[Gordon H]

QUOTE (Mike Button @ 25 Jul 2007, 12:31) <{POST_SNAPBACK}>Good choice of topic Doug, I'm fairly naive when it comes to the internal workings of the loco's I run, but as I understand it capacitors are used to stop interferance with other electrical systems, such as TV's, radio's etc. I'm running a DCC layout in my shed, another passion of mine is cricket, and listening to the test match on long wave requires delicate positioning of the radio so that the interferance from the layout is kept to a minimum.
From a purely selfish viewpoint so I can listen to the cricket whilst the trains go round, I would be in favour of capacitors being fitted, but have no idea of the impact upon a DCC system so will look forward to hearing other viewpoints.
The capacitors and other suppression devices that manufacturers fit to their locos are there to make their products comply with EMC regulations.
The most effective place to locate these components (especially the capacitor) is directly on the motor terminals, or even better, inside the motor itself as Maxon do. Having them so close to the source of the interference minimises the current path and the antenna effect of the motor wires thereafter.
Using a DC controller with this arrangement has no effect on the performance of the motor, as the capacitor simply charges to the set DC voltage, and suppresses any AC noise that comes back to it from the motor. The loco will have been tested and approved for EMC on the basis of this set-up.
When we add a DCC decoder into the system, the method of controlling the motor current is radically different to that of a DC controller. For reasons of efficiency, low power dissipation, etc, all DCC decoders drive their motors using pulses of full voltage. The variability of speed is created by altering the 'on time' relative to the 'off time' of the drive pulses.
In effect, the drive has now become an AC signal too, and the capacitor will try to filter out the sharp edges of the pulses, which it sees as high frequency noise, just like the interference.
However, the circuitry needed to generate these motor drive pulses often doesn't like this kind of extra 'capacitive' loading, especially if the decoder is using a 'back-emf' control method which tries to measure the actual motor speed while the pulse is off. The apparent effect on the motor is that of poor or unpredictable running, because what the decoder tries to put out to the motor is not what actually happens.
It is often claimed that the original motor capacitors can be removed because the decoder now does this for you. However, any such suppression built into the decoder is at least a wires length away, arguably two if you count wires to both motor terminals, and these can now transmit their interference into the air, the effect of which will vary depending on wire length and the way they are laid out etc.
The only real answer to all of this aspect is for decoder designers to allow for capacitive loading on their outputs.

Then there is the 'conductive emissions' aspect, whereby the action of pulsing the motor so quickly causes the current drawn from the track to vary in sympathy, so the track itself will act as a transmitter of the pulses (mostly the odd harmonics). The only way round this one is again to build into the decoder sufficient filtering to prevent the motor current reaching back through the decoder power supply stage. However, to do this effectively would likely require some fairly enormous inductors, which are rather impractical in the sizes of decoder we expect to use.

I bet you wished you hadn't asked now!

 

[Gordon H]

QUOTE (BromsMods @ 25 Jul 2007, 13:58) <{POST_SNAPBACK}>However, as a non-electric expert, can you explain why it is that, in general, I don't find it necessary to remove capacitors from Hornby and Heljan locos but Bachmann always run better with them removed. Indeed, Bachmann even recommend removal on their website. Has this something to do with the type of motor and/or circuitry used by different manufacturers?
It must be. Each combination of decoder and motor (and suppression components) will yield different results, and clearly Bachmann feel that their internal electrical arrangement (presumably when used with with their own decoders) will benefit from removal. Another way of looking at it might be that they know their own decoders can't cope with a capacitor, so advise their removal. I guess they are not really in a position to comment on the use of anyone elses decoders with their locos. Either way, the result could be a locomotive which no longer conforms to the original EMC specification it was tested to.
What I would be interested to know is how manufacturers arrange all this in a loco that comes ready fitted with a decoder. This implies that the whole unit, i.e. loco with decoder, has passed the EMC requirement. This being the case, I would have thought it sensible to be advising users to adopt that arrangement (whatever it is) for retro-fitting conventional locos too.

