Loving it...

I think I learned more about DCC with this aticle than all of my internet research so far!

Well done!

Warflight

Glad to help!

DCC Primer

Best artice I have read for DCC since I started in this crazy zaney hobby in the last 2 years. A ton of thanks for the information and the way it was presented Bruce.

DCC

Bruce,

you seem to do NCE a dis service?

when consisting NCE asks you the direction of each loco.

you do not need to add127 to cv29 etc

NCE is very intuitive with a Procab

NCE consist address

In the caption for the NCE consisting pic you say that the system chooses the consist address.  Not entirely accurate.  The system suggests the address (the next one available in descending order from 127) which you can either accept, or overide by keying in one of the other unused short addresses, before moving on to the lead loco screen.

Jeff Lee

No dis-service intended. I LOVE the NCE consisting methods. I was discussing the NMRA compliant aspects of CV19, not NCE's internal manipulation of those numbers.

Those who read my February column will attest to the fact that I love having things done for me when it comes to CV manipulation. However, knowing what they are saying is sometimes helpful.

Rick

We are in accord.

NCE does pick the consist address, as you detailed. The user can, as you say, override that manually, or can just press ENTER to accept the allocation.

If the user has all the system locomotives on long addresses (which NCE allows, for even addresses such as 1, which is entered 01 to make it a long address, then blind acceptance of the chosen consist address is viable. Otherwise, a manual list of used addresses needs to be kept to avoid duplication.

Current limiting resistors for LEDs

Good article Bruce. I seem to always be learning something new about DCC and this article is no exception --- thanks!

One comment if I may. Most people suggest the use of 1K resistors for loco LEDs (as you did here), but I use a much smaller one. Since decoders will typically provide 12-13 volts DC for lighting and LEDs operate at a nominal voltage of 3.5 VDC, you only need a 425-475 ohm resistor so that the current through the LED does not exceed the maximum 20 milliamps (so I use a standard 453 ohm one). This ensures that I get maximum intensity from the LEDs. If the headlights are too bright to my eye, I just dim them using Decoder Pro (which all newer decoders provide for). Anyhoo --- just my 2 cents worth.

Don Tanner

re: Current limiting resistors for LEDs

The advantage of the 1K resistor is that you only need a 1/4w resistor which is easier to get into tight locomotive spaces. And it generates less heat then the 1/2w resistor needed at the lower resistance, the 1/2w probably doesn't generate enough heat to melt plastic, but better safe than sorry.

You also state using a standard 453 ohm resistor, I don't know where you are getting your resistors, but standard would be 470 ohm, 453 is a unique and specific value.

Also running the LEDs at half their maximum current gives room for any variances in the voltage/resistance/current, giving the LED a better chance at its rated hours of life expectancy.

Typo?

I believe on page 12 of the article there is a typo. The statement reads "The track power comes into the decoder and is converted to a DCC voltage about 1 volt less than the track voltage"

I believe that should read DC voltage.

Confused

I tend to disagree, but perhaps that is because I am a novice to this DCC subject. To me this article is meant for the more advanced persons. It totally confused me, particularly with the mixture of stationary and loco decoders, and all the other 'terminolgy'

I need to start at the basics before I read something like this.

I have a number of years ago read a number of articles by Bruce and found them very informative, so I certainly am not disparaging his work,...it just wasn't what I was hoping to find when I opened the article.

i find that I most likely need to be actually playing with these DCC locos while I am learning about DCC, otherwise my memory of what I read escapes me. Hopefully that will soon happen when i get my new layout up and running, and drag all of those locos out of storage that I have been collecting (but not using).

1K resistors

Running an LED at a lower current also increases its lifespan, which is already likely a lifetime for most of us. I used a lower resistance once and couldn't get the intensity down to a reasonable level by adjusting CVs. I've also used 1/8 watt resistors with good results. No noticable heat given off.

@Brian

I too learn by doing, but after a couple of installs, this article helps a lot.

By installs, what I mean to say, is I started by just plugging decoders into the 8 pin sockets. I tried to pay attention to what was inside. Then I got a LifeLike Proto 2000 engine, that had a weird 8 pin socket, and I got to learn a bit more about how one might hard wire DCC by looking up where each wire in that socket went... then it dawned on me a bit how the decoder was working (I think of it like a sort of filter... power goes into the decoder, and comes out as DC power for your motor, and lights)

After understanding that, this article cleared up a lot of the gaps I had!

