I'm finally getting back into model railroading after a number of years. Despite being a model railroader for practically all of my life, I finally have a dedicated space to build my first ever permanent layout, in a 10' by 9' spare bedroom. Previously, I'd sometimes set up an oval of Kato Unitrack with some turnouts to run some trains. After seeing how complicated the wiring would be for a DC layout, I decided to go with DCC, using an NCE Power Cab starter system. My question for today is, what kind of power pack should I use to power the track? I have an old MRC Tech II 1400 power pack Would that work okay? Thanks for any help.
Do you mean to power the turnouts? The track is going to be powered by the DCC system. You absolutely do not ever want the DCC system and a DC power pack attached to the same track at the same time, that's a very good way to smoke the DCC system. If you are talking about the then turnouts, then yes, an old Tech II will do just fine.
Thanks, CSX Robert! Due to my appalling ignorance of electricity, I couldn't interpret the technical jargon on the Digitrax and NCE web pages.
To oversimplify things, DCC operates at a constant voltage and the track is hot ALL the time. The signal coming from the DCC system is a square topped sign wave so it has a positive and a negative component.....sort of like AC, but it isn't true AC. The system changes the length of the pulses to send "messages" to the locomotives. As you probably know with DCC you can run trains in opposite directions on the same track......trains running in one direction are using the positive part of the signal and trains going the other way are using the negative. As Robert said, mixing a DC power supply into all that is a great way to smoke a DCC system.
Thanks @MRLdave . I'm kind of a DCC newbie too (with 50+ years of DC experience in N) and that's fascinating. I thought that DCC-supplied track had an AC current and that the DCC circuitboard in a locomotive had some sort of rectifier, but was then puzzled as to how waste heat might be handled. Clearly, that was all wrong. I realize there's no need to understand the technical details, but I'm drawn to this sort of thing. Again, thanks! So then DCC voltage is constant in the track, but the DCC system varies the wave height (amplitude?), width (frequency?) and wave shape to suit?
Actually, you were right. These is a rectifier on the decoder that converts the AC to DC. The decoder uses the DC to drive the locomotive (and to power the decoder's processor and other electronics), through an H-bridge circuit to control the direction. The AC is a constant amplitude square wave, with a varying frequency to encode the digital information.
Yep, one way to think about it is that the decoder in the loco does the same thing as a DC throttle that you are familiar with. It changes the AC current from the track to DC and varies the DC voltage to the motor as if you were turning the throttle up or down. It does that by following the DCC commands the Command Station is sending through the AC on the tracks. The commands are generated by twisting the throttle on the Command Station if there is one or via a handheld throttle that is connected to the Command Station or a wireless one that is communicating with the Command Station. One shouldn't let what seems to be a complicated process intimidate them as all of that is transparent to the user and you don't need to understand any of it to enjoy the freedom a DCC layout will give you. Sumner
Amen to that I don't understand nor want too as long as it works, which it does! All I know is DCC is awesome, for me!
Thanks @CSX Robert and @Sumner for sharing your knowledge with such clear writing and filling in the many blank areas in my DCC knowledge. Interesting that my rudimentary assumption that AC to DC conversion happened on the locomotive's circuit board was correct. I looked up an H-Bridge Circuit and the explanation made sense. All very interesting stuff. I'm looking forward to getting my mainline complete so that I can start to explore DCC for real.
Slight correction. It does not vary the DC voltage to the motor to control the speed of the locomotive. The voltage is constant, e.g., 12 volts nominal. The speed of the motor/loco is through pulse width modulation or PWM as it's commonly known. Basically it varies the time that the motor sees the voltage. Longer "on" time means faster RPM. Shorter time means slower RPM. In a nut shell...
Yes and no... the amplitude of the pulses is not varied, but their duty cycle (percentage of on time during the pulse, vs off time between pulses) is varied to control motor speed. But if you hook up a DC volt meter to the motor leads of the decoder, it will measure the equivalent DC voltage. PWM is actually the same way many DC power packs generate their variable DC output. The pulse width modulated (controlled) waveform has an AC and DC component, but the motor itself acts like a low-pass filter, rejecting the AC and using the DC component for power. The means of filtering out the AC is merely the the physical momentum of the motor's armature (and any driven mechanical loads), and the inductance of the motor windings. Note there are different types of PWM, some are on-off (half-bridge), and others are bipolar (full- or H-bridge*), with either positive or negative voltage applied at all times to the motor. Bipolar is often used for bidirectional motor drives, whereas on-off is simpler and useful for uni-directional variable speed motor drives. When the bipolar waveform is 50% duty cycle (equal periods of + and -), the resulting DC component. Bipolar is also often used in uni-directional applications where braking is needed (rather than coasting to a stop). Bipolar is most likely what is used by DCC decoders driving motors. (*) The "H" in H-bridge refers to the electrical topology of the driver circuit, NOT "half" bridge!)
When driving a motor using PWM with a full H-bridge, you generally do not send it pulses of alternating current, it's still just on-off pulses with the polarity set to go in the desired direction. Sending opposite polarity pulses will draw more current and cause unnecessary heating. A DCC command station does send alternating current pulses to the decoder and can directly drive a DC locomotive by varying the width of those pulses, but that also causes the motor to buzz and can easily over heat it if it is left no moving for too long.
If the PWM frequency is high enough, then continuous, alternating polarity pulses at ~50% duty cycle each will cause the motor to draw very little more current, due to motor inductance and Back EMF, the intrinsic voltage generated by a spinning (coasting) DC motor). Some DC motors (especially coreless) are not designed for PWM drive, and will draw too much current and overheat if left on and motionless for a long period. However, the switching losses due to gate capacitances in the bridge will be greater when switching all four transistors, rather than when leaving one transistor on, and switching the opposite-corner transistor on/off. (With an H-bridge driver, the motor/load is inserted in the cross-bar of the 'H'. Only opposite corner transistors should ever be on at the same time, never both transistors on the same side of the H bridge.)
