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Home  >>  Articles  >>  How To

Adjustable Voltage Regulator

Published: September 19, 2009

Adjustable Voltage Regulator

by Rich Dumas



While reasonable attempts have been made to verify information provided in this article, the author will not be responsible for errors, omissions or contrary interpretation of the subject matter contained herein. Working with electricity, power tools and hand tools is inherently dangerous and can result in serious injury or death. The reader is solely responsible for following applicable safety guidelines and precautions when working with electricity, power tools and hand tools. The reader assumes ALL liability for any damages which may result from use of the material contained in 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 reverse connection a 6 amp diode could be placed between the plus input and pin 1 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.

  • 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

A circuit diagram for the adjustable voltage regulator is shown below. Please note, to simplify the diagram the locations of pins 2 and 3 on the LM 338 have been switched. When you fabricate the adjustable voltage regulator, the middle pin (2) will be connected to the plus (+) output.

A 150 ohm and a 470 ohm resistor are wired in series to get 620 ohms. The wires from the plus input to pin 1, pin 2 to the plus outlet and from the minus in to the minus out should be at least 16 gauge. The other wires do not carry any significant amount of current so smaller gauge wire can be used.

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.

About the Author

Rich Dumas is an active member of the Shoreline Model Raceways club in southern Connecticut (United States). Rich is a frequent contributor to many of the Slot Car Illustrated forums.

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