DCS Choke Tutorial (semi-analytical)

Mistake! At the start I meant to say the short circuit current isn't infinity (not zero). Also the approximation of 18V/10A isn't so bad for 1 MHz because the frequency response is actually quite flat. The 2-3 ohms TIU Zth is just measured.

Also "choke" and "inductor" are 100% interchangeable words. When I'm being analytical I use "inductor", and when I'm thinking trains I use "choke" because that's what everyone else calls them

rad400 posted:

From the last part of your tutorial, I am gathering that any incandescent light bulb in a passenger car, caboose, engine lights that is fed from track power will short out the DCS signal and will require a 22uh choke?

Bob D

Almost perfect. Yeah any incandescent bulb fed from track power needs to be choked. It’s not “shorted out” because it’s not drawing all the current, just some so we usually say “it’s loading the DCS signal” as oppose to “shorting” it. You do know what’s going on though  so the video can’t be so awful!

Matt Makens posted:

So then why does the “MAGIC LIGHT BULB” improve DCS signal?

That’s going to be the next one probably. The bulb is a treatment of a problem with the channel response (ISI or inter symbol interference), but it’s a patch fix that doesn’t address the root problem of long unmatched lengths, and it actually does degrade the DCS voltage, just not as much as it resolves the ISI problem already present.

Overall the bulb is a bad way to go since you are loosing signal voltage on it.... and it’s much better to just clean up the channel so you don’t get isi effects in the first place. We just did this with the SD3R folks.

Thanks again Adrian! I appreciate and enjoy your tutorials and maybe someday it will all rub off on me and even sink in.  Please keep doing those too, the more the merrier! 

ogaugenut posted:

A 22 uh choke has been talked about and recommended on the forum for years.  In light of all the new research and analysis that has been conducted recently, does 22 uh still seem like the best value for a DCS choke?

Thanks

Bill

Lets calculate some cases!

 22uH is going to be about 2pi 1 MHz x (2.2x10^-5) = 138 ohms

The TIU is about 2 ohms. So the voltage divider will be about:

(138/140) so basically 1 meaning all the TIU voltage appears on the train.

If we made it 220uH it would be 1380 ohms (everything X10) so we'd have

1380/1382 so basically 1 meaning all the TIU voltage appears on the train.

At 2.2uH it would be (13.8/15.8) so about 90% of the voltage appears at the train.

So you want something you know... above the 5-10uH. Practically speaking, 95%, 99%, 100% are all the same thing since it's not an ideal noiseless environment.

Moonman posted:

One forum member replaced "magic bulbs" with these filters. Are these no longer needed with new TIU versions (Rev L)? Do these provide any benefit or harm?

Probably not helping you in Rev L.  Lowering the Zx if you don't absolutely have too for some special reason is generally never helpful!

carl552 posted:

Do you have a recommendation for the choke to use on the transformer?  A link to ebay, Digikey, Mouser,,,,

Thanks, Carl

 The one I've been using is this one (link below) with an SRF up at 9 MHz and a capacity of 10A. 

https://www.murata-ps.com/en/p...uctors/1422311c.html

Don, the choke is for any mode, it keeps the load in question from attenuating the DCS signal.  Like he says, almost anything loads the signal.  It so happens that any half decent capacitor will really load the signal, but any load that has a low impedance at the 3.27mhz frequency of the DCS signal will affect it.

As GRJ says, it's for any mode.  It's just that the TIU has 4 chokes built-in (one for each channel) for use in the "normal" or non-passive mode when transformer power feeds the input side of the TIU.

Additionally, the choke is needed for any modern engine or piece of electronic rolling stock powered by a track that has DCS signaling on it.  It's just that MTH PS2/PS3 engines already have the choke built-in.  But if using a non-MTH engine or a passenger car you're converting to LED lighting, or fill-in-the-blank, you will in all likelihood achieve better DCS signaling/control if you install the choke.  It's truly a case of your-mileage-may-vary where the technical analysis as to "why" can get tedious - with absolutely no disrespect directed at Adrian!

 

I watched the video and read the postings.  This was a great presentation for help understanding why a choke would be needed. 

I still have a question, are there different choke specs that are needed for each type of application, Non MTH engine, Passenger car lighting and Transformer, or is it a case of one choke spec (22uH) will fit all these types of applications.

A. Infinito

President & Chief Operating Officer

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Serving The Basement.

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The Dude posted:

I watched the video and read the postings.  This was a great presentation for help understanding why a choke would be needed. 

I still have a question, are there different choke specs that are needed for each type of application, Non MTH engine, Passenger car lighting and Transformer, or is it a case of one choke spec (22uH) will fit all these types of applications.

You’re just looking for an inductance >10uH so the impedance is high enough, and that that works up to maybe 10MHz. The top frequency of an inductor or choke is specified as self-resonant frequency or (SRF).

The Dude posted:

...

I still have a question, are there different choke specs that are needed for each type of application, Non MTH engine, Passenger car lighting and Transformer, or is it a case of one choke spec (22uH) will fit all these types of applications.

