Practical PIC Projects



 Dual Output PSU
with adjustable voltage
for Scalextric and other slot cars


I designed this PSU to use with the Scalextric Sport slot car racing set my nephews got for Christmas.  Being technically minded when the kids got the set I couldn't help but have a look at the AC power adapter and how it delivered DC power to the track.  When I stuck my multimeter across the track I discovered that when two cars were going flat out down the track and one of them crashed, the voltage delivered to the remaining car increased by as much as 2 volts.  The fact that even the supplied power base is designed to allow connection of separate PSUs to each track hints that, as is so often the case, it was specified on price not performance.  

I also noticed that my youngest nephew had trouble controlling the speed of the cars and I figured that if we could reduce the overall voltage delivered to the track he'd be able to control the car more easily and keep it on the track longer.

The final piece to this project was the Race Start Controller (RSC) that I had also been working on.  It occurred to me that with a custom PSU, it should be possible for the RSC to shut-off power to the track when a car jumps the  start.

Finally, if you just want to build the PSU and don't want to use it with the RSC you can do that too. It will work just fine on its own.

12ft x 4 ft layout   

Control draw on sliding rails

Front panel - I've ditched the 3.5mm jacks for the throttles and replaced them with 3-pin XLR connectors


The wiring and circuit schematics show how to connect the PSU to the track and to the Race Start Controller

The PSU comprises two identical voltage regulator sections. The schematic only shows one, the second one is identical and on the PCB component overlay parts names appended with the letter 'a' refer to the second voltage regulator section.

Wiring Schematic
click here for PDF version

Race Start Controller Schematic
click here for PDF version


Circuit Description

The Dual PSU is built around two LM317T adjustable voltage regulators.  These devices are capable of supplying a regulated output voltage from 1.25 to 37 volts at up to 1.5amps.  They feature current limit protection and  thermal overload protection so are fairly bullet proof making them ideal for this application.

At the input to the circuit is RF1. This device is a self-resetting fuse and it is included as a safety feature.  If this fuse trips, the power LED1 will go out.  You will then need to remove the cause of the overload and also remove any other loads from the PSU before it will reset. In other words don't have a throttle wide open waiting for the power to come back - it won't.

Diode D1 protects the circuit from accidental reverse polarity connections to the DC input. Since this circuit is designed for use with any suitable 18V PSU it is not inconceivable that some may have a DC plug wired for centre negative.  It also protects any attached PSU from C1 discharging into the input.  I've found that the output from some SMPSU's will dip by a couple of volts when the load goes from zero to 400mA, a scenario typical of two cars accelerating from a standstill. Capacitor C1 helps reduce this dip at the input to the regulators, you may find that you can omit this without any adverse effects.

The components around VR1 follow the basic LM317 reference designs.  Diode D2 prevents damage to the voltage regulator in the event that the voltage at the output exceeds the input voltage. Capacitor C2 provides decoupling at the input to the regulator.

The components around the adjust pin of the regulator set the output voltage.   Resistor R4 sets the minimum output voltage to ~7volts.  The preset PR1 in parallel with P1 is used to set the maximum output voltage.  Once PR1 is set, P1 allows the operator to vary the output voltage between the minimum and preset maximum. 

Transistor Q1 is used to shut-off the output.  Q1 is turned on when the 5 volt signal is applied to the base via R5 from the 'Power Control In' input.  This pulls the adjust input of VR1 to  ground, bypassing the resistor network and causing the output of VR1 to drop to 1.3volts. In practice this is insufficient for a slot car motor to actually move the car, particularly from a standstill.  Resistor R5 and R2 pull the base of Q1 low, ensuring that Q1 is turned off when there is no control input. 

Increasing the output current

The LM317K regulator can deliver up to 1.5amps. If you replace RF1, the 1.1amp resettable fuse with a 3 amp rated part (Rapid Electronics Part No 26-0805 ) and find a 3 amp or greater rated PSU you can deliver up to 1.5amps to each lane.  Diode D1 is only rated at 3amps so the design is not really suitable for delivering more than 3amps total.


The PCB was designed using CadSoft Eagle. You can download a free version from their website

Eagle PCB Artwork file

PCB artwork in PDF format

On the component overlay, component names with an 'a' appended are the duplicated parts of the schematic for the second PSU.

If you use the artwork from the PDF to make the PCB, check that the output is to scale.  Overall board dimensions are 2.7" x 3.85". 

PCB Component Overlay

DC Input Power Supply

I deliberately chose not to build a transformer based power supply for the input to this circuit for the following reasons

  • It avoids having AC mains voltages present and all the safety issues that involves.

  • For the cost of the transformer and associated components you can buy a suitable Switch Mode PSU.

  • Switch Mode PSUs are more efficient and generally have over voltage, short-circuit and thermal protection designed in.

For these reasons I chose to use a switch mode PSU to use as the input to this circuit.  One reason not mentioned above is that in my work I have access to assorted redundant SMPSUs.  The one I've used on the final version of this is an 18volt 1.1amp PSU from an HP Scanner.  I've tried a 22volt 2amp PSU I bought on E-bay for 6 which also worked fine.  

