Epson LQ 1050 - B/W Dot-matrix Printer - Page 92

REV.-A
Detailed Operation (Figure 2-39.)
g,:,
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When the + 5V supply voltage is applied to the STK6722HZ (IC2A), the triangular waveform circuit “’”
*
in
the H-IC starts oscillation and outputs the reference signal (approx. 24KHz) for chopper control.
Because the output of comparator
IC2 (point Q) is high (input at the plus side > input at the minus
side) when the printer power is turned on,
TR7 turns on and base current flows to TR 1. When a high
signal (AO 1: low) is input to the base of TR3,
TR3 turns on, and coil current ICA flows from VCCI to
TR 1 to phase A to D5 to
TR3 to R 13. ICA gradually increases due to the reactance of the motor coil.
Voltage
VR13 across limiter resistance R13 increases. When VR13 becomes the same as the VREF at
pin 8 (from the reference voltage generation circuit), the output of
IC2 (point Q) goes low, TR7 turns
off, and TR 1 turns off. Then
ICA starts decreasing. When VR13 becomes less than VREF, the output of
IC2 goes high again.
The surge voltage (also called
flyback voltage) generated when TR3 is cut off by the zener diode ZD 1
(approx. 47V) between the collector and base of transistor Q5, and is absorbed by transistor Q5 to
protect
TR3 from being damaged.
When the carriage motor stops, the motor drive pulses are fixed at a set value to hold the carriage
motor. At this time,
H-IC is powered down to prevent it from overheating. When the power down ;
operation is performed (when the carriage motor is held), PD 1 of the E05A 16GA (IC7A) goes low, ‘“
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and TR 11 turns on to drop the reference voltage at the plus side of IC2. As a result, the load on VCOMAB
is reduced and H-IC is effectively powered down, At this time, approximately 5V is supplied to VCOMAB
by PWM control.
cot
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-8
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t II
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Figure 2-39. Carriage Motor Control Circuit
2-48