Making resistor wheel sets
Metal wheel sets such as those available from North West Shortline, Jaybee, Kadee, and others can easily be converted to draw current from the rails for block detection and other current sensing purposes. Our method makes it easy connect a resistor from one wheel to the other without drilling or soldering. The suggested parts list includes 3.3K ohm resistors which will draw about 5 milliamps per wheel set. Use a larger value resistor to decrease the current. If you want to increase the current you will have to use a physically larger resistor such as a 1206 or 1812 size in order to dissipate heat generated in the resistor by the current flow.
Parts to obtain from Mouser Electronics (1-800-346-6873):
Circuit Works Conductive Ink Pen - part number 5168-2200ST $11.95
Circuit Works Overcoat Pen - part number 5168-3300G $ 9.95
Surface mount resistors (size:0805 1/10 watt 3.3K ohms) - part number 260-3.3K $2.40 per 100
Selecting the correct value for resistor wheelsets is not an exact science. The actual value needed is different for each situation. The variables are many: based on the type and code size of rail, type of detectors, how your track is scenicked and ballasted, how the track was fastened to the sub roadbed. Etc…
My own personal experience in N-scale is the lower values worked better 3.3k. You mileage may and probably will vary. You will need to do some expirmentation.