TM 5-6675-316-14
(4) A single Darlington driver circuit associated with each of the four coils
allows current to flow in one direction through the motor coil, the current either
flows to ground via diode CR15 or to +26 V via Q39, depending on whether Lead 1 is
high or low.
(a) When Lead 1 is biased high and a stepping signal is received at
Darlington Q13, then Q39 is on. This allows full current to flow from the -26 V
supply through emitter resistor R9 to Q13 which is also on, through the motor coil
and Q39 to the +26 V supply.
(b) When Lead 1 is biased low, Darlington Q39 is off and the motor coil
return is through diode CR15 to ground. This allows only half of the current to
flow through the motor coil.
(c) The current in each motor coil remains the same regardless of
temperature or motor impedance, the base of each Darlington being clamped to a
reference voltage.
(d) Constant current regulation is achieved in this manner: Emitter
resistor R9 provides negative current feedback which changes the bias of Darlington
Q13 according to the current drawn by the motor coil. Current in the coil can be
measured by sensing the voltage drop across emitter resistor R9.
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