While reading the Thinking of DCC column in the November issue, I noticed a few things that probably should be clarified:

The caption for the photo of the MRC diode board is inaccurate (but the equivalent text in the article is correct). Silicon Rectifier Diodes typically have a fairly constant voltage drop of about 0.7 volts (roughly 2/3 of a volt, not 1/3 of a volt as stated in the caption) when current flows through them in the forward direction. Some other silicon diodes (like Schottky Diodes) have lower voltage drops (in the 0.3 to 0.5 volt range is common), while other diodes have other voltage drops determined by their material composition and junction designs. (We see this all the time with LEDs, which are also diodes.)

It can be noted that a silicon bridge rectifier can be used to replace four of these silicon rectifier diodes. To do this, connect the (+) and (-) together on the bridge, and connect the two AC inputs (~) in series with the load. At one time, there was a diagram showing this use of bridge rectifiers on the Atlas DCC web site, allowing the user to drop the track voltage from the Atlas system to a lower voltage. High current bridge rectifiers are available at Radio Shack. That MRC Circuit is probably the functional equivalent of three silicon bridge rectifiers connected in series. Armed with this knowledge, a modeler can have any voltage drop that is an integer multiple of 0.7 volts by just connecting enough rectifier diodes or bridges together.

By the way, the classic “diode drop” constant intensity lighting circuit is the same as 1/3 of that MRC circuit, or one bridge rectifier. Judging from your description of the lighting in the review of the Athearn/Walthers HO Scale F7, that’s what’s used for the constant intensity lighting circuit in that locomotive. (There appears to be four rectifier diodes on the left side of the circuit board in the upper photo on page 15.) You get your 1.4 volts from the diode drop circuit, hook up a 1.5 volt lamp across it, and you have a constant intensity headlight using a lamp. Unfortunately, this won’t work for LED headlights, but LEDs inherently have an apparent brightness which doesn’t appear to vary as much (probably due to some characteristic of the human eye).

Regarding the voltages seen by motors in DCC applications, the motor will “see” a peak voltage of typically a couple of volts less than what is on the track. The higher track voltage actually is passed on to the motor through the decoder (less whatever voltage is lost in the decoder’s input rectifiers), so the motor “sees” nearly the full the peak track voltage. The decoder switches the rectified track voltage off and on thousands of times per second as you stated, so that the average voltage the motor sees is less than the full peak track voltage. Motor speed in model railroad locomotives is primarily a function of average applied voltage.

Resistors don’t waste excess current with heat — they waste excess voltage with heat. The rectifier diodes do the same. The difference is that the voltage drop with diodes is essentially constant and independent of current (within limits), while the voltage drop across a resistor varies with the current. NOTE: A group of diodes dropping about four volts at some specific current will create exactly as much heat as a resistor dropping the same voltage at the same current. A group of diodes will probably spread out the heat more than a small resistor experiencing the same current and voltage drop, due to their greater volume and surface area. Again, the advantage of using the diodes is the essentially constant voltage drop regardless of the current.

Even if speed is not an issue, I personally think modelers in the smaller scales like N and Z should avoid applying peak voltages above about 12 volts to their motors. There is a lot of arcing at the commutators of typical motors in model locomotives. As the commutator rotates and switches the current, the inductive kick from the armature current causes arcing that can heat the commutator and cause pitting of the commutator segments. This can be readily observed when an oscilloscope is used to view the voltage across the brushes, and the inductive pulses start looking pretty nasty in N scale locomotives when the peak applied voltages start going above about 10 volts. My best guess is that HO and larger motors have enough mass and volume to soak up and dissipate the heat, but things should be less rosy for smaller motors.

Jim Hinds
Richmond TX