module (Figures 3 & 4). This arrangement is called a voltage divider network. As the temperature being sensed increases, the resistance in the thermistor decreases and the voltage drop across that thermistor lowers in value (Figure 5). An easy way to remember how the circuit's analog voltage reacts to the resistance decrease as temps rise is to apply Ben Franklin's Fluid Flow principles to the circuit. Franklin correctly theorized that electrons react to movement much like water in a pipe. Impede the flow with restriction and you'll have a drop in the water pressure. Voltage is much like water pressure and resistance (measured in ohms, symbol: Ω) is much like a restriction. The larger the restriction the more the pressure drop. Using the same analogy, the larger the resistance the larger the voltage drop. If General Motors General Motors thermistors have been a key piece of technology for checking temperatures in automotive applications. That short list doesn't even begin to include the thermistors that are built into electronic components such as MAP sensors, MAF sensors and the electronic voltage regulator inside most alternators. All kinds of electronics have built in thermistors to compensate for changes in temperature. Thermistors themselves are quite simple. They're made of a semiconductor material that reacts to temperature changes with resistance changes (Figure 1). Most automotive thermistors are of the NCT (Negative Coefficient Temperature) design that reacts to a temperature increase with a resistance decrease (Figure 2). Thermistors are connected in series with another fixed value resistor internal to a Figure 3 & 4: Most module analog inputs are pull down switches (left) or thermistors that complete a circuit which is tied to a 5 volt reference voltage in the module. Pull up circuits are often used as discreet logic inputs that provide modules with voltage in order to report data such as brake pedal or door latch status. April 2015 2 MACS Service Reports