DCC Explained

[VicZA]

Hi All,

 

I am new to the hobby so learning as I go.

 

Reagrding DCC: is there a "stcky" somewhere that explains the concept? 

 

I amstruggling to understand the practical difefrence between locos that are sold as:

(a) DCC Ready

(b) DCC Fitted

© DCC Sound

 

Also, what is required in the way of wiring up and powering your tracks?

 

Thanks

Vic

[johnb]

Hi All,

 

I am new to the hobby so learning as I go.

 

Reagrding DCC: is there a "stcky" somewhere that explains the concept? 

 

I amstruggling to understand the practical difefrence between locos that are sold as:

(a) DCC Ready

(b) DCC Fitted

© DCC Sound

 

Also, what is required in the way of wiring up and powering your tracks?

 

Thanks

Vic

 

(a) DCC Ready  -  Loco has a socket to fit a DCC chip, but does NOT have a chip, will run on analogue (DC) supply but NOT on a DCC supply

 

(b) DCC Fitted  -  Loco has a chip fitted and will run on a DCC supply. If the chip is suitable and set to do so it may run on DC.

 

© DCC Sound  -  As it says it has a loudspeaker inside and makes noises, steam, diesel, whistles, horns, etc usually one appropriate to the model.

 

Wiring up, well search through the threads on here there's lots on wiring. There's lots of views, but I and others have been converting a DC layout to DCC by the simple method of removing the DC supply, plugging in a DCC supply and switching all the sections on. DO NOT try to mix DC and DCC on the same layout!!!

 

John

[Bill37]

Some very useful information in here;

 

http://www.brian-lambert.co.uk/index.html

[Phil S]

Wiring up, well search through the threads on here there's lots on wiring. There's lots of views, but I and others have been converting a DC layout to DCC by the simple method of removing the DC supply, plugging in a DCC supply and switching all the sections on. DO NOT try to mix DC and DCC on the same layout !!

Phil:  IT IS VITAL if converting from an ANALOGUE (DC) layout that has used 'commercial' Power Feeders' to REMOVE the Radio Interference Suppression Capacitors fitted to them** - as these would appear as a short circuit to the (alternating current) DCC signal  !!!  [ just cut them out ! ]

(If power feeds have been 'home made', they are unlikely to have the RF suppression capacitors which commercial vendors are required to include to comply with Radio Interference Regulations   **[or replace them with the 'digital version']- which is the 'same' but without the capacitor included

 

Also,  for your continued safety: Since many DCC Controllers are designed to power more than one train at a time, they may have higher MAXIMUM outputs than a typical analogue controller: THEREFORE the wiring should be checked that it matches the capability of the chosen controller !!!   IF a '3 - 4 or 5A' output controller is bought, then the wiring should be capable of continuously conducting that current without getting hot .... and the layout-track-and-wiring should have a sufficiently low resistance so that the 'trip' current (which will be ABOVE the normal 'maximum' rating ) CAN FLOW and therefore cause the electronic cutout to operate instantly (-also known as the 'coin' test - place a coin anywhere on the track and the resulting short circuit should cut the power immediately ).

 

There are 'starter' control systems which only have about 1-1.5A output (dcc) which is close to the 'presumed' 1A of an analogue loco and controller - and these may be the layouts where users have reported 'successfully using their old analogue wiring - perhaps just switching all sections on at once' ..... but if they had ALSO changed to a multi-loco capability control system with 3-4-5- or even 8 or 12A capability... then they would need to reconsider / test their wiring for the new operating conditions: of more than 1 loco in any 'section', and the need to protect the layout/stock  with the fast electronic system cutout   (no slow acting  thermal cutouts for dcc)

 

But don't be put off by the simple checks required above - just remember that 'DCC' is an AC (alternating current / voltage - ie bidirectional signal of 'typically' 12-16V almost a square wave *max 22V .... but because it is a square wave, most cheap voltmeters will give misleading values for the track voltage.

[DCC stands for Digital Command Control and has NO CONNECTION to 'DC' direct current whatever]

 

ALL DCC-fitted locos/devices have a DECODER which both converts the incoming dcc square wave ac voltage to dc to power the microprocessor and drive the motor etc, and contains the digital information to identify the loco, its speed, direction etc. 

BEWARE that 'old controllers' of the 'pre-electronic era' - such as the 'popular' H&M Duette, Clipper etc have outputs which are LABELLED as '12V or 0-12V (Triang was labelled 0-14V) - the ACTUAL PEAK voltage which may appear can be 28V* at 100Hz (full wave rectified, unmoothed ) - and this will NOT allow a decoder-fitted loco to operate properly,and may cause damage due to the high voltage

*my measurements.

 

One of your best early investments is a good digital multimeter - it will soon repay it own cost in save decoders  etc.

[johnb]

Phil S makes a good point about the number of locos being run. The DC layouts that I and others have 'converted' to DCC have all been on the small side and would not see more than about three modern locos operation at one time. Generally we have used DCC controllers with a 2 amp max capability . I use an NCE Powercab (with a 1.4 amp power supply) which is set to read the amperage being drawn and even with three modern locos running I have yet to see the current approach 1 amp.

 

Certainly big layouts with lots of locos running need careful consideration in the wiring arrangements.

[WIMorrison]

You also need to consider the current drawn by any accessories that you may have such as points, signals, lighting.

