Abstract: A pen controller in a thermal printer generates a pulse width modulated (PWM) signal based on a reference voltage. The PWM signal is passed through a filter, resulting in a first voltage that varies with the duty cycle of the PWM signal. Additionally, a temperature sensing resistor on a pen of the printer generates a second voltage that varies with the temperature of the pen. A comparator is used to compare the first and second voltages to determine when the pen is at a proper temperature for printing. As the actual reference voltage in a printer may vary from its intended value due to variations in the fabrication process used to create the components of the printer, the printer is calibrated to account for such variations. The printer is calibrated by the pen controller changing the duty cycle of the PWM signal (thereby changing the first voltage) while the second voltage remains relatively constant.