TM 5-6675-316-14
(2) When the stepper signal input is high, the Darlington pair Q25 is biased
on. This allows current to flow from ground through emitter resistor R15 and
through the motor coil to the +26 V supply.
(a) With the stepper signal input low, the Darlington pair Q25 is biased
off and no current flows.
(b) With the Darlington pair Q25 on, current flows through R15 and, as
current flow increases, the voltage drop across R15 increases, reducing the for-
ward bias on the Darlington pair Q25 and the current flowing through the motor coil.
This effectively controls the coil current.
2-27