 

Also be aware that locos that have sound and lighting draw a greater current to service these capabilities and it is easy to reach a higher current draw without realising what has been done

[mikeg]

Hi I have recently fitted a rameter to my layout and I can say that with 12 colbolt point motors, 6 O scale locomotives fitted with sound chips (not running) that the current draw is 0.75 to 0.9 amps and with 1 loco running it goes up to 0.96 amps so I do not think that unless you have some very high current draw locos you’re likely to have any problems.

 

Mikeg

[cravensdmufan]

As well as Brian Lambert's excellent site, I have found DCC Wiki useful.

 

https://dccwiki.com/

 

CDF

[Junctionmad]

A few things ,

 

While the ability of the wire to carry the full rated DCC short circuit current is good advice , it is rather overkill and can mean in theory, the droppers become impractically large. For 4mm scales the largest practical size is around 7/0.2 , for O 16/0.2.

 

Most DCC short circuit limiters work very fast , hence the wire has only to carry the fault current for a few milliseconds , hence these wires are suitable . However the DCC “ bus” wires should as large as practical , I use 2.5sqmm cable

 

However if you have a large layout or for whatever reason a large capacity DCC system, you need to consider breaking up the layout into DCC “power districts “ and installing DCC cutoffs locally in each power district, this keeps the individual power districts short circuit currents within manageable levels.

 

Note that DCC is more correctly called a DC pulse train , it may or may not have alternating feeds , if it has it’s an AC pulse train .

 

The voltage is specified as peak to peak and the NMRA publish reccomended voltages for each gauge , this is 15v for HO/ OO, ( that’s not 20 v peak, just 15 )

 

A multimeter will give you an inaccurate voltage reading on DCC , what it will read will be a function of the individual meter

 

Note that most DCC decoders don’t produce DC to the motor from the DCC signal , what they typically do is rectify the DCC , to DC , then chop that DC into a PWM ( pulse width modulated ) AC signal and feed that to the motor

[VicZA]

Wow! as always fantastic answers and you really take time to give a lot of detail ... THANKS VERY, VERY MUCH .... got a lot of reading to do

[peach james]

A few things ,

 

While the ability of the wire to carry the full rated DCC short circuit current is good advice , it is rather overkill and can mean in theory, the droppers become impractically large. For 4mm scales the largest practical size is around 7/0.2 , for O 16/0.2.

 

Most DCC short circuit limiters work very fast , hence the wire has only to carry the fault current for a few milliseconds , hence these wires are suitable . However the DCC “ bus” wires should as large as practical , I use 2.5sqmm cable

 

However if you have a large layout or for whatever reason a large capacity DCC system, you need to consider breaking up the layout into DCC “power districts “ and installing DCC cutoffs locally in each power district, this keeps the individual power districts short circuit currents within manageable levels.

 

 

 

 

I'm missing something here...

 

The droppers have to be able to carry the current without enough resistance to allow the short circuit protection to work.  Otherwise, you WILL end up with a situation that may result in a fire.  (ask me how I know...).  What has to happen is that the DCC circuit limiter must see a high enough current to trip.  The droppers can be quite small- I use 22 AWG wire for the droppers, which is small enough to put into the web of code 75 rail.  The droppers should connect to quite a bit heavier wire in as short as reasonable distance.  I have a bunch of up to 2' long droppers which then connect to 14 or 16 AWG wire.  You do NOT need to break up into power districts until you get to drawing 3-4 amps on a regular basis.  (or at least, the power of the booster that you are using).  I sometimes run afoul of this, using a lot of illuminated buildings with the Lego.  

 

James

 

(22 awg= .33mm^2)

(16 awg= 1.31mm^2)

(14 awg= 2.08mm^2)

[WIMorrison]

Breaking a largish, or complex layout into power districts is very sensible and isn’t related to the current drawn, it is related to the ability to isolate parts of the layout from each other either to trace a fault or to enable work to be undertaken on one part whilst leaving the tea of it running.

 

When you start drawing significant current - somewhere around 75% of you command station capacity then you need to feed the power districts with booster supplies to support the number of locos running. A booster, when configured correctly can also provide the ability to fault find or undertake work by isolating a section of the layout.

[Phil S]

Oh Dear - Junction Mad wrote: "A few things ...," which need a correcting or clarifying response to avoid future confusion or problems

"While the ability of the wire to carry the full rated DCC short circuit current is good advice , it is rather overkill and can mean in theory, the droppers become impractically large."

Phil: '(Over)KILL is what I wanted people to Avoid ! - Seriously !  YOUR WIRING MUST MATCH the capability of the the SOURCE ( here the Power Supply to,  and output from, the DCC Central Controller OR Booster if more than 1 'Power District' (NMRA Terminology).

