For beginner slot car racers, the concept of voltage, current (amperage), and resistance maybe seem foreign to you. I will try to explain these concepts below in layman's terms:
If we may make an analogy between a slot car circuit and a water system, you will see the following:
The voltage from a slot car power supply is equivalent to water pressure; just like the higher water pressure makes water run faster when you open the faucet, the higher voltage makes the slot car run faster on the track. The voltage is measured in the unit of volts, e.g., HO slot cars typically run between 18-19V.
The current is equivalent to how much water flow through your pipes per minute; the current is measured in the unit of Amps, so it is also called amperage. You should also know that the power consumed by your motor is the product of the voltage at the motor and the current flow through the motor. So the higher the voltage at the motor, the faster the car goes; the higher the current flow through the motor, the faster the motor runs. More on this later.
Resistance is equivalent to the size of your water pipes or how clogged the pipes are; a smaller water pipe or more clogged water pipe will make water run slower for the same water pressure, and a circuit with higher resistance will make your slot car consume less current or amperage for the same voltage. Resistance is measured in the unit of Ohms. A resistor is a component to add into the circuit to change the resistance of the circuit. Your controller can be thought of as a device to change the resistance in the slot car circuit, so that you can change the amperage of your slot car motor, which in term changes the power and speed of the motor.
Now we are ready to learn the Ohm's Law. This is a fundamental physics law, which basically says that the current through a circuit or any part of the circuit is the quotient of the voltage and the resistance pertaining to the part of circuit in question. So if we look at the complete circuit of slot car track, the voltage is whatever you apply from the power supply, and resistance is the total resistance you add together in the system, including resistance from each track and track joints and that from your motor.
Now let's apply these concepts to your slot car track.
A good regulated power supply is like a good water company with stable water pressure, the voltage should not change when you add more slot cars to your system, just the same way as more houses turn water faucets on should not change the water pressure. This requires that the power supply has good regulation. Equally important, the power supply should have high enough amperage rating. Now why is that? Let's see what happens if the water company does not have enough water supply, e.g. you have a well that supplies 2-4 families. If all families turn on water the same time, the well will not be able to keep up, and alas, water pressure goes down. Same thing can happen to multi-lane track that's powered by a power supply with less than sufficient current rating. This is why it's always a good idea to get a power supply with higher current rating than you need.
Now let's look at a long track you may have. Each lane plus your slot car completes a circuit. The voltage from your power supply is not the same as the voltage at your slot car motor. This is because each track joint acts like a resistor even when you have good connection, Ohm's law says that there will be a voltage drop at each of the joints when there is current passing through. The more track joints you have between the point the power supply voltage is applied and where your car is, the more voltage drop you have. This is why the car slows down on the far side of the track, and why you need to add power tabs every 12-15 joints. By doing so, you are reducing the effective resistance in your circuit. For the same reason, if you have a car that demands higher current, it is necessary to increase the number of booster or power tabs.
Now let's take a look at the car motors. The car motors are not simple resistors, they are non-linear device, which means that the current through the motor is not directly proportional to the voltage applied to the motors. A DC motor has a working voltage, which is typically a range. If you operate above it, the motor runs too fast and generates too much heat, it can burn out very quickly. If you try to operate the motor at below the voltage, it can also be precarious, especially if the motor is loaded and needs to over come certain level of friction. The motor may not run and act as a direct short, causing it to burn out very quickly as well. Another important spec for the motor is its power rating or current rating. The higher the current rating, the more current it will draw for the same voltage applied, typically making the slot car run faster. For the same motor, increasing the voltage will also make it run faster, but potentially at the risk of reducing its lifespan. A high performance motor usually calls for power supply with higher current output.
The magnet on your car holds your car down and increases the friction of your car on the track. Typically a bigger motor requires a stronger magnet, otherwise the car can fly off the track easily. On the flip side, a stronger traction magnet or even physically lowering the stock magnet for more down force will require you to increase the amperage of your car motor.
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