered the maximum "standard" DC voltage of H0 to be 12 VDC which is correct.

So the typical DCC Voltage for H0 scale was set 2 volts higher at 14 volts to allow for a 2 volt drop through the DCC decoder to be able to apply up to 12 VDC to the loco motor.


For some reason, when the NMRA standard was developed, the maximum "standard" DC voltage for Large Scale was determined to be 16 VDC.

The only place I have seen 16 volts mentioned with respect to Large Scale is on some Digitrax websites.
So again allowing for a 2 volt DCC decoder drop, the typical DCC voltage for Large Scale was defined to be 18 volts.
22 volts seems like an arbitary number that was chosen as the absolute maximum output of a Central Station/Booster to be a comfortable level below the minimum voltage a Large Scale decoder must be able to handle.


You can see those values and their definitions in this diagram from the NMRA DCC spec:









If the NMRA had recognized that the maximum"standard" voltage used in Large Scale was actually 24 volts, they would have set the Large Scale DCC voltage at 26 volts which would have provided the required maximum 24 volt DC to the motor.

It is somewhat fortunate that many Large Scale locos are geared to run much too fast with 24 VDC applied, also many LGB locos have this built in start delay which essentially provides a 6 volt drop in analog operation so the motor only "sees" 18 volts with 24 VDC on the track, but there are a number of locos that either run well below their prototypical scale speed with DCC or don't perform well simply because the NMRA Large Scale DCC track voltage is too low.


This has been an issue for years and some European DCC system manufacturers have provided DCC central stations and boosters that exceed the 22 VDCC NMRA standard but then they are of course not 'standard compliant' 


I'll think I'll leave it at this.

- Knut


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## George Schreyer (Jan 16, 2009)

22 volts is arbitrary. In most cases, that is all that is needed. Greg is a special case... as usual. 

NMRA DCC decoders are supposed to handle 27 volts although many of them will not. 

- gws


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## Greg Elmassian (Jan 3, 2008)

only over N scale has to handle 27 volts... 

Yep I'm a special case for sure! (my shrink will tell you so!). 

NCE ships it's 10 amp boosters set at 16v. 

Yeah, whoever thought up 16v for large scale was clearly not in G scale... 

Greg


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## krs (Feb 29, 2008)

Posted By George Schreyer on 07 Mar 2011 03:20 PM 
22 volts is arbitrary. In most cases, that is all that is needed. Greg is a special case... as usual. 



Really strange "arbitrary" voltage.
24 volts for instance would have made much more sense, that's one standard in electronics, 22 volts isn't.


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## Axel Tillmann (Jan 10, 2008)

Posted By krs on 07 Mar 2011 06:40 PM 
Posted By George Schreyer on 07 Mar 2011 03:20 PM 
22 volts is arbitrary. In most cases, that is all that is needed. Greg is a special case... as usual. 



Really strange "arbitrary" voltage.
24 volts for instance would have made much more sense, that's one standard in electronics, 22 volts isn't.



Not so arbitrary after all I think........

A common power supply value is 24V DC, or 24V AC in either case you need at least 2V for regulated DCC to be able to deliver highly stable DCC signal at maximum Amperage. It is my guess that it comes from that fact. Could they have chosen then the next higher up 28V power supply and hence recommended 24V DCC, but then a lot of existing conntributor screamed **** and it was dropped to 22V.

Just an edcuated guess from some of the other technology comittee works I have seen and the pleasure







of being a part off.


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## Greg Elmassian (Jan 3, 2008)

Well, if they started with 16 volts for LS DCC, you cannot expect a whole lot of sense afterwards in terms of voltage! 

I go by what scale speeds I need. Not everyone wants to run passenger trains at scale speeds. I don't really often, but I want the capability. 

Greg


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## krs (Feb 29, 2008)

Posted By Axel Tillmann on 07 Mar 2011 07:06 PM 
Not so arbitrary after all I think......
A common power supply value is 24V DC, or 24V AC in either case you need at least 2V for regulated DCC to be able to deliver highly stable DCC signal at maximum Amperage. It is my guess that it comes from that fact. Could they have chosen then the next higher up 28V power supply and hence recommended 24V DCC, but then a lot of existing conntributor screamed **** and it was dropped to 22V.