A 22 uH choke comes in different current-handling ratings with corresponding prices.  For passenger car LED lighting a 0.1 Amp rating is suitable and go for, say, 5 or 10 cents a piece.  For an engine, a ~5 Amp rating is typical and go for, say, $1 a piece.  For track power as might be found in a TIU a ~10 Amp rating runs for, say, $2 a piece.  For track power in passive mode to run more than 10 Amps, a 20-30 Amp choke might run, say, $3 or so.  The physical size is roughly proportional to the current rating.  In all cases it's a 2-terminal or 2-wire component typically placed on the "hot" side of the power wiring (though in theory it doesn't matter if placed on the "hot" or "common" side of the wiring).

As Adrian's analysis suggests, the 22 uH specification is itself not a hard and fast spec.  18 uH or 27 uH (common values near 22uH) would work equally well as would other values.  It's just that the cost of an 18 or 27 uH choke of the same current rating is identical and of the same physical size.  For all I know, someone literally flipped a coin to "choose" 22 uH as the magic value and it just stuck and become the gospel.   In fact, if you look inside a TIU, the magic choke value is not 22 uH!

In the case of the DCS blocking choke, higher values are better, but you reach a point of diminishing returns.  Also, higher value chokes of a given current rating are typically larger and usually a bit more expensive.  Since I have a TIU open, I did check and Stan is, of course, right.  The input side blocking chokes are 31.5uh, interesting tidbit.

Urggg. I finally watched the video and I have to say, it was like my teeth were pulled with pliers without novacaine. I just can't focus that far into it.

I never knew that regular bulbs would cause issues. I was told that they help the signal propagation. This makes my basic fundamentals of DCS get erased and that's an overload right now!

 So I'll take a step back and let things sink into my head. It's easier to just choke everything I do from now on and not question why. The equations melt my brain! Oh no..... I'm melting...……..

Thanks, I think?

" on Sour mash and cheap wine " ??

Why go back to DCC when I have DCS!

Engineer-Joe posted:

Urggg. I finally watched the video and I have to say, it was like my teeth were pulled with pliers without novacaine. I just can't focus that far into it.

I never knew that regular bulbs would cause issues. I was told that they help the signal propagation. This makes my basic fundamentals of DCS get erased and that's an overload right now!

 So I'll take a step back and let things sink into my head. It's easier to just choke everything I do from now on and not question why. The equations melt my brain! Oh no..... I'm melting...……..

Thanks, I think?

The light bulbs help waveform shape , but hurt waveform amplitude . See the video at the top of the other thread which has even more equations... Laplace transform and second order forced responses. I know not everyone wants to be super analytical, but I think it's better to just post the complete story and let people take whatever they want from it.

Don Merz 070317 posted:

Adrian and GRJ--thanks for the answers and the great thread. I am such a nerd that I think this is a FUN DISCUSSION! Hah.

Rattling around in my brain is the issue of the train motors. Doesn't the electricity see the motors as inductive?

Don Merz

 

It's a good question but pretty complicated to answer. The hand-wavy answer is the PS2/3 board in the engine takes up the AC, converts it to DC, then applies pulse modulation. The impedance seen looking into the engine is referred through the conversion electronics (which has a big ripple-removing capacitor inside). As a result the natural V to I phase relation of the motor is broken and the load doesn't look inductive.

Also in front of all that conversion there is a choke of a few uH keeping the DCS from flowing into the power electronics in the first place.

Joe,

I never knew that regular bulbs would cause issues.

They don't.

I was told that they help the signal propagation.

They can.

This makes my basic fundamentals of DCS get erased and that's an overload right now!

At the risk of being accused of being less than a scientist, I will state what experience in the real world of DCS has taught me:

18 volt light bulbs, when used properly and appropriately with TIUs earlier than a Rev. L, are an excellent way to eliminate  DCS signal issues, and to maximize DCS signal strength.

What a scope shows is irrelevant if your DCS works and operates properly.

Barry

 

DCS Ambassador & author of The DCS Companion series of books

Train-Ca-Teers - All For O and O For All!

 

Barry Broskowitz posted:

Joe,

I never knew that regular bulbs would cause issues.

They don't.

I was told that they help the signal propagation.

They can.

This makes my basic fundamentals of DCS get erased and that's an overload right now!

At the risk of being accused of being less than a scientist, I will state what experience in the real world of DCS has taught me:

18 volt light bulbs, when used properly and appropriately with TIUs earlier than a Rev. L, are an excellent way to eliminate  DCS signal issues, and to maximize DCS signal strength.

What a scope shows is irrelevant if your DCS works and operates properly.

Thanks Barry,

I still have one older TIU that I have bulbs just on the outputs. It does still work fine for me. I don't use anything with my newer version L TIUs. I wanted to get another TIU and relegate the oldest to my workbench. I had actually waited as it still does the job, and I had expected MTH to release a new version with the WIFI built into the TIU. That never happened and now I just have too much stuff!

 

" on Sour mash and cheap wine " ??

Why go back to DCC when I have DCS!

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