A Brief History of Racing Power Supplies - Carl Dreher, April 11, 1996 The DC (direct-current) power supply provided in an HO racing set consists of a step-down transfer, which reduces the approximately 120 volt AC (alternating current) house line to a safer 12-18 volts AC, and a rectifier to turn the AC to DC. They also always include a fuse of some kind for safety. In many cases the fuse is the transformer itself, which is designed to burn out on overload. (Cheap for the manufacturer, expensive for you.) An AC voltage looks like this: (bad ASCII art!) ** ** ** <--- positive voltage * * * (repeats on...) * * * * * * * * * ------*----------------*----------------*------ ("ground") * * * * * * negative voltage --> * * ** ** ** Notice that although the voltage goes positive at some point, the AVERAGE voltage is zero, because the upper and lower half-cycles cancel each other. Your DC permanent-magnet motor cannot run on AC! It requires a DC voltage that looks like this: ***************** ^ | positive voltage | | ----------------- ("ground") Running a DC motor on AC is a recipe for a burned out armature. A rectifier circuit is built of diodes, which are a solid-state "one-way valve", allowing current to flow only in one direction. Very cheap power supplies (and I use cheap in all senses of the word) perform what is called "half-wave" rectification. The diode is used to remove the negative side of the AC, giving this: ** ** ** ** <--- positive voltage ** ** * * * * * * * * * * * * * * * * ------* ****************** *********** Notice now that although the voltage still isn't a straight-line DC, the AVERAGE voltage is now some positive value, which your motor can use. However, your DC motor isn't happy about this, because every time the current rises and falls, the magnetic field in the armature pole also rises and falls, sometimes in direct opposition to the direction necessary to keep the motor turning. The result is a motor that runs slower and worse, hotter. In the early 1960's, diodes big enough to handle the current drawn by a slot-car motor were expensive, so half-wave rectification was used. Now, diodes are very inexpensive, so a slightly more complicated rectification circuit is used. It is called a "full wave rectifier", and gives: ** ** ** ** ** ** ** ** ** * * * * * * * * * * * * * * * * * * * * * * * * ------* *** *** * The AVERAGE of this waveform is a higher value than before, which means your motor runs faster, bit it still suffers from all those humps of rising/falling voltage. Unfortunately, this is where most commercial HO slot car power supplies stop. OK, the power-supply lesson is over. What can we do about this? These best solution is to run on batteries. Batteries product pure DC. Unfortunately, they also are messy and in the case of car batteries, can produce hydrogen gas, which is explosive. Despite this, I used car batteries very successfully for years, combining a 12V and 6V to give an 18V supply. Car batteries are inexpensive. Six volt batteries are harder to find, since no car has used 6V for over 20 years. However, fork-lifts use them, and many battery specialty stores can get you one. Also, stores that cater to the antique car trade are a good source. You will also need two small battery trickle chargers, one 12V and one 6 volt, to keep the batteries charged. (Available from motorcycle shops.) You can also use a regular, large charger to top-up the battery between racing-heats. HERE'S AN IMPORTANT POINT: DON'T use a large battery charger on the batteries when you are racing. These can induce a full-wave AC ripple voltage on the DC battery, which defeats the purpose of using batteries! Gell-cell, or "sealed" batteries are also available, but much more expensive and (I believe) they take a bit longer to recharge. If you don't want to go to batteries, these is another solution. The goal is to fill out those valleys in the full-wave rectifier waveform. You can do this very easily with a capacitor. A capacitor is nothing more than a charge storage "bucket". When the voltage is high and the power supply is providing current to your motor, the bucket fills up. When the voltage is low and the power-supply cannot meet the demands of the motor, current is drawn from the bucket. With a big enough capacitor, the waveform looks like this: *** *** *** ** **** ** **** ** ** (etc.) * ********* ********* ***** ----------------------------------------------------- The average voltage is now higher and there is less ripple to the voltage. Your motor will run faster and cooler. (Well, that isn't exactly true. Since the average voltage is higher, you will be feeding more power to your motor and it will run hotter. But for a given speed, as set by your controller and racing hand, the motor will run more efficiently and waste less power turning the AC ripple into heat.) OK, so how do I get these magic capacitors? Radio Shack, where else! Capacitors are rated in micro-Farads (abbreviated uF) and volts. FOR SAFETY, YOU SHOULD CHOOSE A CAPACITOR THAT IS VOLTAGE-RATE 100% GREATER THAN THE VOLTAGE OF YOUR POWER SUPPLY. This is important!!! Using a low-voltage capacitor will cause it to overheat and explode. A standard rating is 35 volts, which is perfect. Don't use less. You want to buy the maximum uF's you can get. The more the better. 5000 and 10,000 uF capacitors are easily available. Don't bother with anything less than 1000. Furthermore, if you wire capacitors in parallel, the uF rating adds, that is, two 5000 uF caps wired in parallel gives 10,000 uF. To wire them to your power supply and gain all their benefits, simply connect the positive side of the capacitor to the positive side of the power supply and the negative side of the capacitor to the negative side of the power supply. That is, it goes ACROSS the power supply, NOT in-line with a wire going to the track. WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!! LARGE CAPACITORS HAVE A POSITIVE AND NEGATIVE SIDE, JUST LIKE A BATTERY. THE POSITIVE SIDE OF THE CAPACITOR MUST GO TO THE POSITIVE SIDE OF YOUR POWER SUPPLY. IF YOU REVERSE THE CONNECTION, THE CAPACITOR WILL OVERHEAT AND EXPLODE WITH THE POWER OF A LARGE FIRECRACKER. YOU CAN BE SERIOUSLY INJURED! DOUBLE CHECK THIS BEFORE YOU TURN ON THE POWER SUPPLY! Try this and I think you'll be surprised what a difference it makes. --------------------------- One last funny story: Back when most HO tracks were powered by the cheap power supplies that came with racing sets, someone had the bright idea of putting a capacitor inside a controller. It was wired across from the hot lead to the ground used for dynamic braking. This had the desired effect and made the car faster. It also had a strange after-effect. As you know, most races are timed, with a master switch cutting the power. With the capacitor in the controller, when the power was cut, the car would continue to run an extra second, because the charged capacitor acted like a battery. Everyone's car would stop except this other car would travel on a few extra feet. I saw a race won by this extra amount.