 

FAULTS WILL OCCUR (despite what some people appear to write) - and the (fast) internal protection of the Central Controller is designed to protect both the User AND the equipment in that case: by stopping the current flowing ONCE THE EXCESS HAS BEEN DETECTED ....  This is why I drew attention to the importance of realising that this is related to YOUR CHOICE of Controller and is also of its MAXIMUM NORMAL** (Continuous maximum) Current output Rating:  For some controllers this may be only 1A, but for others it might easily be 3,4,5,8, 10 or 12 Amps. {**This has nothing to do with how much current your layout is usually taking }

 

YOUR WIRING (and your responsibility) MUST ENSURE that this FULL MAXIMUM current can flow CONTINUOUSLY to ALL PARTS of your LAYOUT [ EXCEPT beyond  Local sub-district breakers giving a lower local limitation: THIS IS THE EQUIVALENT of the Mains Electricity reaching your house [100A continuous capability]  and then passing through Fuses/RCCBs with 5A, 15A 30A etc breakers for the local sub-district circuits beyond (eg lighting, ring main, cooker)  AND ALSO for the 30A Ring Main with 13A Sockets, via Plugs (3A-13A fuses) to devices which then have smaller protective fuses inside them .... thinner wire being acceptable in each of the later stages providing it can take the  (Protected) MAXIMUM Rated current CONTINUOUSLY.   

Note that an old fashioned fuse is a thin piece of wire - designed to melt in an enclosure which will not catch fire.  A 'Dropper' ( or should that be 'riser' wire 8-) )  is short, and is connected to adjacent metal which helps conduct heat away from it - its resistance is. or should be 'negligible' - especially in comparison to the TOTAL RESISTANCE of the  Electrical LOOP from the controller via busses  to ANY PART of the track.   FOR THE SHORT_CIRCUIT PROTECTION to WORK - the time taken is irrelevant to this calculation -  V=IR.  Simple Ohms Law:   

Example: If the NORMAL MAXIMUM CURRENT is 1A, and the TRACK VOLTAGE  12V: From V=IR ...  R= V/I  ...   12/1 = 12ohms BUT

IF THE TRIP CURRENT is then 2A (just 1 A or 100% over 'normal maximum')  then from V=IR...  Rmax = V/I ...  12/2    <  6 ohms MAX

 

BUT if a larger output controller is chosen -  eg 5 Amp Normal Maximum Output:  From V=IR ... R= V/I ... 12/5  =    2.4 ohms BUT for the

SHORT CIRCUIT DETECTION -  requiring (even a MOMENTARY current of )  say 7 A or 10 ..:  R= V/I ... 12/7 or 12/10 ohms < 1.2ohms

As a check: My garden line (125m total) has a loop resistance of 0.5 ohms .  20V = I x 0.5 .. would allow a 40A short circuit current to flow  [ Note also, that the slightly higher track voltage allows for more resistance in the track, and motor currents are proportionally lower ]

My 00/H0,H0e and G-scale portable layouts all use 16Vdcc [ by which I mean 0-peak measured by 'scope or Rrampmeter etc ]

 

For 4mm scales the largest practical size is around 7/0.2 , for O 16/0.2.

I do not mention any size of wire, particularly for the final connection to the track - as the scale is not relevant to the electrons heating up your copper wire as they flow around.

However, as a GUIDE, the quoted ratings on wires sold for 230Vac use are based on the heating effect/dissipation when embedded in a plaster wall or other appropriate environment -  For my loft layout I use '3A' or '6A' twin flex (using the coloured outer sleeves for identification)  - PROVIDED the circuit passes the 'coin test' as calculated above -- AND ALSO checking for less than a couple of volts drop in any section when  'the maximum load'  is present in the loop --- decoders might not work below 7V (many current designs do -  this is not usually a problem if the 'coin test' has been passed).

 

 

Most DCC short circuit limiters work very fast , hence the wire has only to carry the fault current for a few milliseconds , hence these wires are suitable . However the DCC “ bus” wires should as large as practical , I use 2.5sqmm cable

Phil: ALTHOUGH - 'heatwise' they SHOULD only be carrying the EXCESSIVE current for a few millseconds - if the LOOP RESISTANCE is TOO HIGH then the TRIP VALUE will NEVER be reached ....and heating will continue - revealing itself first at the weakest part 

{My experience of this was a failed internal switch inside a Fleischmann point - it was taking a continuous 3.5A - and therefore NOT tripping the ZTC511 Controller with a 5A normal-maximum continuous output capability - result - 1 melted point} 

 

However if you have a large layout or for whatever reason a large capacity DCC system, you need to consider breaking up the layout into DCC “power districts “ and installing DCC cutoffs locally in each power district, this keeps the individual power districts short circuit currents within manageable levels.

Phil: Agreed except for the 'However' - the SUB-DISTRICTS (if from the same power supply) or POWER-DISTRICTS ( own power supply ie Controller and Boosters)  are desirable to assist FAULT_FINDING by Localising the problem: 'divide and conquer' - and are best chosen for that purpose, and not  for historical 'cab-control' sectionalising reasons. In the same way your house has a 'consumer unit' with a range of circuit breakers as described earlier, I strongly recommend the use of PSX or similar intelligent circuit breakers because it permits 'more modest' wiring  BEYOND the protective device  BUT especially once Sound and lighting is used as these are designed to cope with their surges ( in the same way as there are Type 1,2,..3  circuit breakers available)

 

Note that DCC is more correctly called a DC pulse train , it may or may not have alternating feeds , if it has it’s an AC pulse train .

The voltage is specified as peak to peak and the NMRA publish reccomended voltages for each gauge , this is 15v for HO/ OO, ( that’s not 20 v peak, just 15 )

Phil: Where does this mish-mash of confusion and error come from?  

IT IS NOT HELPED by a COMMON ERRONEOUS ASSUMPTION that 'dc' means a smooth steady single value supply like from a battery, or that 'ac' means a Sinusoidal waveform like the Mains Electricity supply:  these are both just specific EXAMPLES of dc & ac.

