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3.5.3 Head temperature
sensor
circuit
When the printer prints continuously for a prolonged time, the print head faces the risk of getting
overheated. There, this head temperature sensor circuit finds out the internal temperature of print
head by sensing the resistance value of a thermistor located inside the head.
Make the printer execute the following operations, according to the internal temperature of its print
head.
a. T< 105"C
. Execute regular print operation (bidirectional printing).
b. 105°CsT
Stop print operation. As soon as the internal temperature of print head lowers down to below
105"C, resume the bidirectional printing of above item 'a'.
Below is given a conversion table of internal temperature of print head and resistance values of
thermistor. The resistance value of thermistor is sensed every alternate line.
AN voltage is controlled to set 3.868V at 105"C.
Temp.
Resistance value of thermistor (Kfl)
("c)
Min.
Typ.
Max.
T
105
2.622
2.926
3.232
One example of head temperature sensor circuit is given in Fig. 3-8. This circuit senses the
temperature by employing an A/D converter built inside CPU.
TH
Thermistor TM541OH(HITACHI)
Vcc
3 [
R1
10KO * l~o
+5V +
t
cl
O.OIWF Ceramic capacitor (noise elimination)
VREF
*
+5V
TH
5!
AN
cl
RI
CPU
777
A
Fig. 3-8 Head temperature sensor circuit (example)
-13-
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