TM 5-6675-316-14
(9) Carriage escapement board J12 supplies 10 signals to control the carriage
stepping motor. These 10 signals control 10 constant current Darlington driver
circuits which supply current to the five motor coils to step the motor.
(a) Pairs of Darlington driver circuits associated with each of the five
motor coils enable current to flow in either direction through the motor coil.
(b) When a signal is applied to U11, and it is high, it turns on the
Darlington pair Q7. This allows current flow from ground through motor coil,
Darlington pair Q7 and emitter resistor R6 to the +26 V supply. When the input
signal is low, the Darlington pair Q7 is biased off and no current flow takes place.
(c) When signal input is applied to U3, Darlington pair Q6 is turned on.
This allows current flow from the -26 V supply, through R5 and the Darlington pair
Q6 to the motor coil. When the input signal goes low, the Darlington pair Q6 is
biased off and no current flow takes place.
(d) The current in the motor coils is regulated to 1.35 amps, regardless
of the temperature or motor impedance. When current flows through R5 or R6, a
voltage is developed across the resistance. If current tries to rise above the
preset level of 1.35 amps, the voltage drop across the resistors will increase.
This will reduce the forward bias on the Darlingtons. This in turn reduces the
current passing through the motor coil. Current in the circuit can, therefore, be
measured by the voltage drop across R5, R6. It should equal 1.35 V, as R5 and 6 = 1
ohm.
(e) When the Darlington is biased off, the resulting collapse of the
magnetic field around the motor coil can give rise to voltage transients which could
damage the Darlingtons. Bridge rectifier CR1 bypasses the voltage spike back to the
+26 V and -26 V supplies. The other driver circuit pairs are identical to the one
described.
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