There are many situations where CONTEXT is the best selector of the terminology quoted- and it usually depends on HOW YOU PROCESS the SIGNAL  - as 'analogue' or 'digital' for example.

 

DC - Direct Current  = Flowing in one direction only.  It may vary in value - slowly or quickly, turned on or off or even get reversed in polarity (direction of flow) in some devices - CONTEXT is used to select some aspects of the terminology to best  relate to the environment OR PROCESSING in which it is encountered. DC 'can be considered' as AC with a frequency of 0 Hertz if more convenient.

 

Simply put: BLOCKED IN ONE DIRECTION by a DIODE ('semi-conductor) - BLOCKED (after charging up),a CAPACITOR,but passed through un-impeded by an INDUCTOR.  The type of input required by a motor-with commutator for continuous rotation to take place.

 

AC - Alternating Current = Flowing in both directions - A Diode in series will remove the reversed-part of the current,  and a capacitor across the supply will be seen as a short-circuit  (which is why the interference suppresion capacitors NEED to be REMOVED for dcc)

 

So a Battery Voltage is 'dc' - pure and simple.   Sound pressure waves are 'AC' ( Barometric Pressure is 'DC' ) A loudspeaker coil responds ONLY to the AC content of the 'sound' waveform applied and an old analogue TV/video waveform is 'DC' although it varies from (below black) to white. -   'DC restoration' (clamping) was required to maintain black  as black on a TV when the average value varied.

 

A Modern Electronic Controller - such as that from Gaugemaster or Roco, produces a 'dc' output which is a series of pulses of varying width and/or amplitude - to apply to a motor with a commutator (that requres a 'dc' supply.) - a diode could be placed in series,and the motor would still be able to run in that direction  - but not the other way.

An old H&M Duette produces a rectfied-sine wave shape waveform - peaking at 28V 100x a second - but is labelled 12Vdc by H&M..

Under load, at  'maximum speed' the average voltage is 12V, and, for any one direction of the control  knob it is DC - one direction - but certainly not smooth  because it ALSO HAS a (large) AC component.

 

A DCC WAVEFORM IS AC - it is a series of pulses at a frequency a bit below 10,000 Hz -  alternate pulses cause current to flow in opposite directions. THE AVERAGE VALUE  is also ZERO   ( ie NO 'DC COMPONENT) - except when/if a 'Loco 0' unfitted analogue loco is being driven - but this is not recommended  and not allowed on many systems.  

 

It is a 'square wave' in that it has fast-rising edges whose timing indicates the data it s carrying - although,as with ALL devices, the rise is controlled to  minimise interference etc.    It is 'digital' in that the duration of the pulses is measured by the receiving decoder, and classified as either a    '1' or a '0' .  In the Decoder, the track AC signal is Full-wave rectified to produce a DC power supply for the decoder to operate.  From this internal supply,  Function outputs and the MOTOR can be controlled - as stated by PWM for efficient variable speed  - and this is commonly via an 'H-bridge - whereby swapping which pair of ( 4 )  transistors are used, the direction can be selected.  However,decoders are also available to drive Servos and AC motors - this is NOT a requirement.  Many european decoders are also multi-protocol and multi-standard for motor types.

 

THE VOLTAGE may be measured and described EITHER as a 0-peak value   eg 12V   16V up to a maximum of 22V,  or, as is common on an oscilloscope; as peak-peak which would be double these figures.   There is no Fixed DCC track voltage specified in the NMRA standard  - just recommendations and a maximum limit (23V) which keeps it on the safe side of certain regulations. Prior to modern electronic controllers and switched-mode power supplies, Motor/Track Voltages varied considerably - eg Tri-ang controllers stating 0-14V BUT also being higher off-load due to lack-of voltage regulation.    RocoDCC default outputs 16V (dcc0-peak) which gives a maximum of about 12V at the motor. 

 

The control-data of DCC is carried by the MODULATON of the voltage using 'FM'/ Bi-phase-Mark or Manchester1 or other names which can describe the idea of a 0 and 1 being sent by pulses of (2) different durations - 1 half the other  (Except during 'Loco0'mode )

 

[HOW a waveform is BEST described usually depends on HOW YOU intend PROCESSING IT !! - For example, 'Digital TV Transmissions'  are transmitted by a process which has some very analogue characteristics !!! - which can be decoded or displayed as a 'grid' of 16 x 16 dots .. "256 values"' - not just 2 binary levels ....and therefore, if you have too much amplification between your aerial and TV receiver - such that old-fashioned 'clipping' occurs - then your TV will show good signal strength BUT low quality - and be unable to decode the picture 8-)]   { And different ,TV multiplexes use different settings - so reception of some is more difficult than others )

 

A multimeter will give you an inaccurate voltage reading on DCC , what it will read will be a function of the individual meter

Phil: Agreed with many 'cheap' digital meters which are only calibrated for Sne Waves, and a t 50-60Hz. eg Flukes are accurate on DCC.

Note that most DCC decoders don’t produce DC to the motor from the DCC signal , what they typically do is rectify the DCC , to DC , then chop that DC into a PWM ( pulse width modulated ) AC signal and feed that to the motor

Phil: Earlier the writer wanted to call the AC DCC waveform DC, and now appears to be saying the feed to a dc motor is 'ac' !!!