 

[Gordon H]

QUOTE (Doug @ 25 Jul 2007, 15:07) <{POST_SNAPBACK}>I placed the radio right next to the terminus at my turntable. I tuned in (BBC Radio 4) and got quite a good amount of static when I powered up the DCC system. I ran a Hornby steam loco that had a capacitor at the motor terminals on address #0. A little more static and whining on the radio. I snipped the capacitor and there was a considerable drop in static. I wonder why?
Loco address zero is a special 'worst case' for pulsing of motors. Having no decoder in the way, the capacitor on the motor experiences a pulse edge transition of twice the voltage that one driven by a decoder does, and this will cause large current spikes at the start/finish of each pulse.
Decoders only drive pulses positive or negative one at a time (depending on which direction you want to go).
The effect of the 'Address Zero' technique is that zero speed is achieved when the positive pulses on the track are balanced exactly by the negative pulses on the track, i.e. the motor sees an actual AC square wave all the time, and what makes it move one way or the other is an imbalance between the positive and negative 'on' times.
It is sharp pulse edges, and the harmonics that they generate, that tend to cause most RF interference in cases such as this.
Curing it is all part of the issue of designing a decoder motor driver stage to cope with such a situation, and limit these current spikes somehow, which is one of the reasons why you will often see inductors as well as capacitors fitted to the little PCBs that come in a standard unfitted loco.
I had a situation recently (not in railway modelling) where I found a motor driver chip objected to having a certain length of cable connected directly between it and the motor. Fitting an inductor at the driver chip end cured the issue because it was no longer seeing the capacitance of the cable.
There is a lot to this subject, which it is why it crops up so often on forums such as this, because although there is clearly a desire for a definitive universal answer, no such answer is available. Each combination has to be taken on its own merits.

 

[Gordon H]

QUOTE (dbclass50 @ 26 Jul 2007, 10:05) <{POST_SNAPBACK}>You also get similar noises when pickups go from one power district to another.
When you go between districts (or cause a reversal on a reversing unit) you are essentially causing a direct short circuit between two high power amplifiers (boosters) for the period the wheels straddle the boundary. Ideally, these boosters will be operating exactly in phase with each other (i.e. they will always change polarity absolutely simultaneously), but the chances are there will always be a few tens of nanoseconds difference between them, during which time some very high currents will flow, causing the interference that has been noted. This period of pulse overlap will hardly be noticed by a decoder, which simply rectifies whatever it finds beteen the rails, but the effect on emissions is potentially enormous. The boosters might not like it much for extended periods either!

 

[Gordon H]

QUOTE (BromsMods @ 26 Jul 2007, 23:06) <{POST_SNAPBACK}>Technical explanation. The capacitors are placed in the circuit by Bachmann for the purpose of reducing the possibility that electrical noise generated by the motor when running would interfere with a TV or radio or cell phone or possibly some other electronic device. All TCS decoders have this capacitor built into the decoder and there for external capacitors are not needed. The TCS decoder is tuned to work with its own internal capacitor in operation. With external capacitors applied in the circuit the result is a de tuning which results in poor speed control. That is why the Bachmann applied capacitors need to be removed. The internal decoder capacitor reduces the possibility of electrical interference so there is no harm in removing the external capacitors.
This is all very well, but the capacitor on their decoder is not in the right position physically to do the most good. It really needs to be on the motor terminals. Note that they only say 'reduces the possibility of interference', rather than claiming to give the same level of suppression that existed before. A better solution (though probably impractical from their point of view) would be for them (TCS) to provide the capacitor they are tuned for as a separate item, so that you can fit it yourself in place of the original. If that is too difficult, they could at least suggest what value of capacitance they have tuned their decoder for, to allow those who feel able to work at that level to do it properly.

