Variable Power Base

Note Nov 2003: I build a wood track and the variable power controller pictured below has been disassembled to make one Drivers Station per lane. However the information is still valid just the enclosure changed.

I recently purchased the SCX Acropolis/Le Mans 1/32 scale slot car sets. I purchased them thinking it would be a great experience for my 2 young boys. My experience with slot cars was limited to a set that I received in my childhood. I wore the metal contacts out on the cars and once they did not run anymore I lost interest. Well that was then and this is now I immediately set up my new track on my old ping-pong table and started to turn some laps. I was having a blast when my wife brought my oldest son down stairs to the basement and once downstairs of course he wanted to see what daddy was playing with. I gave him a controller and tried my best to explain to him about how these little cars went around the track by pressing the trigger on the controller. He caught on really fast but he did not catch on to slowing down and before long one of the cars was looking pretty battered and beaten, missing the rear wing and pretty much any piece that protruded from the body of the car. I immediately set upon repairing the car and building something that would prevent the boys from destroying the cars until they understood how to control the cars. The following is something you can build to control the voltage on your slot car track so you to can let your youngest pick up a controller with you favorite car on the track.

This article explains how to create a variable voltage controller for your track. This project will not only help you prevent the kids from destroying the cars as well you can use the controller to supply the correct voltage to the track for the specialty cars you may have. I recently purchased a TSRF car, the motor is rated @ 12 volts but my stock SCX transformer was rated at 18.5 volts. By lowering the voltage I was able to use my stock controller.

Click on the images for a close up.

I rewired my stock SCX powerbase to accommodate separate power for each lane. When used with 2 power supplies this mod will give each car the power it requires without having to share it with each other. In combination with the voltage controller this allows you to control the power to each lane separately in turn you can race at full power while your little ones get a little less.

The schematic on the left is the circuit in it's simplest form. The LM350 is a 3 amp voltage regulator all built into one nice package. It's easy to work with and has many features worth while to this project, one of them being it is inexpensive! To see the complete specifications of this chip please visit National Semiconductor.

This schematic was taken from the PDF on the LM338. My final schematic is very close to this one except I used different resistor values and I added a diode after the output of the LM338(LM350). The capacitors are very important to help filter the input so the regulator does not prematurely die due to ripple in the input voltage. I used different resistor values because the input voltage calls for >= 28vdc but the average home set power pack is 18vdc or less.

This is the circuit board I completed as you can see it is very bare if fact I decided to build a separate controller for each lane so I could play at full voltage but my son would only get enough juice keep him happy. This circuit has 2 controllers on it.

This is the underside of the board it's not very complicated in fact if I'm sure a 4 lane version could probably fit on this board however the meters would require a larger housing as you will see.

Below is the project box with the meters mounted in it. I used a project box that was just large enough to accommodate the 2 meters mounted side-by-side. If you were to build a 4 lave controller a larger box would be required.

If you did not want to get as fancy as I did one meter would suffice with a switch where you could control which LM350 you were monitoring.

This is the top of our project box with the two variable resistors (pots) mounted and breathing holes drilled. The breathing holes were a safety precaution since the LM350's might get warm during operation and I don't want heat to build up inside the box. I also added heat sinks to the LM350's to help dissipate some of the heat please read on.

This photo shows a dry mount of most of our components. I do not have the heat sinks on the LM350's yet but most everything else is here.
Voltage input is located at the bottom of the picture, voltage output is located on the sides of the unit and you can see the perfboard fits perfectly.

I have everything mounted and tested. Notice the LM350's they each have a heat sink added. When you use high current motors in your cars the current draw on the LM350 causes it to heat up and this is not good however for stock cars the heat sink is probably not necessary.

Here we have a top view of the almost completed unit. I have not drilled the holes in the top cover yet but what I wanted to show here are the power output plugs, notice them protruding from the side of the box. I used these plugs because the price was right FREE but had I went to purchase plugs I would have gotten 90 degree plugs. This would keep the plugs and wires flush against the side of the case minimizing the over all foot print of the unit.

This image shows the rear of the box with the computer power plugs hot glued into place. If you want to use the transformer that came with your set without cutting the end off just buy the proper plug. I was even thinking about adding different plugs just so I could let my friend use it with his Scalextric transformer and why stop there for total compatibility you could add plugs for all transformers if you really wanted.

Parts list, this is not a complete listing since I used a few parts I had kicking around in the spare parts bin and you may not use the exact same parts as I do and because of this I will have to guess on the price of some of the parts. However if you scrounge around you may find a lot of the parts in the discount bin or if you have a computer buddy he may have some of the parts.

Misc Parts: hot glue, wire, solder, screws, old heat sinks from old motherboard and anything I thought would work.

This list is for a dual controller.

Quantity	Item	Approx Price
2	LM350	$5.00ea
2	620 ohm R	$.25ea
2	Automotive Voltmeters	$9.99ea
1	Plastic Project Box	$10.99
2	10k pot	$2.49ea
1	Computer Power Splitter	$4.99
1	20pin connector strip	$.50
1	2x3 inch perfboard	$.99
8	2pin connectors	$1.00ea
2	Power jacks	$.99ea
2	Knobs for pots	$1.25ea

Additional points: I will at some point add a switch to the box to reverse the voltage supplied to the base this will reverse the direction of the track WARNING some controllers may not be compatible with this feature.
The power supplies I'm using are two old Compaq laptop supplies rated at 18.5 volts @ 2.2 amps I'm going to add a 4700--> 6000 microfarad @ 25 volt capacitor to each output. this will provide that little extra power at take off for the current hungry cars out there.

Tools: soldering iron, multimeter, electrical tape, hot glue gun, drill and bits, exacto knife.

Requirements: For a dual controller your power track unit must be wired for separate power for each lane.

Recommendations: For optimum car performance you should think about a separate power source for each lane.

Useful Links:
Very good Slot cars Forum
Slot Car Garage tech stuff
Fantasy World Hobbies Tech Tips
Gregory Braun great tips and free lap timing software
Slot Car Illustrated great reviews!

For exact part Numbers from Active Electronics just e-mail me here.

Dated: Feb 1 2003


	These are my current drivers stations. The information below is what was used to build these.