As described above - PWM chopping DC as in most decoders results in a DC waveform to the motor - many now operate at ultrasonic frequencies ( early decoders continued the 50-60Hz (100/120Hz) mains relationship as with Zero-1 - producing noticeable 'jerks' n the motor, and unsuited to coreless motors ) - this has now allowed smaller components to be used - saving space. The motor's inductance helps smooth out the pulses.   In between the 'digital' pulses, as the motor continues to spin,it acts as a generator - creating a voltage - a back emf - and this can be measured by the decoder to trim the speed - this is an example of analogue measurement on-board by an A to D converter

 

I never suggest a particular size of wiring - as the real question is does it meet the NECESSARY REQUIREMENT - and the first requirement is your continued safety - and simply checked with a 'coin test' short circuit - does it protect ???  THIS TEST can be done on a reel of cable .... if the 100m reel passes the test - then any part of it will meet the test.  However - don't forget the resistance of Nckel Silver Rail - especially in smaller gauges / 'code' eg 55 or 80 in N,  75,80,100,120 in 00/H0 332 in G and in brass - a better conductor!!

Edited September 13, 2018 by Phil S

[Crosland]

Different diameters of wire have different resistivity. Thicker the wire, the lower the resistivity.

 

The resistance of a piece of wire is resistivity x length.

 

It should be immediately obvious that a short, thin, wire can have the same low resistance as a longer, fatter, wire. That's why droppers can be quite thin, even down to 1/0.7 if they are kept short, without affecting the ability of the booster or cut out to detect an overload or short circuit.

 

Pedantically, ANY waveform can be broken down into DC and AC components superimposed on each other. Most sources of "pure" DC will have some very small AC component, generally referred to as noise or ripple.

 

DCC is definitely AC when all you have to measure are the two rails (e.g. from the point of view of a decoder). There is no DC component in any sense (unless you are drivine a DC loco on address 0 - not recommended these days).

 

The PWM to the motor can be thought of as a DC pulse train. It can also be though of as AC with a DC component of half the track voltage.

[Crewlisle]

Vic,

 

I converted my 00 gauge 8ft x 7ft 6ins three level DC layout 'Crewlisle to DCC 10 years ago.  The attachment is the article which was published in October & November Model Rail about how I converted 'Crewlisle' from DC to DCC.  I did not go into the technical details as in previous comments in this forum but what I have written has given me trouble free operation for the last 10 years in spite of dismantling & reassembling for numerous exhibitions.

 

My loco numbers have now increased to 52 & I have split the layout in to two power districts using two DCC Supplies PSX-1 modules - one for the high level & one for the two lower  levels.  This means that if there is a short/accident on one level, the other section can still be operated.  Very useful at exhibitions!

 

TO DCC OR NOT TO DCC.doc         1640 Edit:  See Comment 24 where you can read the complete article with photos.

 

Peter

 

.

Edited September 14, 2018 by Crewlisle

[Junctionmad]

Oh Dear - Junction Mad wrote: "A few things ...," which need a correcting or clarifying response to avoid future confusion or problems

"While the ability of the wire to carry the full rated DCC short circuit current is good advice , it is rather overkill and can mean in theory, the droppers become impractically large."

Phil: '(Over)KILL is what I wanted people to Avoid ! - Seriously !  YOUR WIRING MUST MATCH the capability of the the SOURCE ( here the Power Supply to,  and output from, the DCC Central Controller OR Booster if more than 1 'Power District' (NMRA Terminology).

 

..............

 

I never suggest a particular size of wiring - as the real question is does it meet the NECESSARY REQUIREMENT - and the first requirement is your continued safety - and simply checked with a 'coin test' short circuit - does it protect ???  THIS TEST can be done on a reel of cable .... if the 100m reel passes the test - then any part of it will meet the test.  However - don't forget the resistance of Nckel Silver Rail - especially in smaller gauges / 'code' eg 55 or 80 in N,  75,80,100,120 in 00/H0 332 in G and in brass - a better conductor!!

 

 

 

 

YOUR WIRING (and your responsibility) MUST ENSURE that this FULL MAXIMUM current can flow CONTINUOUSLY to ALL PARTS of your LAYOUT [ EXCEPT beyond  Local sub-district breakers giving a lower local limitation:

 

Firstly wire ratings are based on  ohms/Km resistivity rated at a certain temperature , for given ambients and derated for bundling , There are many guides on the internet that will guide you to properly evaluate temperature rise in wires carrying current , so that you can size a wire correctly and allow a certain rise in temperature above ambient in fault conditions.

 

Layout are not mains and its incorrect to transfer " code" from one industry to another 

 

 

There are several ways ( including car light bulbs etc ) , and commercially available DCC breakers that allow you to segregate your layout so as to reduce short circuit currents from a potentially large booster.  I pointed this out ( i.e. power districts , or local district cutoffs) 

 

 

 

Example: If the NORMAL MAXIMUM CURRENT is 1A, and the TRACK VOLTAGE  12V: From V=IR ...  R= V/I  ...   12/1 = 12ohms BUT

IF THE TRIP CURRENT is then 2A (just 1 A or 100% over 'normal maximum')  then from V=IR...  Rmax = V/I ...  12/2    <  6 ohms MAX

 

BUT if a larger output controller is chosen -  eg 5 Amp Normal Maximum Output:  From V=IR ... R= V/I ... 12/5  =    2.4 ohms BUT for the