 

[Gordon H]

QUOTE (John Buckland @ 28 Jul 2007, 15:28) <{POST_SNAPBACK}>I suppose MRC would say the Prodigy is NRMA conformant but not compliant? I could stand corrected but I don't believe it is compliant.
Other way round. Compliant implies designed to work to the standard, but has not been tested to confirm it. Conformant means it has been tested and proven to comply by measurements specified and performed by the NMRA.
Therefore Conformance can be regarded as a higher state than Compliance.

 

[Gordon H]

QUOTE (Gordon H @ 2 Aug 2007, 13:06) <{POST_SNAPBACK}>Other way round. Compliant implies designed to work to the standard, but has not been tested to confirm it. Conformant means it has been tested and proven to comply by measurements specified and performed by the NMRA.
Therefore Conformance can be regarded as a higher state than Compliance.
Hang on a minute - I'm getting my 'Compliance' confused with my 'Compatible'. Compliance is virtually the same as Conformance. If something has been proved to be Compliant, then it must by definition be Conformant as well. It is when something only claims 'Compatibility' that there need be any concern.
That's what comes of rushing your replies during a lunchtime!

 

[Gordon H]

QUOTE (Richard Johnson @ 5 Aug 2007, 12:26) <{POST_SNAPBACK}>Hornby doesn't really understand the DCC world, didn't read the standards or simply chose to ignore the key parts of them and so their digital products are not made to any standard that is recognised by anyone else so can neither be called conformant, nor compliant:

As I understand it, Hornby didn't actually design their own 'DCC' items - they got a design house somewhere in Cambridge to do it for them. Quite what their remit was from Hornby is unknown. It doesn't take that much effort to get hold of and interpret the specs to do it all properly, which leads to the suspicion that keeping the price low was all important, with performance playing second (or even third) fiddle. I still find it hard to believe that someone in their position would actually come out and say that they didn't think scope traces were important!

 

[Gordon H]

QUOTE (Howzatt @ 8 Aug 2007, 01:18) <{POST_SNAPBACK}>I've read about adding a terminator network elsewhere to cure this problem and I will certainly give this a try, but I really don't see why I should need to go down this track (sorry!).
The problem with termination networks in this kind of application is where to put them. With one or more moving loads (i.e. the locos) the line characteristics the booster sees will be constantly varying. What it really points to is the need for the design of the track booster to be sufficiently robust to cope with a wide range of load situations - and that may add somewhat to the cost.

QUOTE A scope connected across the track may shed more light on this when I can borrow one.

It is often quite revealing to probe the track waveform with a scope, but can also be awkward from a triggering point of view if you want to analyse packet contents.
You will certainly see any ringing or other loading effects on a pulse by pulse basis, but because the content of a DCC packet directly affects the time the packet lasts, timebase selection and 'hold off' settings become critical - and might need adjustment every time you turn the controller knob.
The only truly reliable way to avoid this is for the command station to output a separate trigger pulse each time a new cycle of packets starts. Quite straightforward in a home brew system like mine, but I doubt whether any commercial systems provide such a facility.

 

[Gordon H]

QUOTE (Richard Johnson @ 3 Aug 2007, 06:03) <{POST_SNAPBACK}>I work with CE a lot. CE is written as a protective legislation: Actually as long as it (the loco) complies on the day it is sold it doesn't matter what you as a consumer do as long as you don't affect others....
<snipped>
re CE: Its irrelevant to consumers... just do what you wish to your loco. No draconian interpretation of legislation affects it once you own it, you'll not create a social disaster by removing the capacitors and jails will never be full of CE offenders .....so body cares except you really.
re: removal of bits and bobs such as capacitors: I install 500+ loco's a year, all brands and scales from Z to gauge 1.
Just out of interest really...
Whilst I can see the legislation couldn't realistically be applied to the end consumer, if an individual (such as Richard) installs decoders into locos on a regular commercial basis (i.e. gets paid for doing it), does that individual not become a 'new' manufacturer in the chain of supply, and therefore responsible for the revised CE/EMC situation?
Does it make a difference if the end user has already bought the loco elsewhere and simply 'lends' it for conversion whether or not money changes hands for that conversion?