Sorry Axel, you're off base here....

To start with, if a system power design is required to work with already existing product, then one always designs the new power appropriately to work with the existing equipment, not the other way around.

In this case, 24 volts DC had been used as the maximum track voltage on Large Scale for years and DCC for Large Scale should have taken that into account.


Using your logic, the DCC voltage for H0 should have been 10 volts ( a standard 12 volt supply minus the 2 volt drop) not 14 volts. For H0 using a standard 12 VDC supply to power central stations and/or boosters wasn't a criteria, why then should a standard 24 volt supply be a criteria for Large Scale DCC/


Another point - when DCC first started the power source was just a simple transformer, not a DC power supply of any description. Power transformers come in a wide range of secondary voltages, usually in 2 volt increments in the 12 to 24 VAC output range, there was no specific transformer secondary that would have driven a 22 volt DCC llimit


And finally, I'm happy to see that Zimo, something right up your alley, decided to continue to provide an adjustable DCC output voltage on their new MX10 from 10 to 24 volts.

Outside of the NMRA spec., but NMRA has never taken Large Scale seriously anyway.

Best regards,

Knut


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## George Schreyer (Jan 16, 2009)

arbitrary in the sense that most of us that use 22 volts iterated there. I kept turning the DCS100 up until I got the speed that I wanted. After I upgraded the booster to the PB110, I set it at the same voltage, which happens to be pretty much all it will do without some modification.


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## Axel Tillmann (Jan 10, 2008)

Knut:

Actually the adjustable voltage is standard already in the MX1 generation - as a matter of fact you can adjust today's MX1 (I should probably say - yesterday's MX1) between 12V and max power in my case I slapped a 28V transformer behind it so I get solid 24V/25V (ZIMO needs about 3V from the AC input to deliver at maximum Amp Voltage). I can get 28V but not at 10A (has something to do with the regulator circuit).

While engine run proably at 14-15 Volt at their prototpyical speeds, I like the extra V for the long track, so that I can use the ZIMO decoder feature of Max Speed as a factor of absolute voltage rather than a percentage of track voltage. So setting that valaue e.g. to 12V makes the max speed independant of the voltage fluctuations over long distance.


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## rreiffer (Jan 14, 2009)

Ok all,
I think I have figured out where the 22V comes from. Ready to put your GEEK hats on? Here it is!:
The VP = the voltage peak of an AC signal. Most supplies are rated in RMS voltage so a 24V RMS will follow the following equations for a full wave bridge rectifier:
VP =(V secondary rms x 1.414) - 1.4 V (the 1.4V is for the diodes)

So 24 X 1.414 = 33.936


The second equation is: 


Vdc av = 2VP/π 
So we take 2 times the 33.936 which equals 67.872

Now take the 67.872 and devide it by PI (3.141592654) and you have 21.6 Volts or roughly 22V! 

Now the bottom line is that they should have just simply put it at a maximum of 24VDC which would have been a lot easier!


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## krs (Feb 29, 2008)

You forgot to subtract the 1.4 volt for the diode drop to get the peak voltage.

And then of course you need to multiply the resiult with the double variable Xi or








to get the right answer.


Gives you exactly 22 volts every time!


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## Road Foreman (Jan 2, 2008)

Rich, 

Please remember that most systems have a maxium AC voltage of 18 volts for there boosters.. 
Digitrax, NCE & Lenz are some that have an 18 volt maxium.. LGB, Massoth will handlle a higher AC voltage.. 

BulletBob


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## George Schreyer (Jan 16, 2009)

Digitrax will take 22 VAC just fine. Mine ran that way for quite a while. 

- gws


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## StanleyAmes (Jan 3, 2008)

Posted By George Schreyer on 07 Mar 2011 03:20 PM 
22 volts is arbitrary. In most cases, that is all that is needed. Greg is a special case... as usual. 

