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Adjustable Voltage Regulator

Copyright (c) 2009 by Rich Dumas - All Rights Reserved.

Copyright (c) 2009 by Slot Car Corner L.L.C. - All Rights Reserved.



While reasonable attempts have been made to verify information provided in this article, neither Slot Car Corner L.L.C. or Rich Dumas assume any responsibility for errors, omissions or contrary interpretation of the subject matter contained herein. Neither Slot Car Corner L.L.C. or Rich Dumas make any representations or warranties of any kind, express or implied, as to the information, content, materials or products, included in this article. To the full extent permissible by law, Slot Car Corner L.L.C. and Rich Dumas disclaim all warranties, express or implied, including but not limited to, implied warranties or merchant ability and fitness for a particular purpose. The reader assumes ALL liability for any damages which may result from use of the material contained in this article Under no circumstances will Slot Car Corner L.L.C. or Rich Dumas be held liable for any damages of any kind arising from the use of the material contained in this article, including but not limited to direct, indirect, incidental, punitive and consequential damages.

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The following article was written by Rich Dumas. Rich is a member of the Shoreline Model Raceways club in southern Connecticut (United States) and a frequent contributor to many of the online slot car forums. Slot Car Corner would like to thank Rich for writing and sharing this article.

Having an adjustable voltage regulator for each lane lets you tune the lanes for equal lap times and to tame those overpowered cars. An added bonus is that it also adds voltage regulation if your power supply does not have it. This article describes how to fabricate your own adjustable voltage regulator including a list of parts required.



At the heart of this project is a LM 338T voltage regulator. The LM 338 has an internal current limiting circuit to keep the current to a maximum of 5 amps. For most 1/32 slot car racing applications, 5 amps should be sufficient (particularly if you plan to provide a separate voltage regulator for each lane of your track). A regulator with greater capacity could be used or two LM 338s could be wired in parallel to increase the current capacity. The maximum input voltage is 40 volts. Other key points to note about the adjustable voltage regulator are listed below.

  • The minimum output voltage is 1.24 volts.
  • There is a 1.6 volt drop across the regulator, so the input voltage has to be 1.6 volts higher than the highest voltage that you would want out. For example, to get a 10 volt output, you would have to set the input voltage at 11.6.  You can use the voltage meter on your power supply to adjust the input voltage or if more precision is desired, use a multimeter.
  • To protect against a reverse connection, a 6 amp diode could be placed between the plus input and pin 3 of the LM 338T. The input voltage would have to be increased to compensate for the 0.7 volt drop across the diode.
  • If the power supply is not filtered it would be a good idea to put a 2200 µF 35 volt polarized capacitor across the plus and minus inputs. The minus side of a polarized capacitor is the one that is marked with a stripe.


Parts List

All of the parts except for the LM 338T and the 1µF capacitor are available from Radio Shack. Radio Shack part numbers are shown in parenthesis.  The 5K ohm potentiometer is not the perfect value, but it is easy to find at a reasonable price.  The two (2) resistors were chosen to work well with input voltages ranging from 10 - 12 VDC.

  • LM 338T adjustable voltage regulator
  • 1µF 35 volt polarized electrolytic capacitor
  • 5K ohm linear taper potentiometer (RS# 271-1714)
  • 470 ohm 1/2 watt resistor (RS# 271-1133)
  • 150 ohm 1/2 watt resistor (RS# 271-1109)
  • 0.1µF 50 volt ceramic disc capacitor (RS# 272-135)
  • Perfboard (RS# 276-1395)
  • heat sink (RS# 276-1363)
  • knob (RS# 274-407)

Top row: 5 K ohm potentiometer, heat sink

Bottom row: resistors, 0.1µF capacitor, 1µF polarized capacitor, LM 338T


Design and Fabrication Notes

A circuit diagram for the adjustable voltage regulator is shown below.  As you review the circuit diagram, please note the following:

  • A 150 ohm and a 470 ohm resistor are wired in series to get 620 ohms.
  • The wires from the plus input to pin 3, pin 2 to the plus outlet and from the minus input to the minus output should be at least 16 AWG stranded copper. The other wires do not carry any significant amount of current so smaller gauge wire can be used.
  • The 1uF capacitor is marked and the negative side should be connected to the negative side of the circuit.
  • The center tap of the potentiometer is connected to the wiper, the other two taps are the opposite ends of the resistor.  Only use one of the end taps!

Here is a picture of the completed adjustable voltage regulator.


Further Enhancements

Several additional enhancements can be easily incorporated into the adjustable voltage regular shown in this article. Here are a couple of examples:

  • A small panel mount voltage display could be wired into the output side of the circuit. This would provide a visual indication of the output voltage as the adjustment knob (potentiometer) is turned.
  • The adjustable voltage regulator could easily be mounted/packaged inside a small plastic project box.



While it is possible to fabricate an adjustable voltage regulator using diodes and a rotary switch, the design above is much more robust and offers a far greater range of adjustment. If you are comfortable with a soldering item, this is not a difficult project. The completed adjustable voltage regulator can add a great deal of flexibility to any slot car layout. Here are just a few examples:

  • Adjusting lane voltage (generally downward) for new and/or inexperienced racers. This is particularly helpful for younger racers.
  • Matching lane voltage to the cars/motors being run. Some cars run better at higher/lower voltages than other type of cars/motors.
  • Adjusting lane voltage for magnet and non-magnet cars. Magnet cars can generally be run at higher voltages than non-magnet cars.
  • Equalizing lane performance. On most layouts, there are varying degrees of difference between lanes. Some lanes are faster (maybe because they are shorter or less technical) while other lanes are slower. Lane voltage can be adjusted to "normalize" performance as measured by lap times. Equalizing lap times generally leads to closer racing.