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DT
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Discussion Starter · #1 ·
I am opening up this topic as there is a demand for this information and there is some good information around at the moment. I am hoping that one or two members will add some good and useful information here in due course.

Please do not bring into this topic any arguments about the NMRA etc.
 

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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.
 

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DT
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Discussion Starter · #3 ·
QUOTE (Mike Button @ 25 Jul 2007, 13:31) <{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.

You are in a perfect position to do some real world, live testing for us.

Set up your radio next to the track, run a DC loco that has it's capacitors fitted. clip the capacitors and run again. Report findings. Add a DCC decoder, run again and report findings.

I'll go and do the same, if I can get the spiders out of my radio. I'll tune into 198 LW and see if there is any cricket on.
 

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QUOTE (Doug @ 25 Jul 2007, 12:45) <{POST_SNAPBACK}>You are in a perfect position to do some real world, live testing for us.

Set up your radio next to the track, run a DC loco that has it's capacitors fitted. clip the capacitors and run again. Report findings. Add a DCC decoder, run again and report findings.

I'll go and do the same, if I can get the spiders out of my radio. I'll tune into 198 LW and see if there is any cricket on.

I'll give it a try tonight, I've got a loco that still needs converting DC to DCC. No cricket til Friday though Doug, It will have to be a French radio station instead!
 

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The capacitor should be left in place.

This is actually alot more important than it used to be-paticularly on the smaller motors found in shunters. modern digital TV is much more suceptable to interfearence from high revving motors. we had a man call in to our station and complain about his reception-his son was using his vespa in the garden!

With modern Can motors its not such a problem as the can acts as a faraday cage and stops interfearence from the brushes.

Peter
 

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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!
 

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ZTC 511 controller.
Zimo decoders.
Hornby and Bachmann loco's
Capacitors left in place.
No problems at all.

Darren
 

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Gordon

That is extremely useful and informative and explains a lot to me.

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?

Regards

John R
Bromsgrove Models
 

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DT
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Discussion Starter · #9 ·
I have just tested - with the long wave radio - the effect of removing the capacitor.

Note, I have never had interference problems before on TV and radios in the house due to the trains in the garage. I do listen to BBC on the Internet in the garage, so the long wave radio has been gathering dust over the years.

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?

I added a Lenz Gold decoder and the static increased a bit. Not quite as much as without the decoder. Reconnecting the capacitor whilst the decoder was in place made no difference to the static.

I do have a video of this experiment. If the sound of the radio comes out OK, I'll post it here when I have some time to transfer it to the PC.
 

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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.
 

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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.
 

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QUOTE (Doug @ 25 Jul 2007, 15:07) <{POST_SNAPBACK}>I have just tested - with the long wave radio - the effect of removing the capacitor.

Note, I have never had interference problems before on TV and radios in the house due to the trains in the garage. I do listen to BBC on the Internet in the garage, so the long wave radio has been gathering dust over the years.

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?
The subject is complicated enough, without bringing DCC address 0 into the equation ;-) Any loco with capacitors fitted for EMC will only have been tested on DC. To see the difference between cap/no cap you need to test with a DC controller.
QUOTE I added a Lenz Gold decoder and the static increased a bit. Not quite as much as without the decoder. Reconnecting the capacitor whilst the decoder was in place made no difference to the static.
The decoders PWM motor drive is drawing current at a different frequency from and asynchronously to the DCC waveform (ie not in time with it). As you add more chipped locos (each slightly out of phase with the others, and drawing different currents) you end up with quite a complex current waveform which leads to more (or at the very least different spectrum) noise. Try adding more chipped locos and operating them at different speeds with different loads.

I would like to know what constitutes a representative layout for the purposes of EMC approval of a DCC booster. Anyone?

Andrew
 

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We have found that locomotives vary - a lot - even "identical" ones.

If a loco operates as it should with the caps, then fine - leave them in place. If it does not & it is improved without them then leave them out. We have found the Lenz Silver & Gold do not like caps - they often "see" them as a motor short & just sit there flashing the lights ! There are far too many variables for hard & fast rules.

As Andrew says - "Any loco with capacitors fitted for EMC will only have been tested on DC." to which I agree - don't forget, as soon as you modify something (like hard wire a decoder) the CE complience is now effectivly "out of the window" anyway. Interference to radio/TV can be a problem but look at the other electrical "noise" around - mobile phones for one !

Now I'm not too familier with the insides of H & B loco's, but for example all Roco loco's I see with factory fitted decoders do not have caps, but they do have chokes between the decoder outputs & the motor - maybe there is a difference between H & B in this respect.

