3.6.7Detecting Abnormal Temperatures of the Thermal Head
To protect the thermal head and to ensure personal safety, abnormal thermal head temperatures must be detected by both hardware and software as follows:
zDetecting abnormal temperatures by software
Design software that will deactivate the heat elements if the thermal head thermistor (TH) detects a temperature of 80°C or more (thermistor resistance RTH ≤ 3.80 kΩ), and reactivate the heat elements when a temperature of 60°C or lower (RTH ≥ 7.45 kΩ) is detected. If the thermal head continues to be activated at a higher temperature than 80°C, the life of the thermal head may be shortened significantly.
zDetecting abnormal temperatures by hardware
If the control unit (CPU) malfunctions, the software for detecting abnormal temperatures may not function properly, resulting in overheating of the thermal head. The overheating of the thermal head may cause damage to the thermal head or cause skin burns.
Always use hardware in conjunction with software for detecting abnormal temperatures to ensure personal safety (this may not prevent damage to the thermal head).
Using a window comparator circuit or similar detector, design hardware that detects the following abnormal conditions:
(a)Overheating of the thermal head (approximately 100°C or higher (RTH ≤ 2.09 kΩ))
(b)Faulty thermistor connection (the thermistor may be open or
If (a) and (b) detected, immediately deactivate the heat elements. Reactivate the heat elements after they have returned to normal.