Required Max Voltage for DCC

E

ernie

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For my N and OO gauge, I use Gaugemaster Prodigy Advance 2 (squared). This is actually MRC system badge engineered. I am used to using it and I like it. Now I see that MRC have a Prodigy Elite that comes with a 10amp supply. MRC say "it handles the high power demands of G gauge, while being able to safely run HO, N and other gauges".

But I cannot see how it handles say 20 volts (or even more) that G-gauge should get and also be suitable for HO and N gauge?

Am I missing something?

Thanks
 
ernie said:
But I cannot see how it handles say 20 volts (or even more) that G-gauge should get and also be suitable for HO and N gauge?

Am I missing something?
Click to expand...

Yes and no Ernie. First of all, you've mentioned amps in one sentence and then volts in the other. As you'll know the two are not the same. G scale demand in terms of current use by motors is considerably higher, so any G scale power supply needs to have the ability to provide more amps (current) for a given number of locos than a supply for smaller scales. The voltage is (theoretically) irrelevant, though the G scale standards for DCC chips allow slightly higher maximum voltages than HO/N.

For reference, the standards for voltages in G scale are as follows:
Minimum at DCC central station: 7 volts
Expected voltage at central station: 18 volts (14 volts HO; 12 volts N)
Maximum voltage at central station: 22 volts

Minimum voltage required by DCC chip: 7 volts
Expected voltage of DCC chip: 18 volts
Maximum voltage tolerated by DCC chip: 27 volts (27 volts HO; 24 volts N)
 
My knowledge of electronics is almost exclusively theoretical, from pg research at Imperial. I rarely even switched on an actual psu or got involved with electric motors but spent my time doing paperwork and computer aided circuitry design.

The motors in most locos are DC, usually now 5 pole to make them perform smoother. Their speed of rotation is primarily achieved by increasing the voltage, by often rotating the knob on most DC controllers. MY LGB starter pack psu provides just over 18V with 36VA, so ~2 amps. The DC controller then outputs +16.5V max turning clockwise and -16.5V turning anti-clockwise. All measured with my true rms meter. As the voltage increases, so does the loco speed. And so will the amps (not actually measured by me).

Assuming an infinite amount of volts and amps, ultimately the top rotational speed of the motor will be limited by back emf or the coils on the motor overheating and the insulating varnish, on the motor coils, melting. The increasing voltage will, of course, require more and more amps being drawn through the motor and hence more power (VA) until the motor blows.

The DC motor on a DCC loco does not know whether it is being driven by DCC circuitry or a simple DC controller, so the rotational speed of the motor must still be dictated by DC voltage (and also DC pulse width for DC pulsed supply). The decoder creates the DC from the DCC square wave by full rectification. The square wave varies from negative to positive. The advertised spec of Prodigy Elite psu is 10 amp/ 15 volts. By the time this has gone through the central controller, I guess the track will see a max of under +/- 14 volts square waves. I just don't see how any fancy circuitry on a small DCC chip can generate higher voltage. Clearly the amps that the DCC controller must supply is higher for G gauge in order to provide sufficient power. But I still don't see how the higher speeds of the loco are achieved if the max volts reaching the motor is only 14 volts.

I suppose it may be the the way that manufacturers quote the voltage. I use a pretty accurate true rms meter. I suppose I am going to have to get a storage scope (not too expensive nowadays if not Tektronix) in order to see what is actually going on. And scope able to store umpteen samples.

Thanks
 
Maybe the output voltage of the MRC system is adjustable? My NCE system has adjustable voltage, as do other systems I've encountered such as Massoth and CT Electronik.

My NCE is set at 18V and I find that is plenty for the top speeds I want with my locos (various types and manufacturers).

<edit>hmm... the manual for the Elite says max input voltage 16V dc, output 14.5V. It can display the track voltage but I see no mention of an adjustment function - only a current limiter.</edit>
 
The signal sent to the track is 'data' used by the decoder, not the actual voltage sent to the motor..
If you have +/- 18V then you have the potential (pun intended) to have a total voltage of 36V. - It is all relative to what you measure from..
The output to the motor from the decoder is PWM DC. - That is why analogue loco's make such a horrible noise on a DCC track.
NOTE: Do not let analogue loco's sit for any length of time on a DCC powered track. Run one if you must, then remove it from the track.
 
As mentioned in another post, it all works by magic anyhow - as proven by the fact that if the magic smoke escapes, it stops working - so don't sweat the theory too much, just enjoy running trains! ;) ;) ;)

Jon.
 
From memory, I measured across the rails with my LGB MTS2 as 18V, using my DMM.

Can't remember if this was DC or AC range but I understand that it is just a modulated signal on a DC level.

If it was AC, it would have very strange effects on an analogue loco.

I am not sure what the Progidy Advance 2 output would be, but it would be interesting to measure across the rails with a DMM on both DC and AC ranges to get an idea....
 
DCC is AC voltage. As it's square sine wave you can't get an accurate measurement with a multi meter. They are useful for checking voltage drops etc tho as they consistently read the voltage incorrectly.
 
ernie said:
. .

The motors in most locos are DC, usually now 5 pole to make them perform smoother. Their speed of rotation is primarily achieved by increasing the voltage, . . . As the voltage increases, so does the loco speed. And so will the amps (not actually measured by me).

. . .
Click to expand...

I'm not sure that your assumption about speed, voltage and current is the full story.

I observed the following on my Massoth 1200Z set at 24V output.

The DCC voltage is ‘rectangular'. It can be measured with a true rms meter, such as my Fluke 175, and reads 24V. Thats AC of course. Zero Volts on DC as required by the NMRA Standard.

