Sometimes things simply don't work, despite everything looks good. In such cases it's the best idea to work step by step to find the cause.

Typical thermistor circuitry.

Resistors change their resistance with temperature. For fixed value resistors this effect is unwanted. Thermistors take advantage of this effect and do so in a predictable fashion.

A typical thermistor circuitry is shown to the right. About all RepRap controllers use the same design.

• CT1 means to smooth out noise, it's not directly involved in the measurement circuitry.
• Connector is where the thermistor is connected.
• RT1 makes up a voltage divider together with the thermistor.

Connecting components this way gives a voltage on the signal line which depends on the thermistor's resistance and accordingly, on the thermistor's temperature. If the thermistor is very hot, its resistance is near zero, so the signal's voltage will be close to zero. If the thermistor is very cold resistance is very high, signal's voltage will beclose to the supplied voltage, 3.3 volts (or 5 volts on other controllers). In between is our measurement range.