NMRA DCC decoders are supposed to handle 27 volts although many of them will not. 

- gws 



No 22 volts was a very deliberate decision. It was based on the CE and US electrical code at the time. At that time 24 volts was an absolute maximum without a whole lot of required testing. The voltage limits have changed somewhat since then but the standard has not changed. For a system to conform it must not put any voltages above the +- 22 volt limit.

The figure in the Standard is an example only and does not specify any additional restrictions.

Stan Ames


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## Greg Elmassian (Jan 3, 2008)

So, if 24 volts was the limit, why not pick 24 volts? Yeah, 2 volts safety margin.... but there were a number of 24 volt consumer systems... but I can see that... 

Greg


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 08 Mar 2011 11:58 PM 
So, if 24 volts was the limit, why not pick 24 volts? Yeah, 2 volts safety margin.... but there were a number of 24 volt consumer systems... but I can see that... 

Greg


When it comes to the definition of what constitutes a safe low voltage, the Electrical Codes are a can of worms, especially if one includes global requirements.
This is a pretty good article skimming the surface:
http://findarticles.com/p/articles/mi_qa3726/is_201004/ai_n53505531/

I don't know which particular requirement NMRA decided to follow at the time, but for all the power design that I was involved in, any power unit that was meant to be used on customer premises could not exceed 30Vrms or 50 VDC at any unprotected output to qualify as "non-hazardous".

With DCC, I would think it's the peak-to-peak voltage that would have to meet any specific requirement so it's really the 44 volt peak-to-peak maximum that's the consideration, not the 22 or 24 volts.

After all, we have has 24 VDC power packs in Large Scale for a long time and they, at least theoretically, all meet code.

Knut


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## Greg Elmassian (Jan 3, 2008)

That is a pretty good article, nice to find one updated from the old 24v. 

All I said was I can "get" someone choosing 22 volts, by operating from the belief that 24v was the max safe voltage outside. I know that part of history. 

But even at that time, 22 volts could have been marginal for things like the LGB track cleaning car. 

Greg


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 09 Mar 2011 10:01 AM 
That is a pretty good article, nice to find one updated from the old 24v. 

All I said was I can "get" someone choosing 22 volts, by operating from the belief that 24v was the max safe voltage outside. I know that part of history. 


I don't think 22 volts or 24 volts DC is the issue, it's the total voltage swing of 44 volts.

Knut


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## George Schreyer (Jan 16, 2009)

peak to peak means virtually nothing because the "peaks" don't happen at the same time. The 0-P is what is important from a component rating point of view and the RMS value is important from the energy delivered point of view. 

- gws


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## rreiffer (Jan 14, 2009)

All,
It took some time for me to get it through my thick head







but after looking at the theory behind the H-Bridge it really helped to explain how I was getting such a "large" swing in voltage from a single power source that was 1/2 of it (approx). Here is the page: http://www.mcmanis.com/chuck/roboti...index.html
Now I see that we are truely just sending plus and minus (with respect to ground) swings of the SAME DC source. This is what generates the "double your voltage" that one sees on an oscilloscope. 
Now I still agree that they could have come up with a maximum voltage on the plus and minus side of 24V.


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## krs (Feb 29, 2008)

Posted By George Schreyer on 09 Mar 2011 10:46 AM 
peak to peak means virtually nothing because the "peaks" don't happen at the same time. The 0-P is what is important from a component rating point of view and the RMS value is important from the energy delivered point of view. 

- gws 
We're talking about the Electrical Code, the hazard of receiving a shock, not any component rating.

30 volts (rms) AC will give you more of a shock than 30 volts DC even though the energy delivered is identical. 

And with an AC signal the peaks don't occur at the same time either.

Knut


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## Road Foreman (Jan 2, 2008)

My understanding was if 24 volts was used you could have 48 volts across 2 power districts & that was to close to the 50 volts maxium for low voltage circuits.. The powers @ that time then reduced the voltage by 2 volts giving you 22 volts as the maxium.. 

BulletBob


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