Volts, Amps, and Ohms

A brief introduction to basic electrical concepts

 By Bob Ward

You may be one of the many new hobbyists getting involved in slot car racing who haven’t yet acquired an understanding of the basic electrical principles that determine how a slot car runs and responds to control inputs. So… here’s a little primer that should help with common questions.

The race set’s power system plugs into a wall outlet and takes in 110 to 120 volt alternating current (AC). It delivers to the contact rails on the track 12 to 18 volts (depending on the system) direct current (DC) on which the cars’ motors run. In most track power systems this conversion takes place entirely in the wall-mount power pack. On Scalextric’s, however, the “wallwart” contains only the transformer, which lowers the voltage, while the rectifier, which turns AC to DC, is in the terminal track section, most commonly called a connection straight or power base track.

Electric current is measured in both voltage and amperage. Voltage is sometimes compared to the pressure in a water main, while amperage is analogous to the volume of water passing through the pipe.

The more voltage you feed to a motor the faster its armature will turn until the motor overheats or the centrifugal force of the armature’s rotation exceeds its structural capabilities and some part of it flies off. Either way critical parts of the motor self-destruct. When that happens with the motors commonly used in home set slot cars, it’s time for a whole new motor.   Running a 12-volt motor at 14 or even 16 volts probably won’t shorten its life too much. Bumping the power to 24 volts will let the smoke out in rapid order.

Amperage affects performance differently. A motor will only draw as much amperage as it can use. If the power supply is not delivering enough amperage supplying more will increase motor performance but only until the motor’s needs are satisfied. After that, all the amperage in Grand Coulee Dam won’t make the motor run faster. It won’t harm it, either, but it will increase the probability of catastrophic damage to track and controller wiring in the event of a short circuit.

To drive a slot car you have to vary its speed using a hand-held controller. There are two kinds of controllers in common use, resistance controllers and solid-state electronic controllers. Race sets come with resistance controllers because they are less expensive to produce than electronic ones and keeping down the overall cost of the race set is essential

resistor.jpg

A resistance controller varies the cars speed by introducing resistance, measured in ohms, into the circuit. The controller acts like a valve in a water main, preventing a greater or lesser portion of the available current from reaching the car’s motor and thereby changing the car’s speed.   When the controller trigger is in the off position the circuit is broken completely and no current is flowing. When the trigger is pulled back just a little, the resistor holds back most of the current, converting it to heat, and the car moves slowly. As the trigger is pulled back, more of the current passes through and the car goes faster.     Finally, the wiper button on the trigger makes contact with the full power band on the resistor and all the power reaches the car’s motor, just as if it were hard-wired to the power supply.

The biggest single factor determining whether the driver’s experience will be fun or frustration is his ability to control the car effectively. To do that, the controller’s resistance must match the requirements of the car. If the resistance is too low the car will take off at high speed and reach full throttle before the controller trigger is pulled all the way back. If the resistance is too high the driver may have to pull the trigger as much as halfway back before the car even begins to move. In either case he will have only a portion of the trigger’s travel over which to vary the car’s speed and it will be much harder for him to drive the car competitively.   When controller and car are properly matched the car begins to move slowly as soon as the trigger is pulled even slightly. It continues to accelerate until it reaches top speed just as the wiper button reaches the full power band.

Most 1:32 scale race sets available today come with controllers that have too much resistance for the cars, typically around 60 to 70 ohms. Non-magnet cars do fine with such controllers, but virtually all race set cars now come with traction magnets and the magnet-equipped cars are happier with 45-ohm controllers. Modified cars with strong magnets and/or more powerful motors would be better off with 30-ohm controllers and more power pack amperage as well. Parma controllers, available pre-wired with plugs for the various manufacturers’ terminal tracks, are a popular and reasonably economical upgrade for many race set owners. In choosing resistance controllers there’s a big subjective factor at work. One driver may be most comfortable driving a particular car with a 60-ohm controller while another may prefer 40 ohms with the same car.   A controller can be adapted for different types of cars by changing the resistor to one of a different ohm rating.

A solid state electronic controller bypasses these issues because it uses electronic circuitry instead of a resistor to control the car’s speed. This means that one controller of this type can properly control cars with a much wider range of driving characteristics and also accommodate a wider range of driver preferences. This allows one set of electronic controllers on your track to work well with an assortment of cars that might require two or three sets of resistance controllers. Electronic controllers are also available with plugs for all the common track systems. Higher-end electronic controllers also offer adjustments such as variable braking and variable sensitivity that further increase their versatility.

circuitry.jpg

Most race set power packs don’t really deliver enough amperage to completely supply the needs of two cars. With the set’s own cars and others with similar current requirements this isn’t too much of a problem, but when the racer starts buying strong-magnet cars or puts powerful aftermarket magnets or motors on his race set cars, he finds his cars starved for amperage. Neither car achieves full performance when both are running and when one stops or deslots the other gets a blast of power that often sends it off the track.   A power system upgrade that feeds each lane separately with its own power supply will meet the requirements of virtually any kind of car a hobbyist is likely to run on his plastic home track and ensures that nothing one car does can affect the amount of current received by the others. This can sometimes be done using the manufacturers’ standard power packs.   Scalextric, unfortunately, has stopped making its C8217 and C8241 power bases that offer the option of powering the lanes separately but they can still be found on the Internet if you look for them.

The other alternative for providing adequate amperage is to buy an aftermarket power supply and adapt it for use on your track by grafting the appropriate plug onto the power supply-to-track wiring.

Power issues can be some of the most confusing ones confronting newcomers to the hobby, and determining the best combination of components for your specific needs may require some expert help, at least until you acquire some experience. We’ll be glad to help with advice and information. You can call us toll-free at (insert phone # here) or e-mail us at (insert address here).

Copyright © 2014 Robert M. Ward. All rights reserved.

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