SHORT CIRCUIT DETECTION -  requiring (even a MOMENTARY current of )  say 7 A or 10 ..:  R= V/I ... 12/7 or 12/10 ohms < 1.2ohms

your knowledge of ohms law is impressive 

 

talking a typical 00 dropper, 7 strands of 0.2mm wire , this has a resistivity of approx 0.04 ohms metre at 20 degrees and  is rated for 85 degrees , hence you are at over 30 metres before you run into I2R heating issues ( and its I2R thats determines the short circuit carrying capacity  as its directly related to temp rise ) 

 

I would suggest that 30 metres in DCC is one hell of a dropper , garden railways are a different kettle of fish entirely 

 

The fact is that overall loop resistance is far more likely to be determined by resistance at soldered joints and  things like frog switches/blade contacts  then it will be by dropper resistance . Its these that will more then likely overheat rather then a couple of metres of dropper 

 

 

Note the " coin Test " is a moniker used in DCC , its has little relevance . The test is a simple short circuit test , The point being as you  point out in a very long piece , is to simply generate a short circuit test , i.e. that the booster trips. Only in situations at the extreme , will there  likely to be a problem ., Someone running 30 metres of 7/0.2 wire in a DCC bus clearly doesn't understand the fundamentals and is likely to have all sorts of issues 

 

Note that the short circuit test ( the badly named coin test, as I use a test lead with crocodile clips ) will show up that the droppers are the least problematic area ( being high quality commercially made conductors) , Nickel sliver rail is a comparatively poor conductor etc 

 

Again I never mentioned a short circuit test , and of course any dcc layout should pass a simple short circuit test anywhere on the layout , I was not addressing that issue 

 

 

 

Most DCC short circuit limiters work very fast , hence the wire has only to carry the fault current for a few milliseconds , hence these wires are suitable . However the DCC “ bus” wires should as large as practical , I use 2.5sqmm cable

Phil: ALTHOUGH - 'heatwise' they SHOULD only be carrying the EXCESSIVE current for a few millseconds - if the LOOP RESISTANCE is TOO HIGH then the TRIP VALUE will NEVER be reached ....and heating will continue - revealing itself first at the weakest part 

{My experience of this was a failed internal switch inside a Fleischmann point - it was taking a continuous 3.5A - and therefore NOT tripping the ZTC511 Controller with a 5A normal-maximum continuous output capability - result - 1 melted poin

 

exactly the issues are rarely experienced in the dropper wires , but elsewhere in high impedance situations often not immediately obvious , I was merely addressing the dropper wires 

 

 

 

A DCC WAVEFORM IS AC - it is a series of pulses at a frequency a bit below 10,000 Hz -  alternate pulses cause current to flow in opposite directions. THE AVERAGE VALUE  is also ZERO   ( ie NO 'DC COMPONENT) - except when/if a 'Loco 0' unfitted analogue loco is being driven - but this is not recommended  and not allowed on many systems.  

as a professional electronics engineer , with 30 years experience , Let me TELL you the definition 

 

AC . means ALTERNATING CURRENT , if the current does not change polarity within defined wavelength IT IS NOT AC , Hence a rectified AC , which still has a full waveform , is DC , not AC , even though on a scope it is not " smooth " 

 

DC : means DIRECT CURRENT , i.e. , current flows ( over a time greater then any waveform ) in one direction 

 

frequency ( or voltage )  has NOTHING to do with it 

 

MOST definitely NOT is DC,  AC with a frequency of 0 , such a supply was never AC to begin with 

 

 

Hence  DCC can be implemented in a number of ways , including being supplied by a DC single sided power supply , hence the current waveforms in the supply are DC pulse trains , it may be supplied by a supply above and below a designated ground , in which case the track power can be viewed as AC in respect of the designated ground , even if the power supply waveforms are DC in respect of earth ( for example ) 

 

The point is that it may be DC or AC pulse trains , depending on what you term as your frame of reference 

 

I will except to the uniformed , the terms AC means a waveform and DC means a smooth continuous feed, the definition and the reality is of course different 

 

Digital circuits ( for example ) are full of DC pulse trains , these are NOT AC waveforms 

 

DCC can be implemented from  a unipolar supply , which means for example it depends on where you take the measurement frame of reference , as to whether you have a AC pulse train or a DC pulse train 

 

Hence unless you have differential probes , if I put my scope on a rail with DCC and and the probe ground is at earth ( nominally 0V) on  the other side, what I am seeing in my frame of reference is a DC pulse train , even though from other frames of reference I might be seeing an AC pulse train 

 

 

 

THE VOLTAGE may be measured and described EITHER as a 0-peak value   eg 12V   16V up to a maximum of 22V,  or, as is common on an oscilloscope; as peak-peak which would be double these figures.   There is no Fixed DCC track voltage specified in the NMRA standard  - just recommendations

Nobody in a professional engineering circle describes digital pulse trains from the 0V point , almost everyone uses Vpp , simply because the ground position may be entirely arbitrarily ,( there may be no  " zero " position at all )  NOTE everything the NMRA does is a recommendation, it doesn't have the force of law, The NMRA spec is not well written in this regard as it assumes a bipolar supply and references everything to 0V  

 

