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. |
