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Curtis PMC 1204/1205 Manual
output and thus hold the current at the limit. Because the voltage across the power MOSFET
switch is high when it is off, the current limit comparison is inhibited during the off interval by the
current limit disable circuit. The current limit is set as follows:
1. During manufacture, the current limit is set by an internal trimpot (current
limit adjust) to the model’s nominal rating.
2. During operation at extreme high or low temperatures, current limit is
reduced to protect the controller from damage. From a thermal sensor
(temp sense) on the heatsink, signals are produced to cut back the current
limit at temperatures above 75°C or below -25°C (overtemp, undertemp).
3. During plug braking operation, the current limit is reduced to give an
appropriate motor braking torque. The plug braking current is set during
manufacture; it is also user-adjustable by a trimpot (plug current adjust).
The plug current limit reference is derived from the current limit reference.
The transition to the plug braking mode is detected (plug detect) by monitoring the voltage across
the plug diode. When this diode becomes forward biased, it indicates that the motor field has been
reversed and the controller has gone into plug mode. The current limit is reduced as described, and
the frequency of the oscillator is reduced from 15 kHz to 1 kHz, to allow finer control of the
controller output while plugging. During plug braking operation, the acceleration circuit is reset
to a low level so that when drive operation resumes, the controller will go through a normal
acceleration ramp. When the motor has come to a stop, the plug diode will again become reverse
biased and the controller will revert to normal drive operation.
A-3
APPENDIX A
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