MERGs DCC for example is generated by a H-Bridge from a unipolar supply  , so depending on how you determine your reference , its a AC signal oscillating around supply/2 or a DC signal oscillating above  power supply ground 

again the nomenclature all depends on what you determine the reference point is . IN other words, you can can an AC signal with a common mode voltage of Vpp/2 applied.   so it has both an AC and a DC components , yet to the decoder wired as it is , there is no DC component 

 

( this is why electronics is profession and not a hobby    ) 

 

Again I except that the general public see AC are a wave and DC as a smooth but again thats not the point 

 

 

 

The control-data of DCC is carried by the MODULATON of the voltage using 'FM'/ Bi-phase-Mark or Manchester1 or other names which can describe the idea of a 0 and 1 being sent by pulses of (2) different durations - 1 half the other  (Except during 'Loco0'mode )

DCC is differential Manchester encoding ( except for the stretched 0 ) bi-phase mark is the same term 

 

 

 

As described above - PWM chopping DC as in most decoders results in a DC waveform to the motor

 

 

 

what you can say is that the PWM signal tends to resemble DC, as an approximation , as the inductance effects are as you describe , it is however clearly not smooth DC 

 

 

 

 

I never suggest a particular size of wiring - as the real question is does it meet the NECESSARY REQUIREMENT - and the first requirement is your continued safety - and simply checked with a 'coin test' short circuit - does it protect ???  THIS TEST can be done on a reel of cable .... if the 100m reel passes the test - then any part of it will meet the test.  However - don't forget the resistance of Nckel Silver Rail - especially in smaller gauges / 'code' eg 55 or 80 in N,  75,80,100,120 in 00/H0 332 in G and in brass - a better conductor!!

This is a nonsense suggestion , most layouts are of reasonable size , and it would be ridiculous to suggest  carrying out short circuit test ( I really dislike the term coin test , as using a coin is not a good test at all , as the resistance in the coin and its contacts can be misleading )  on 100 metres reel of dropper wire , I have suggested particular wiring because I ​know that works  for the vast majority of typical layouts and doesnt leave people in confusion. If you were to seriously use 100m as a metric , dropper would be three or four times the necessary size . the fact remains , typical 7/0.2 used very commonly on layouts is about 0.04 ohms per metre rated at 20 degrees , with a cable rating of typically 85 degrees C derated for greater ambients and bundling , apply the calculations as appropriate  or accept that several metres of typical dropper wire is not going to be an issue 

 

as I mentioned I drew a distinction between dropper and DCC buses , again based on the typical  arrangement most people use for DCC layouts  i.e. a high power bus largely determined by voltage drop considerations and smaller cross sectional short ( 1-2 metres ) droppers 

 

electronics is a engineering discipline , it is not voodoo or rules of thumb , calculations are required , its why you attend college and then spend many years in practice . 

 

Again , as I have above ,mentioned that carrying out a short circuit test is a good thing , I never said it shouldn't be done. But the point is the droppers are typically the least area of concern in a short circuit test. The main reasons these fail are more to do with bad soldering , poor rail connections , frog switching contacts failing ( Ive seen fishplates glow) and other  issues , none of which I was addressing in my comments , and other poor installations . To focus on dropper wire alone is " overkill" and incorrect 

Edited September 14, 2018 by Junctionmad

[Sir TophamHatt]

While I appreciate some of the detail here, look at what the OP has asked and think about their level of skill.

 

I've been modelling for a couple of years now and I still don't understand much about wiring, resistance levels, ohms, wattage, blah blah blah.

 

OP: Look online at videos and ask people for specifics to what they use for wires.

Don't get bogged down with the detail unless it interests you. I bought wires, LEDs and power supplies that others have and it's served me well - without knowing all the resistance details, or what wires are what size or any of that stuff.

 

I have "bus wires", "dropper wires" and "chip wires" <-- three different sizes depending on what I need. That's my sort of level of technical skill compared with 7/0.2 thickness wire.

 

As I say, it's great going into detail but I don't think the OP will have a clue what some people are writing.

Edited September 14, 2018 by Sir TophamHatt

[Junctionmad]

just let me summarise for the average reader 

 

 

The layout must withstand a short circuit test . anywhere on the layout without any noticeable heating effects 

 

The short circuit test must actually cause a short circuit , if it doesn't you could have any number of issues 

 

 

Conventional DCC wiring regimes of large DCC busses connecting to shorter droppers , will undoubtably pass this test , special cases  like large single feed garden railways not withstanding 

 

Loop[ resistance issues ( leading to heating )  are more likely to appear in places other  then commercial wiring 

 

7/0.2 IS good for 00 , 16/0.2 IS good as a dropper for O and 7/0.1 if you can find it , is good for FS 2mm , this is based on average layouts upto about 35 feet by 15 ( with short droppers , i.e. < 3 metres ) 

 

2.5mm IS good for DCC bus feeds as a general comment , hence you can strip mains cable etc , ( albeit I dont like mains wiring colouring on a layout ) , 1,5mm square can be used as a bus on small DCC layouts 

 

testing droppers in 100metre rolls is nonsense 

 

if you are at the extreme , then apply the calculations , for example you might allow a rise to 50 degrees in the cable bundle ( or be conservative and only allow 10 degrees ) , the booster trip times can be factored in if you wish , You may want the layout to withstand the short circuit on a continuos basis , but this is a call , as in most cases your DCC booster will have burst into flames !! . again you can only do the calculations if you understand the design parameters . You will need to know things like the boosters actual trip current and trip time parameters.

