WHAT THE HECK IS A “NEGATIVE TEMPERATURE COEFFICIENT” THERMISTOR?
That was enough to get me started on Google and within minutes I had tracked down the device he had described. It is called an “Inrush Current Limiter” and is a specialized type of thermistor. You may know that a thermistor is a device that changes resistance as its temperature changes. If you have an inexpensive digital thermometer chances are good that the sensor on that device is a thermistor. Most thermistors have a higher resistance as their temperature increases. In our application, however, we need what is called a Negative Temperature Coefficient Thermistor (NTC) that presents less resistance to the circuit as it becomes hotter. The theory behind using these devices to slow the acceleration of our train is simple. The thermistors I chose have a resistance at room temperature of 20 ohms. When power is turned on the train starts out very slowly since much of the power from the transformer is being absorbed by the thermistor. This power must be dissipated and serves to heat up the
