DC vs AC power for DCC - Model Railroader Magazine

Another example of confusing terminology, and you're not the only one who gets totally lost when reading documentation from DCC manufacturers. This will hopefully clarify things:

Conventional DC power packs control a locomotive's speed by varying the amount of voltage on the rail. Although nearly all HO-scale systems are rated at 12 Volts DC, some power packs actually put out up to 18 or 19 volts DC at full throttle, especially the cheaper transformers that are included in train sets.

Most DCC systems can operate from either a DC or AC power source, so you can use your old power pack as the power source for the DCC system, provided it has enough amperage output. The DCC system rectifies AC input and changes it into DC, so no matter what type of power you connect to the DCC system, it changes it into DC.

DCC systems that are National Model Railroad Association (NMRA)-compliant place between 14 and 16 volts continuous DC voltage on the track. A high frequency digital command signal is superimposed onto this DC voltage, and this signal is what controls the locomotives.

The decoder inside a locomotive reads the digital signal, and if it is addressed to that particular locomotive, it does whatever that command is telling it to do -- run at a certain speed, forward or reverse, with headlight on or off, etc.

If you measure the track voltage with a DC voltmeter, you will get a steady DC voltage of around 14 volts. If you measure that same track with an AC voltmeter, it will read around 16 volts because it is now also measuring the high frequency digital command signal.

Because a steady voltage is always on the track, unless you turn it off with a block control toggle switch, lights in passenger cars, etc. will stay on all the time. The lights in locomotives without DCC decoders will also be on, and you may hear a buzzing sound from their motors caused by the high frequency DCC command signal.

What the Roco documentation is telling you is that you can connect a DCC system to a layout that has been wired for conventional DC block control, turn on all of the block control toggle switches, and it will work without any further need for changes to the wiring. Your block control power pack should not be on at the same time, however.

The old locomotive you mention will not run on DCC, but the headlight will come on if it is left on the track, and the motor will buzz and overheat, so it is advisable to remove it from the track or park it on a siding that can be turned off with a block control toggle switch.

I'm not familiar with the Roco DCC system, but some indicate that you can operate one non-DCC equipped locomotive at the same time as DCC locomotives, by setting your controller to address "0." This is not advisable because it can burn out a locomotive's motor. What the DCC system is doing is stretching one of the DCC pulses into a longer than normal output, which then makes the DC locomotive receive what it thinks is a DC voltage, and it moves accordingly. This stretched DCC pulse can be shortened or lengthened to give the DC locomotive more or less voltage, thus controlling its speed. Changing the DCC pulse from positive to negative reverses that locomotive's direction. But again, it is not recommended that you do this because the DC locomotive's motor is still receiving high frequency DCC pulses and it can rapidly overheat.

I hope this clarifies some of the muddy waters created by the Roco documentation.