IMHO we are getting a little too concerned about CE/EMC here - as long as we are not causing a problem to other people why worry. How many people who seem to be concerned about CE/EMC have home made power supplies that would not comply with basic safety, let alone CE/EMC in a month of Sundays ! (Some of the things I see when at exhibitions make me glad I have always declined to carry out Portable Appliance Testing for clubs !). Having said that, to name but 2, I have seen some very well constructed panels (DWB's) & power supplies (Dougs).
 

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My experience while chipping a couple of Steam Loco's, both of which went into "runaway" has led me to leave all caps in place. One particular Loco that had no caps fitted to the motor from new, was only controllable once caps had been retro-fitted. With my set-up, Diesel Loco's don't seem as critical, probably due to their multi-axle pickups with less chance of producing signal corrupting spikes, and will work with or without caps.

It seems that users get different results depending on their DCC System/ Decoder/ Loco combination. What will work for one probably won't work for another.

Regards
 

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QUOTE (dbclass50 @ 26 Jul 2007, 02:45) <{POST_SNAPBACK}>We have found that locomotives vary - a lot - even "identical" ones.

If a loco operates as it should with the caps, then fine - leave them in place. If it does not & it is improved without them then leave them out. We have found the Lenz Silver & Gold do not like caps - they often "see" them as a motor short & just sit there flashing the lights ! There are far too many variables for hard & fast rules.

As Andrew says - "Any loco with capacitors fitted for EMC will only have been tested on DC." to which I agree - don't forget, as soon as you modify something (like hard wire a decoder) the CE complience is now effectivly "out of the window" anyway. Interference to radio/TV can be a problem but look at the other electrical "noise" around - mobile phones for one !

Now I'm not too familier with the insides of H & B loco's, but for example all Roco loco's I see with factory fitted decoders do not have caps, but they do have chokes between the decoder outputs & the motor - maybe there is a difference between H & B in this respect.

IMHO we are getting a little too concerned about CE/EMC here - as long as we are not causing a problem to other people why worry. How many people who seem to be concerned about CE/EMC have home made power supplies that would not comply with basic safety, let alone CE/EMC in a month of Sundays ! (Some of the things I see when at exhibitions make me glad I have always declined to carry out Portable Appliance Testing for clubs !). Having said that, to name but 2, I have seen some very well constructed panels (DWB's) & power supplies (Dougs).

I agree, if it works with cap in leave it there. If it does not work, take it out.
After 12yrs operating DCC and many different types of decoders both sound and non sound, all I know is; decoder plus cap = trouble. Take cap out = no trouble so I always remove caps.
After a few bad experiences I always remove all electronic boards in loco's and hard wire all decoders anyway.
Happy DCCing
Ian sa
 

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Discussion Starter · #16 ·
In my experiments, I found that the nosiest source was the reversing unit. When a loco straddled the track between on phase and the other it created an almighty din on the tranny. I suppose the electronics of the device are working overtime to keep the system in phase.
 

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Well Doug I set up the radio last night next to the track, cleared all loco's off the track and turned on the power, lots of static from the radio which only increased when I put the DC loco on with the capacitor fitted, took the capacitor off and although there was a change to the static it still blotted out the program. I don't know what this means, all I know is that the layout works fine with the DCC hardwired to the track via a main bus circuit, and the radio works fine, just don't put the two near each other!
The loco's i've converted to DCC have all had their capacitors removed prior to fitting with decoders (not sure about my one DCC ready loco.)
Don't know if this is any help to the discussion but I thought I'd chuck it in.
Spaeaking as a fairly new DCC user with limited knowledge of electronics, I'm finding this discussion interesting, even if I don't understand all of it!
Happy shunting Mike
 

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QUOTE (Doug @ 26 Jul 2007, 01:12) <{POST_SNAPBACK}>In my experiments, I found that the nosiest source was the reversing unit. When a loco straddled the track between on phase and the other it created an almighty din on the tranny. I suppose the electronics of the device are working overtime to keep the system in phase.

You also get similar noises when pickups go from one power district to another.
 

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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!
 

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Sort of offtopic...

I was playing around with a 25000MHz Oscilator and had an engine (RoCo #33241) running around my desk. Me, thinking I had wired the Oscilator correctly started to play around with it. all I could pick up was interference on my shortwave between 12005MHz and 29999MHz (no sound on LSB or USB). The loco is supposed to have a Supressor inside to stop it that happening....

Back to playing I suppose
 
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