A DC power pack tutorial

Discussion in 'The Academy' started by ezdays, Jun 16, 2006.

  1. ezdays

    ezdays Out AZ way

    There seems to be a lot of modelers that haven’t gone over to DCC. We took a poll a while back and the split was about 50/50. I can see the advantages and the features of using DCC since I spent a good 40 years building digital control systems. I’ve chosen to stay with DC since my N scale layout is relatively small (about 32 sq. ft.) and there’s not going to be more than one or two consists operating at any one time. It’s economics as well, and I know I’m not alone in that regard.

    Anyway, I’ve seen many questions here regarding DC power packs, so I’ve put together a few diagrams that might be helpful in explaining some basic concepts of DC. Someone recently posted a primer on basic electricity, voltage, current and resistance so it might be helpful in reading that as well. The diagram below shows what different voltage sources do over time. If you had an oscilloscope connected to these power sources, this is what you would see; the technical term for that is called a “waveshape”. It represents what the voltage measurement is at any point in time. For example, waveshape “A” is pure AC, just like what comes out of a wall outlet. The voltage starts at 0 and slowly increases to some value before it goes back to zero and reverses itself and goes negative and equal amount. The frequency it does this at is called Hertz. The basic frequency in North America is 60 Hz, while across the pond and in OZ it becomes 50 Hz. We can thank George Westinghouse for coming up with this concept; Edison would have had us all running on DC if he got his way. Now, in order to get this voltage to be reduced, one must use a transformer. This will reduce the wall outlet voltage from say, 120 volts, to something more usable, like 12 or 18 volts. There are times when this is enough, like running some Lionel O gauge, or operating lamps or other accessories and for switching turnouts. There are times when AC won’t work and DC is required, like running most other scale engines and all electronics. Waveshape “B” shows what happens when you add a diode to the circuit. A diode, or rectifier, will only allow current to flow in one direction. By placing this in series with the load, the result is that the negative half of the cycle is blocked. You will find this on most cheap power packs that come with your basic train set.

    An improvement on that is shown in waveshape “C”, where by using four diodes, it is possible to flip the negative half-cycle and not have the blank space, or 0 volts for that portion of the cycle. I’ve got an early model MRC that uses this. Both these methods are OK for running trains, but are unreliable since the voltage will vary with the load. If you add a capacitor to “C”, then you get what you see in “D”, filtering and a little more stability. The little squiggles are called ripple. What is happening is that the capacitor charges up like a small battery and slowly discharges. The next part of the cycle causes it to charge up again. The amount of ripple is dependent on the size of the capacitor and the amount of the load. The load still has some affect on the voltage output here as well.

    Waveshape “E” shows what pure regulated DC looks like. It’s accomplished by adding some electronics and some more filtering to “D” so that the voltage remains constant regardless of the load, the same as you get from a battery. Most engines don’t need that kind of regulation, but most electronics do.

    Attached Files:

  2. ezdays

    ezdays Out AZ way

    Now there are power packs that are digital in nature, but are still considered to be DC. The ones I have are the MRC Tech 4’s. What they do is produce a DC regulated voltage that is constant, but is pulsed, or turned on and off quickly, so that the average voltage is equivalent to some DC value. For example, if the pulses are 18 volts, and they are there for 30% of the time, than the average DC would be 6 volts. If you look at waveshape “F”, you can see that the pulses are short resulting in a low average value and the engine will run slow. In “G” the pulses are wider and the engine runs faster, and “H” shows it running almost full speed. What’s really cool about this is that it’s possible to do things like “momentum” and “braking”. What happens here is that the electronics in the Tech 4 power pack will start up the train with short pulses, then slowly increases the pulse width until it reaches the maximum voltage the unit is set for. The same for breaking, it will stop the train slowly by decreasing the pulse width until it reaches zero.

    I hope these explanations are not too technical, but I think they might answer some basic electrical questions I’ve seen here from time to time.

    Attached Files:

  3. Russ Bellinis

    Russ Bellinis Active Member

    Don, good tutorial. I'm wondering if this should go in "Tips & tricks" where it won't disappear into a back page in a couple of days? I'm also wondering if it would be worthwhile to have a technical FAQ section with some articles on basic issues for model railroad newbys to peruse? Perhaps it would be worthwhile to have a FAQ section as a subforum here in the Technical Q & A forum?
  4. ezdays

    ezdays Out AZ way

    Thanks Russ. That can be arranged, I haven't given it much thought up until now. We do have a tutorial sub-forum as part of the Academy, but an FAQ sub-forum here does have its possibilities though.
  5. sumpter250

    sumpter250 multiscale modelbuilder

    The Pulse Width Modulation, or Duty Cycle control, is also what DCC uses for speed control, it's why some locos "whine" when running on DCC analog (address 00). It has the advantage of max voltage, max current, which can help start the motor for smoother operation.
  6. kf4jqd

    kf4jqd Active Member

    Thanks EZdays for the refresher course of my Digital and AC courses from my Techincal College days!

  7. Santa Fe Jack

    Santa Fe Jack Member

    Great post, but I have just a couple of minor corrections to add:
    Actually, the technical term is "wave form".
    Westinghouse was an entrepreneur, not an inventor. The man who is to be credited with AC and its development is Nicola Tesla. Westinghouse was a benefactor of Tesla, but Tesla deserves all the credit. Aside: A great biography of Tesla is "Man Out of Time." And Tesla thought Edison was a hack -- they had completely different approaches to their work. Edison would try something 1000 different ways to find one that worked. Tesla, on the other hand, would simply imagine the perfect device in his head, and build it. Amazing.
  8. ezdays

    ezdays Out AZ way

    Opps, I missed this when you posted it, but you are correct, although waveform is used interchangeably with waveshape. Look up one you usually get the other as well. (see:http://www.allwords.com/word-waveshape.html)
    You are also right, Westinghouse didn't invent, or discover AC, but if it wasn't for his perseverance over Edison's entrepreneurial spirit (remember, he was also a businessman and had things to sell), we could be running everything today on DC,:D although I think it's unlikely since someone else would eventually had to take us in that direction.

    Thanks for adding this valuable and interesting information to this.
  9. MasonJar

    MasonJar It's not rocket surgery

    Moved to The Academy|Tutorials 01 March 2007


Share This Page