Best of luck trying to capture the waveform on a storage oscilloscope. Its not regular (periodic) and finding a reliable trigger is beyond me. And that’s using my Tektronix (TDS2001C 50MHz). What I have found is two horizontal lines, representing the 24V peak voltage with varying (random) vertical lines between. These vertical lines indicate switching of polarity and presumably carry the intermittent information to control the decoder.

When set for analog operation the voltage remains at 24V peak to peak, but the duration of the ‘positive’ and ‘negative’ parts of the waveform is varied to give a resultant DC component. At no time does the voltage exceed 24V (in accordance with EU safety regulations and those of other jurisdictions). The Fluke 175 will measure this DC component when set to DC voltage.


With regard to speed of a DC motor, its been a very long time since I ‘lab tested' one, but in practical terms on full size DC traction motors top speed is obtained by switching to ‘weak field’. Many model DC motors are permanent magnet and this facility is not available, but theoretically its not just the applied voltage that determines speed. If I recall correctly (it was a long time ago) there is little difference in applied voltage at various speeds. Its the ‘back emf’ that varies. The mechanical load on the motor (and hence the motor current) is probably the most significant factor. It may appear that you have to turn up the output of a DC controller to compensate for hills or other loads but I think you will find that this is actually to provide the increased load current. On DCC of course it depends how much, if any, back emf control is programmed.

I finally came to the conclusion that so long as I avoid 'magic smoke' then I'm happy to tolerate 'magic speed control'!

As a matter of good engineering practice its a good idea to restrict the applied voltage to reduce electrical stress on insulation, but particularly with dirty track outdoors you may get more reliable operation by increasing the voltage.

Don
 
[quote author=shropshire lad link=topic=299272.msg321536#msg321536 date=1405604563]
DCC is AC voltage. As it`s square sine wave you can`t get an accurate measurement with a multi meter. They are useful for checking voltage drops etc tho as they consistently read the voltage incorrectly.
[/quote]

A &quot;square&quot; sine wave is a contradiction.

The DCC signal looks like this

DCC Signal NMRA.gif
 
Gosh, thanks to all for your input.

I think the bottom line for me (as ntpntpntp mentioned after looking at MRC Elite spec) is that the MRC Elite is not the one for me. So, I'll just have to get a different handset and system from another manufacturer. Ho well.

I did seriously consider NCE as being the king of DCC (sophistication wise) for N and OO but, if you decide to go wireless, you are in trouble with radio regs in the UK. So, I went for Gaugemaster (ie MRC badged) as doing most things and really easy to program. Also, Gaugemaster is well known in the UK for excellent service.

Muns, I see both of us refer to S 9.1 spec. That was the first thing I did when choosing which DCC for N and OO.

Don, when I was at college during the seventies, each student got their own set of equipment and storage scopes were really expensive and not necessary for my research. Things have changed now and DSO's are relatively inexpensive. I thought with a digital storage scope, one can get it just to sample amplitude at the freq you want, up to its max bandwidth. So, why the need to trigger? Am I missing something. I'd better check with a supplier as I agree trying to trigger on a varying waveform will be hopeless. I would need to sample at say 8 times highest expected signal waveform freq (inverse of 104 usec) in order to see a reasonable shape. Twice max freq would just show the freq existed (nyqvist).

Gizzy, my Prodigy Advanced 2 PSU gives 14.5 V DC when open circuit (label says 15 V :)). I put a Hornby Sound Ready 0-6-0 Diesel Shunter on my rolling rails. Using my true rms DMM , gave 13 V AC when loco was going full tilt and sound on. Switching sound off, a slight increase of <.1 V and when motor also at a standstill, increase to 13.4 V AC. All measurements of V DC were +/- 7 millvolts. The stated amps for this PSU is 3.5.

So now, I will need to get manuals for real G gauge systems, much reading and then decide.

Thanks to all
 
ernie said:
For my N and OO gauge, I use Gaugemaster Prodigy Advance 2 (squared). This is actually MRC system badge engineered. I am used to using it and I like it. Now I see that MRC have a Prodigy Elite that comes with a 10amp supply. MRC say "it handles the high power demands of G gauge, while being able to safely run HO, N and other gauges".

But I cannot see how it handles say 20 volts (or even more) that G-gauge should get and also be suitable for HO and N gauge?

Am I missing something?

Thanks
Click to expand...

I looked up the manual for the Prodigy Elite it says it puts out 14.5v at up to 10amps. I wouldn't buy that system if my primary need was to run G since it doesn't allow the output voltage to be varied as many systems do.

The manual for your existing Prodigy Advanced 2 says it puts out 15v at 3.5 amps. While the voltage is somewhat low for G this would still be able to run a few small efficient G scale locos and you might just want to stick with it on a trial basis before splashing out on a different system. You wouldn't be able to run locos at top speed but most of the time you would never want to do that anyway.

In your search for a new system I would recommend looking at all DCC systems, not just those traditional seen as G scale DCC systems.
 
Yes, that is an excellent plan to try out my existing MRC DCC on my new g scale first. I have 2 lgb starter sets (2 x 78302, wanted a rake of 4 red and cream double axle coaches :) ). and am collecting a piko starter set 37120 today. From my reading, I understand all have efficient motors. This experience will certainly be very helpful when I start on my better g gauge DCC search.
 
Sounds like a good plan; the two LGB Stainz locos with their 7-pole Buhler motors shouldn't draw more than about half an amp each under normal running conditions; the Piko starter set loco probably has a 5-pole motor (though Piko are moving to 7-pole Buhlers too on their more expensive locos) and might draw a little bit more than the LGB, but certainly not over an amp.

Jon.
 
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