 

Obviously the layout should operate with no discernible rise in cable temperatures ( i.e. <10 degrees above ambient ) , it should operate that way upto the allowable max continuous operating current in any given power district 

 

for larger layouts with high power boosters , consider breaking up the layout into power districts that have DCC trips or breakers, that are less then the full power of the single booster ( or use multiple smaller powered boosters ) , this keeps the short circuit current ,manageable 

But most people divide up to isolate shorts rather then  control short circuit current , your mileage may vary 

our O gauge is 32 by 15 feet circular with 200 metres in droppers( 16/0.2) ( no dropper over 2 metres ) and bus wire of 2.5 square . ( from memory 24/0.2 I think )  full short testing was carried out no discernible heating and trips occurred within the booster parameters 

 

The reality is that most average layouts will never see a problem ( at 0.04 ohms/metres in a typical dropper ) this is only a problem at the margins and at the margins you need knowledge and do the calculations 

 

Oh and dont use a coin !

 

regards 

 

Dave 

Edited September 14, 2018 by Junctionmad

[Junctionmad]

 

 

I have "bus wires", "dropper wires" and "chip wires" <-- three different sizes depending on what I need. That's my sort of level of technical skill compared with 7/0.2 thickness wire.

7/0.2 is 7 strands of 0.2mm conductor , its a very common wire , in my experience of being whats in peoples layout toolbox 

 

DCC chip wires are often smaller , 7/0.1 being available and other combinations 

 

I use these terms as that is the correct way to specify cables in SI land , and its the way you can  look them up and order them from Rapid electronics, RS, Farnell , etc . Typing in " dropper" wire into these companies product finders isnt going to do much !!

 

dave 

[Pete the Elaner]

 

I use these terms as that is the correct way to specify cables in SI land , and its the way you can  look them up and order them from Rapid electronics, RS, Farnell , etc . Typing in " dropper" wire into these companies product finders isnt going to do much !!

 

I agree.

7/0.2, 16/0.2 is a standard way to describe wiring. Using a widespread standard is the only accurate way to describe things like this.

 

If you are trying to teach someone to cook something, you would not get a very predictable result if you said "a mugful of flour, an egg-cup of water & cook on a high heat until brown", but you would get a predictable result by saying "200g of flour, 50ml of water, baked in a fan oven at 200c for 10 minutes"

 

& I agree about your comment against using a coin for a coin test. I cringe when I hear someone suggest that you deliberately try to create a short.

Edited September 14, 2018 by Pete the Elaner

[Crosland]

...

 

Hence  DCC can be implemented in a number of ways , including being supplied by a DC single sided power supply , hence the current waveforms in the supply are DC pulse trains , it may be supplied by a supply above and below a designated ground , in which case the track power can be viewed as AC in respect of the designated ground , even if the power supply waveforms are DC in respect of earth ( for example ) 

 

The point is that it may be DC or AC pulse trains , depending on what you term as your frame of reference 

 

...

 

Hence unless you have differential probes , if I put my scope on a rail with DCC and and the probe ground is at earth ( nominally 0V) on  the other side, what I am seeing in my frame of reference is a DC pulse train , even though from other frames of reference I might be seeing an AC pulse train

 

Taking one rail in isolation is not really DCC.

 

DCC on the rails is by definition a differential signal on two conductors. There is no other reference voltage. 

 

All you have is two rails, call them A and B. Half of the time current flows from A to B. Half of the time the current flows from B to A. The current flow alternates. It is alternating current.

 

DCC is differential Manchester encoding ( except for the stretched 0 ) bi-phase mark is the same term

 

DCC is simple frequency modulation. Ones and zeros are encoded with a single cycle of one of two frequencies. It's more commonly defined in terms of the bit periods. Ones are 116 us (two half-bits of 56 us), zeroes are > 200 us (two half bits of > 100 us), again ignoring stretched zeros. See https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-9.1_electrical_standards_2006.pdf

 

Differential Manchester encoding is very different.

[tigerburnie]

Most informative thread, helps me make my mind up, I shall stay well away from DCC and remain analogue.

[melmerby]

Oh dear

How did these questions:

Hi All,

 

I am new to the hobby so learning as I go.

 

Reagrding DCC: is there a "stcky" somewhere that explains the concept? 

 

I amstruggling to understand the practical difefrence between locos that are sold as:

(a) DCC Ready

(b) DCC Fitted

© DCC Sound

 

Also, what is required in the way of wiring up and powering your tracks?

 

Thanks

Vic

end up in a definition war!

 

Keith

Edited September 14, 2018 by melmerby

[Crewlisle]

Vic,

 

I converted my 00 gauge 8ft x 7ft 6ins three level DC layout 'Crewlisle to DCC.  The attachment is the article .....................

 

Vic,

 

I have just found the scanned version of the attachment complete with photos.

 

Peter

 

 

 

DCC Article - Part 2.pdf

[Andymsa]

The poor old OP must be even more confused, it's like reading war and piece. There are good points to all the posts but all he wanted was basic advice On what is required.

 

So I just say how I did mine, there are six boosters that each are subdivided, single multi strand 2.5mm mains wire is used these go to bdl168s the feeds from these are 16/02 these are connected to the rail by 0.7 copper wire in the visible sections and direct in non visable sections. All cables are colour coded

Edited September 14, 2018 by Andymsa