Head Burnout Protection - Star Micronics TSP200 Series Technical Manual

PRINCIPLES OF OPERATION
CLOCK
SI
LATCH
COMMON
STB1
STB2
STB3
2-2-4. Head current control
Heat buildup in the thermal printhead can cause print quality to degrade. To maintain uniform quality, the printer varies
the energizing time (time that STROBE remains LOW) in accordance with the head temperature.
The print head's surface temperature is calculated based on the resistance value of the attached thermistor, and energizing
time is controlled accordingly.

2-2-5. Head burnout protection

Misoperation of the CPU may cause head current to remain on too long, resulting in burn damage to the elements unless
current is shut off by other means. The printer therefore includes a burnout protection circuit that automatically forces
current off at some fixed time tw following current startup. Control is implemented through the head's BEO terminal,
which is active on HIGH; setting the terminal LOW forces current off regardless of the STROBE state.
IC1
CPU
HD ON
24V
Fig. 2-7 Timing Chart
C
EXT
B
tw=(C
Fig. 2-8 Head Burnout Protection Circuit
– 18 –
R
EXT
+5V
Q
IC6
) (R )
EXT EXT
CN6
BEO