@Brian

Having something to play with and to physically reference when reading the stuff in magazines always helps me - I remember back in the mid-80s I had been without a computer for some time and I realized that what I was reading the in the popular computer mags was starting to resemble Greek. And I wasn't a linguist.

So I bought a cheap PC clone and got back into it - and the magazine articles started making sense again.

This may be what you'll need to do with DCC.

Two types of decoders

Decoder function output

Why would a 470 ohm resistor generate more heat than a 1000 ohm one?  It will let more power into the LED but it should generate less heat at the resistor.  It is blocking less power.  I understand the need to be careful and select higher watt resistors for a 12 to 14 volt decoder function output with LEDS. I always test the setup on a breadboard first and touch the resistor to see how hot it gets.  You recommend using a 1/4 watt 1000 ohm resistor for decoder powered LEDS, which is great advice.  Sometimes 1/8 watt, 1000 ohm resistors are supplied with LED packs.  I have tried those on the breadboard using 12 volts DC for an LED and they get very hot.  I am not sure about the outcome for a 1/8 watt 470 ohm resistor.

Thank you for your great columns and contributions to our hobby.

Why would a 470 ohm resistor generate more heat ....?

W = I² x R = Ω x A², or the power dissapated in the resistor (W) is equal to the square of the current (I or A) times the resistance (R or Ω).

By Per Mejdal Rasmussen - Own work, inspired by Teaching Ohm's Law to Techs by Daniel Sullivan, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=48171195

Applying above

With the 470 ohm resistor more current flows through the resistor (and LED), and therefore the 470 ohm resistor has to dissipate more power, which it does as heat. The current flowing through the resistor is what generates heat, not the current blocked.

Doing the calculations, well ok I used an online calculator because I was lazy.

With a 12 volt source and the assumption that 3 volts is consumed by the LED the remaining 9 volts is across the resistor the calculations from ( http://www.ohmslawcalculator.com/ohms-law-calculator) yield the following.

470 ohm resistor has 19.5 mA flowing requiring it to dissipate .172 watts (1/8 w is .125)

1000 ohm resistor has 9 mA flowing requiring it to dissipate .081 watts

re: Electrical calculation wheel

Just to add to this, probably most don't care, a note about the wheel in Ken's post. The wheel probably contains all the calculations you will ever need in model railroad electronics. To get the formula from the wheel, look in the center for what you want (W= Watts/Power) then use the any of the three pie wedges depending on what you know.

I don't particularly like the wheel Ken grabbed from wikipedia as it is showing the units and not the items, e.g. the ohms symbol is the unit of resistance.

Of course everyone has their own personal preferences, so I prefer something like this:

https://commons.wikimedia.org/wiki/File:Ohm%27s_Law_Pie_chart.svg, note there are at least three different letters used for Volts/Voltage: E, V, and U, depends on your point of view: physics, electronics.

I LOVE it . . .

when we get meaningful discussions going on these threads.

Keep it up, folks.

Circuit Emulator

If all you want to do is calculate a resistor value, Google "LED resistor calculator" and you will find several fill-in-the-blanks type aids.

If you want something that goes a little farther you can use something like iCircuit.

iCircuit is an inexpensive circuit emulator.  You can actually build a circuit, watch it run, and measure the results.  When I was in college something like this would have cost thousands of dollars.

There is a video on the web page. It's available for Windows and Mac desktops and iPHone and Android phones. The flavors I checked were all $10 or less.    

gs

re: Electrical calculation wheel

Jim B,

I also like the version of wheel you referenced. I was rushed and just used the first one that I found on the web

As far as basic electrical calculations there are dozens of good apps available for Windows, iOS and Android devices. Just search for Ohm's Law Calculator, Electrical Calculator or Electrical Engineering ( App). Most of the fee apps include adds, but there a few inexpensive calculators without adds.

For those that get frustrated with other things electrical, like resistor color codes, there are also apps for that and more (e.g., ElectroDroid).

Dumb question?

Bruce;

One of my pet peeves is a locomotive that starts chuffing as though it has a loaded train right out of the block. Can the starting voltage be adjusted to get a light engine rolling with just the hiss of steam or “drift” away from the stationary stop? Perhaps this is back emf, idk. With a back emf setting adjusted to “feel” a load the wouldn’t a heavy train act and sound differently?

Voltage is not source of heat!

CURRENT is the source of heat.  By allowing more current through the 470 Ohm resistor causes the excess current do dissapate as heat.  By using a higher resistor like a 560 or 680 Ohm you are limiting the CURRENT going through through the resistor.  Less current= less heat= smaller wattage resistor needed.