A thermal printhead has a series of heating elements that will work well under controlled temperature conditions. If a temperature overload occurs due to printing without paper or a fault within the driving circuit or software, one or more of the heating elements may be damaged or burn out.
To avoid this, never drive the printhead without printing media loaded (paper, card, etc.). Care must be taken to ensure the various absolute maximum ratings detailed in the specification are observed. In particular, the stated maximum energy level at the applicable ambient temperature must not be exceeded.
Do not subject the applied element pulses to any voltage spikes, even those of a short duration.
Take care when designing the circuits, including the power supply, to allow for switching large currents. Refer to the detailed specification to establish the maximum current if all dots are energised. Also check how many dots may be energised at the same time as certain wider print width models cannot have all dots energised together.
Use the following sequence to turn on the power voltages for printheads with driver IC’s mounted on the printhead:-
Apply the driver IC power source voltage (Vdd)
Activate the control function and clear any data in the printhead shift register.
After confirming the control system operates correctly, apply the power voltage (VH).
For safety purposes, the timing of large switching current and the timing of the data transmission (input of data to the printhead) should be set so that they do not overlap.
Observe the following precautions to prevent driver IC malfunction which may occur when large currents are switched:-
Ensure the power cable has an adequate current rating.
When connecting the VH power source and the Vdd logic voltage, be careful that no potential difference occurs between them.
For safety purposes, it is recommended that a protective circuit be included in the circuit design to protect the power cable against excessive voltage.
ESD from external sources can reach very high levels, particularly under high speed printing conditions. Such ESD levels can cause damage to the printing elements and built-in IC’s if precautions are not taken. The ESD resistant coating applied over the heating elements on many printhead models will avoid, or help to avoid, damage but it is recommended that if high ESD is an expected phenomena, suitable ground/earth path should be incorporated in the mechanical design.
Kestronics Ltd,Unit 26, North Orbital
Commercial Park Napsbury Lane, St.
Albans Hertfordshire, AL1 1XB