Re: RC Snubber question


Vollrath, Don <dvollrath@...>
 

Ed,

You need to read Mark’s website stuff again. There are just too many variables to support your all inclusive statement of ‘no matter how large’.

 

The real problem of voltage ‘spiking’ starts with the inductance of your wiring, which will never be zero. This is related to the length of wiring run no matter how it is constructed. [Yes, twisted pair wiring does help.] The second part is the ability of your booster to provide instantaneous amperes into an unwanted short circuit out on your layout. [This is not the same as the so called product ampere rating.] The third part is what happens when that track short becomes interrupted (because the loco wheels are still moving) and the sensitivity of any given decoder to absorb the short circuit energy stored in the inductance of the wiring. That energy will be ½ x L x i^2 where the current, i, is the instantaneous current that was flowing into the short circuit… before the booster (or electronic CB) trips. [this can easily be 3-5 times the rating of your booster, or more, depending on brand and how it is constructed.] As the short circuit becomes broken that energy must and will go somewhere… either into a voltage spike (the L x di/dt part) causing a spark, or be absorbed into an R/C filter, or into a nearby DCC decoder where it may be simply absorbed by filtering internal to the decoder, or somehow cause damage in the form of CV memory corruption or actual physical voltage breakdown destruction to the decoder. [Evidence tells us that some decoders are less robust and more susceptible to damage than others.]

 

Use the basic principles of the best practices guidelines. If you have long DCC bus runs, add R/C snubber/filters. How long is too long? Evidence indicates that problems seem to start at 30+ ft (>9 meters) from the booster. Your luck may vary.

 

DonV… the guy who took the scope pictures on his home layout and posted them on wiringfordcc.com.  

 

From: WiringForDCC@... [mailto:WiringForDCC@...]
Sent: Monday, August 11, 2014 7:17 AM
To: WiringForDCC@...
Subject: Re: [WiringForDCC] RC Snubber question

 



Mark,

Great response and web post.  It appears that you are saying that IF I construct my layout with proper wiring (twisted pairs and solid connections) I do not have to worry about RC filters at all no matter how large my layout is - is this correct?

Ed Robinson


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