Slot Cars Rule !!!

Cheap and easy Adjustable Voltage

A word on the voltage adjustment through the diode rotary switch described here

• Diodes are non-linear electronic components. This means that the amount of voltage dropped (reduced) by a diode is actually a non-linear function of how much current is going through that diode. As soon as some significant current is flowing (say at least 10 mA), the voltage drop will be about 0.6-0.7 V per diode, however at very low current (no load) it will be only 0.2 to 0.3 V.
• Thus, it is not like an adjustable voltage source where voltage can just be dialed-in and be guaranteed. In this case, to know how much the voltage setting really is, the rotary switch should be adjusted while having a car at full throttle (or close to full) on the track (I am assuming that the car will be held it with the rear wheels off the track ☺). Another way is to just adjust the switch while and after running a couple of laps with a car to what feels like the best setting.
• Not all diodes are created/manufactured equal, so no two circuits built as described in this article will drop the exact same amount of voltage. When all eleven diodes are in use, variations from one circuit to another could be as much as 0.3 to 0.5 V
• Of course, there are other ways to actually have a regulated voltage drop that is the same at all current values, but they are a bit more expensive and mostly a bit more complex to build, requiring a voltage regulator chip along with several resistors and capacitors. The goal here was to have a quick, cheap, and easy to build solution.

Details on the assembly of the diodes and rotary switch:

• Here are the required components with their cost. They can also be found easily at any on-line retailer or any electronics shop

Rotary switch, 1 pole, 12 positions
1RS-117 $1.75

Two knobs for 1/4 round shaft
KNB-123 $0.33

Eleven rectifier diodes
1N4001 $0.73

• The diodes in the table above are only rated for 1 Amp, which is plenty enough for my 1/43 set-up and probably good enough for many tracks and cars. However, more could be required for hot motors in 1/32 or 1/24 cars. Here are higher current diodes at
o 3 Amps/50V 1N5400 $0.15 each
o 6 Amps/50V 6A05 $0.21 each

• The pins on the rotary switch are numbered 1 to 12, with an additional pin at the center of the switch. Pin 12 will be the power input; the center pin will be the output. The way the switch works is that as the knob is rotated, it connects successively the center pin to any of the pins numbered 1 through 12. Since each of these diodes will drop about 0.7 V of voltage, the rotary switch can be used to decide how many diodes in series are desired between the power supply and the track, thus dropping the voltage by a multiple of 0.7 V, up to a maximum of about 7.7V. This, by the way, is very similar to the way the Professor Motor controllers work.

• To assemble, take the diodes, and bend the pins of the diodes as close to the body possible. Then insert the first such diode through the pins numbered 12 and 11, making sure that the silver ring on the diode points towards pin 11. Insert the second diode through the pins 11 and 10, making sure that the silver ring points towards pin 10.


• Then solder pin 12 and pin 11.

• Next insert the diode between pin 10 and 9 and solder pin 10

• Keep going all the way until pin 1 and 11 diodes should then have been inserted. Solder the input (red) wire to pin 12, this one will go to the positive output of the power supply. Solder the output (white) wire on the center pin, this one will go to the controller.


• Next, add the knob to the shaft of the rotary switch, that's it.


• Finally, you need to insert this into your track wiring. Here is an example on how to do that. Pin 12 of the rotary switch is connected to the power supply side while the center pin is connected to the controller side (white connection on a three wire controller, for positive polarity tracks). In the example shown below, cars would travel from right to left.