DCC Misinformation Debunking

 This page is under construction and needs updating as the DCC Wiki has been seriously improved.

Typical misinformation:

• DCC is not AC but DC
• DCC is not bipolar
• DCC track voltage never goes to zero
• my $5 voltmeter reads DCC track voltage just fine.

DCC Wiki

My overall advice is that this is just like the Wikipedia, but worse. You have too many people putting information forth as facts, when they are often opinons.

Worse, while the Wikipedia almost always has lots of references, the DCC Wiki has none.

The intro is good, and a good page to read on basic concepts:https://dccwiki.com/Introduction_to_DCC

Note: previous incarnations of this Wiki were awful, it's been a lot improved.

Twisting track power wires

The DCC Wiki starts off well enough:

"This topic is fiercely debated on internet forums and DCC-related mailing lists, endlessly and without any real conclusions. This article tries to provide concise reasoning for either. "

https://dccwiki.com/Twisting_Power_Bus_Wires

But before getting to the reasoning, the BS starts:

This hell right away, stating:

"There is always a large debate on the twisting of track power bus wires. Keep in mind that DCC power buses are an 'Unbalanced Pair' - one wire (A) is held to ground while the other (B) is energized, then they flip when A carries a signal and B is held to ground. This is happening constantly and at a high rate. "

Complete crap. Spent several days arguing this with a person recently.

Yes, if you are INSIDE a DCC booster circuit board, you can typically measure from the internal ground to one of the 2 output connections, and you will see it go positive and negative, BUT THERE IS NO GROUND OUT ON THE LAYOUT, THE TWO RAILS ARE CONSTANT ALTERNATING IN POLARITY PLUS TO MINUS AND BACK AGAIN THIS IS THE VERY DEFINITION OF AC (ALTERNATING CURRENT).

Complete insanity. Not only is this a balanced pair, but this is WHY twisting wires helps reduce interference from outside sources and why twisted pair has been used from telephones to Ethernet wires for data.

I have only been able to determine that this idiocy has come as a necessary support for some other misinformation.

Look at the many examples of DCC waveforms on an oscilloscope, it's clearly a bipolar square wave.

Other than this BS, the DCC wiki does an OK job of identifying pro's and cons of twisting. They should have stopped there.

Back to twisting the wires:

Inductance

So a benefit given for twisting wires is that it reduces inductance.

(aside, having inductance in a DCC power circuit "rounds off" the sharp corners of the square wave, which is not the desired effect.  Additional inductance would normally reduce high frequency noise, if the right inductive value. In general you want to minimize inductance)

So, it is known that to minimize inductance is to place 2 wires of a balanced pair (we have established this is a balanced pair) closer together. So the basic "common wisdom" is that twisting together will keep the wires closer, and thus minimize inductance.

That is indeed true... but in general, twisted wires will increase inductance since twisting significantly uses more wire, so often the inductance INCREASES.

But, the "experts" then recommend just a few twists per foot, like 2 or 3. So, that is reasonable for HO trains. It probably does little harm at worst case, and MIGHT help installations where the 2 main DCC wires can get far apart.

Noise reduction:

An external magnetic field can induce voltage in a wire. Basic law of physics. How magnetism is generated in a motor. So a wire or other device close enough to your DCC wires can induce an unwanted voltage, and this is generally called "noise".

You see telephone wire, and more recently Ethernet cables made of twisted pairs. Any magnetic field that hits a twisted pair will induce a voltage in the wires, but since each wire is twisted in a different direction (one clockwise one counterclockwise) the voltage in each wire is equal in magnitude but OPPOSITE in polarity, and effectively cancel each other out. This is called common mode rejection.

All is well here, except that the number of twists needed is way more than the "common wisdom" Look at an Ethernet cable, open it and convince yourself.... don't listen to "experts". The people making these cables know what they are doing, since Ethernet works.

Bottom line:

I think the best way to approach this, is keep your wires close to each other and away from other stuff. If you have issues, you can try moderate twists, but I would reposition wires and look elsewhere first. At these frequencies, a cheap oscilloscope can be had for very little money. LOOK at the waveform yourself and you can stop guessing. You can see directly what the issue is from looking at the ACTUAL waveform.

For garden railroads, most people use twin lead landscaping wire, where the 2 wires are nice and close (and consistent) and will do perfectly fine. My wires are separate 10 gauge wire used for wiring 220v circuits in a house, but in conduit. Runs up to 60 feet. No issues!

Back to the DCC Wiki on twisting wires:

Well they should have stopped there, with the pro's and con's.

But it continues with this:

DCC Issues... most make sense, but some seem nuts.

"Excessive track bus impedance can cause a multifunction decoder's PWM pulses to be superimposed onto the DCC signal, distorting the DCC waveform."

I'm going to research this.. seems nuts... how can the PWM get BACK from the H bridge, through the full wave bridge, back into the rails? Seems completely nuts, driving voltage backwards through a full wave bridge into the rails? How can that be?

They rest are fine, but I'm going to find the person who wrote this and ask him where he got his data.

Noise:

I think this is by far the bigger problem over the inductance in your wiring. Noise get in by a magnetic field. This can come from a nearby wire carrying heavy current (making a strong magnetic field) or possibly motors, or something putting high frequency noise IN THE DCC CIRCUIT. There are some possible outliers, like having an RF transmitter nearby and one of your wires is a "tuned length" for that frequency, but this is rare.