Brother PT-2500PC, Service Manual, Page: 61 / 79

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3.2.8 Head Voltage Detection Circuit and Ambient Temperature Detection Circuit
Fig. 3.2-9 shows the head voltage detection circuit and ambient temperature detection
circuit. Through these circuits, the CPU monitors the voltage applied to the thermal head
and the ambient temperature and then controls the STR pulse width.
(1) Head voltage detection circuit
This circuit, which is composed of divider resistors R30 and R29, steps down the
power source VH and then feeds the output to the analog input port AN1 of the
CPU. According to the sensed voltage, the CPU controls the STR pulse width.
(2) Ambient temperature detection circuit
This circuit consists of thermistor TH2 which is pulled up to Vcc through resistor R25.
It converts the resistance of the thermistor (which will vary depending upon the
ambient temperature) to the voltage level and then feeds its output to the analog
input port AN0 of the CPU. According to this data, the CPU determines the optimum
STR pulse width.
Fig. 3.2-9 Head Voltage Detection Circuit and Ambient Temperature Detection Circuit