adj vr howtobuild

Adjustable Voltage Regulator

Published: September 19, 2009

Adjustable Voltage Regulator

by Rich Dumas

Disclaimer

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.

Introduction

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.

Description

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.

Page 1 of 6Adjustable Voltage Regulator

4/14/2015http://www.slotcarillustrated.com/plugins/p2_news/printarticle.php?p2_artic...

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 F 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 1F capacitor are available from

Radio Shack. Radio Shack part numbers are shown in parenthesis.

LM 338T adjustable voltage regulator

1F 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)

0.1F 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, 0.1F capacitor, 1F 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.



of 6Adjustable Voltage Regulator


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.

Summary

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.

Discuss this article in the Forum!



adj vr howtobuild

Documents