Resistive Wheel Sets
Adding Resistors to
Metal Wheel Sets
One way to add resistance to a freight car with Jay-Bee
metal wheels, is to "hang" a 1/4-watt resistor between
the two axles as shown below.
This illustration shows the bottom
side of the truck.
This requires the two axles to be installed in opposite
directions from each other, with one insulated wheel on
the right rail, and the other on the left rail - pointed
out above. With the resistor running between the two
axles as shown, power will travel from the non-insulated
wheel on one rail through the resistor to the
non-insulated wheel on the other rail.
Wrap the resistor leads loosely around the axles until
they overlap themselves, solder the lead to itself, then
snip the excess resistor lead off - also shown above.
When properly done, there will be no binding of the
resistor to the axles. A drop of Conducta Lube or
Automatic Transmission Fluid (ATF) on each axle will
assure they run smoothly, get clean, and stay clean for
adequate contact.
While "hanging" resistors on axles is a very
cost-effective way of getting freight car detection, you
can also purchase Jay-Bee wheel sets with resistors
already in them. This costs a little more, but is a whole
lot faster and easier.
But do you use one resistor or two?
Current-sensing block detectors generally require a
certain amount of current to be drawn in order to trigger
the detector. This is where the resistance requirement
comes in. The higher the resistance, the less current is
being drawn. Ultimately, with DCC, you want to draw the
least amount of current possible and still have your
block detectors work reliably. Also, block detectors,
once triggered, will stay triggered with less than half
of the required current - which is twice the resistance.
The theory is that if you use two resistors of twice the
resistance, you'll be drawing the full current most of
the time, but only half the current if either one of the
wheel sets loses power - such as with dirty track or
wheels. This is irrelevant while the train is moving
because block detectors take this into account.
This really only matters if you have a single detected
car on a detected block and it is not moving. In some
cases it's better, in other cases it's worse. In any
case, I think this issue is irrelevant. I don't see that
you gain or lose one way or the other. That being the
case, I vote for one resistor - it's cheaper and easier
to do.
Team Digital note: When our DBD22 detector is powered
with 5 volts the typical trigger (track) current is 3
ma. When powered with 12 volts it is 6 ma. So with a
HO track voltage of 14 volts, a resistance of 4700
ohms (14 / 4700 ~ 2.97 ma) will trigger the block
detector when powered by 5 volt.