Resistive (Heat) Elements and Dot Pitch (Terminology)
Thermal Printhead Terminology
A brief explanation on commonly used terms related to thermal printheads, such as heat elements and dot pitch.
- Heat Elements / Dot Pitch / Dot Density / Print Speed / Print Cycle / Print Energy / Print Supply Voltage / Clock / Data In / Latch / Strobe / Print Life / Print Division / Thermistor
Resistors that are used as heat sources for printing. The size and configuration of the heat elements must be taken into account depending on the usage conditions. In addition, there are differences in characteristics between thick-film and thin-film printheads.
Indicates the distance between heat elements.
Dot Density (Dots/mm, DPI)
Refers to the resolution of heat elements, which is typically expressed in one of two ways: dots per millimeter (dots/mm) or dots per inch (dots/inch or DPI). Since 1 inch = 25.4mm, the following relationship holds between the two.
Print Speed (IPS)
Print speed denotes the actual print speed on thermal paper. IPS here stands for Inches Per Second, which represents the distance printed per second in inches.
Print Period (SLT)
SLT, short for Scanning Line Time, indicates the drive cycle of the heat elements. The following equation shows the relationship between SLT, print speed (IPS), and dot density (DPI).
SLT ＝1 / （DPI ｘ IPS）
Applied Energy (Eo, mJ/dot)
Refers to the energy (product of power and time) provided in order to raise the temperature of the heat elements to a level required for printing. The applied energy listed in the datasheet (specifications) denotes the energy consumed (required) by the heat elements, and the total energy that must actually be supplied to the thermal printhead must take into account the above energy plus the reactive energy consumed by the drive circuit and other electrodes. These calculation conditions are defined in the specifications for each product. The applied energy defines the life of the printhead. Care must be taken to never exceed the maximum applied energy value (energy limit curve shown in the datasheet) for each head.
Print Supply Voltage (VH)
The supply voltage required to energize the heat elements. The value of the heat element resistance of the printhead must be selected based on the print supply voltage used. Typically this is 5V, 12V, or 24V. In addition to this print supply voltage, logic supply voltage (3.3V or 5V) is needed for the drive IC.
A signal used for inputting a serial signal to the driver IC. When the CLK signal input goes from Low to High (and when the DI signal voltage is High), High data is input to the thermal printhead. Conversely, when the CLK signal is input (and when the DI signal voltage is Low), Low data is input to the thermal printhead.
Data In (DI)
Serial data input signal of the print pattern.
Latch (LA, LAT)
One of the functions of the driver IC, this refers to the control signal from the shift register to the register that temporarily holds data from the externally supplied print data.
A signal for supplying current to the heat elements corresponding to the signal (held by the LAT signal) input to the printhead. The pulse width of this signal determines the time the heat elements are energized, making it possible to control the applied energy. The STB signal is set to positive or negative logic by the printhead.
The lifetime of a thermal printhead is defined as the point when the resistance of the heat elements changes by more than 15% from the initial value. Lifetime is usually specified in one of two ways.
・ Pulse life: The number of pulses applied under standard print conditions
・Running life: The life when thermal paper is run under standard print conditions
Line-type thermal printheads contain many heat elements. When these heat elements are energized at the same time the current consumption can become very large, so in order to reduce the power supply load and voltage drop within the head itself print division is typically performed. Print division is achieved using the strobe signal.
A separate thermistor is mounted to monitor the temperature of the thermal printhead. Control by referencing the thermistor detection temperature is necessary to ensure that the substrate temperature of the printhead does not rise above the threshold value.