If you are in O scale, and you signal your layout using DCCOD train occupancy detectors offered by Dr. Bruce Chub, you might over time find one or two charred resistors on a board. An O-scale permanent fix is easy and cheap, only adding two tiny and inexpensive parts. Don’t panic!
The cause of burned resistors was excellently and thoroughly analyzed by Mark Gurries, moderator of the NCE Yahoo group. In his analysis Mark showed how the current transformer works and its relationship to the “burden resistor” wired across its output. Mark showed what was happening using both theory and actual measurements.
Much of Mark’s excellent analysis may sound like techno-babble for those who did not study Electrical Engineering. I appreciate Mark’s work because he dead nailed the analysis. The problem is far more likely to occur on an O-scale layout than N because our larger motors draw more current.
Some suggest increasing the wattage of the two resistors that fail. Unfortunately, for us in O-scale that would not be sufficient.
However, two tiny Schottky diodes can be added to the back (foil) side of the Rev E board. If your soldering skills and iron are fairly good, adding these should not be a problem. These diodes clamp excessive voltages across the secondary of the current transformer that can eventually burn up the two resistors. Just order two “Surface Mount Schottky Barrier Rectifiers”, Vishay part number SS26S, Mouser.com part number 625-SS26S-E3, for each DCCOD detector board. Solder them on the back of the board according to the photo below. Note that one diode faces to the left, and the other to the right. They are connected directly across the secondary winding of the current sense transformer. When the transformer produces high currents through its secondary winding, these diodes safely provide a path for the current to flow and thereby protect the burden resistor.
So why use this particular diode? There are lots of Schottky diodes that can handle the current, but these also fit perfectly between existing Rev E PC board traces.
The above photo is of a Rev E DCCOD board, built by Vince, and retrofitted after installation. Also note that Vince installed a male connector on the detector board. This made it easier to remotely mount in the layout.
We use axle detection resistors as high as 8.3K. None of the modified detectors have any trouble detecting a single resistor axle with only 4 wires through the center of the core of the current transformer. More turns are not required for robust detection. That said, we usually install two resistor axles on each car, one on each end. Doing so makes them easier to detect if only one end is where “it shouldn’t be.” Also, a resistor on each end of a car helps maintain detection while traveling through a short interlocking.