You need a PSU that can supply at least 1 amp, a bit more won't hurt but it won't make your cars go any faster either.  You need to supply the circuit with at least 4 volts more than the maximum output at the track.  In theory you could get away with a 16volt input but I recommend 18volts.  You also need to make sure that it will work without any load, most of the power-brick type SMPSUs will, but make sure the one you buy does.

A good place to look is CPC at I found the following, all of which appear to be suitable (though I haven't tried them)

18V / 1.66A - part  # PW01446
18V / 1.1A - part # PW00752
18V / 1A - part # PW00526

Circuit construction and setup

When constructing the circuit on the PCB, don't be tempted to install the big components first, they make it difficult to install the small parts afterwards.

  • Install RF1, resistors, preset, diodes and capacitors except C1 and solder to the PCB

  • Next install the DC power connector and PCB screw terminal blocks

  • Now fit both heat sinks and solder the lugs to the PCB.

  • With the heat sinks fitted install the voltage regulator IC's.  You might want to apply a bit of heat sink compound to the back of the regulator.  Push the legs through the PCB holes but don't solder them in place at this stage.  Using an M3 screw and nut attach the regulator to the heat sink.   Tighten the screw being careful not to twist the regulator. A tip here is to ensure that the screw is fitted so that it is facing the edge of the PCB otherwise you can't get a screw driver on it

When the voltage regulator is bolted to the heat sink, the heat sink is electrically connected to the regulator output. It is important that the heat sinks do not touch any other components or metal work inside an enclosure.

  • Once the regulators are attached to the heat sink, solder them to the PCB and then fit capacitor C1

  • With all the components fitted, have a good look at the track side of the PCB to ensure there are no solder bridges and all the joints are sound.

Once you're happy with the assembly of the board connect a suitable DC power supply to the DC input.  

  • Measure the voltage at the DC_out connector on the PCB. This should be the same as the rated output of the DC power supply.

  • Now measure the voltage at the THR1 and THR2 connectors.  The voltage should be somewhere between 7 and 16 volts. If you adjust preset PR1 / PR1A you should see the output voltage change.

  • Next attach the potentiometers P1 /  P1A to the terminal connectors on the PCB.

    • Adjust preset PR1 to its mid position.

    • While measuring the output voltage at the THR1 terminal adjust P1 fully clockwise then fully counter clockwise.  Measure the voltage at the THR1 connector in both positions, then leave P1 set at the position where the measured voltage was highest.

    • Now adjust preset PR1 until the voltage measured at the THR1 connector is 12.5 volts.
      (You can adjust the maximum output up to about 14 volts which will make the cars go faster - however you do so at your own risk - if you burn the cars motor out or damage it in any other way it was your choice)

  • Repeat the last three steps with THR2/P1A/PR1A to setup the maximum output on the second PSU.

The circuit is now complete and ready to connect up as shown in the wiring schematic above

Notes for use with the Race Start Controller

  • If you're using the Race Start Controller with this PSU, remember to fit 1K2 1/4watt resistors in positions R6A/B & R7A/B on the controller PCB.


Don't follow my wiring colours here, I hooked this up before I did the schematics


A schematic diagram of the internals of the Sport Powerbase track part C8217.

If you want to connect the PSU to the track via the Powerbase, rather than wiring directly to the underside of the track you can.  Details of how to do this are shown on the Wiring Schematic above.

If you do this you must ensure that the Switch is set for dual PSU and you don't connect anything to the 16V inputs on the Powerbase.

  PDF version  

Parts Listing

  • All Rapid parts/descriptions correct at 19-Sept-2008.  You should check part# and descriptions are correct when ordering in case I've made a mistake transferring them onto this page.

  • The resistors are sold by Rapid in packs of 100.  You may find it cheaper to buy some or all of the parts from other sources - I make no recommendation.

  • You will also needs some short lengths of wire for the interconnections between the PCB, potentiometers.  Rapid sell a pack of 11 colours x 2M lengths of stranded equipment wire, Part # 01-0108


Qty Part Number Description
1 55-0120 5mm green LED
2 47-3318 LM317T voltage regulator
2 47-3130 1N4001 diode
1 47-3144 1N5401 diode
2 81-0066 BC548B
1 26-0780 or 26-0740 1.1A polyswitch fuse
2 67-0608 2K, 6mm cermet preset potentiometer
2 65-0710 4.7K, 16mm potentiometer
2 (sold in packs of 100, order Qty 1) 62-0382 3.3K ohm 0.25 watt resistor
1 (sold in packs of 100, order Qty 1) 62-0374 1.5K ohm 0.25 watt resistor
1 (sold in packs of 100, order Qty 1) 62-0410 47K ohm 0.25 watt resistor
2 (sold in packs of 100, order Qty 1) 62-0354 220 ohm 0.25 watt resistor
2 (sold in packs of 100, order Qty 1) 62-0370 1K ohm 0.25 watt resistor
2 11-1046 10uF / 25volt Tantalum Capacitor
1 11-0765 2200uF / 35volt Radial Electrolytic
2 08-0235 100nF disc ceramic capacitor
6 21-0440 2-way, 16A interlocking terminal
2 36-0316 TO-220 heatsink, 3.9oC/Watt
1 (to suit DC power supply) 20-0962 2.5mm PCB mount DC power socket
20-0960 2.1mm PCB mount DC power socket
  • M3 screws, nuts, knobs for the potentiometers etc are not listed so you'll need to source these to suit.