![]() The circuit within the CPLD provides the functions of a digital control algorithm that controls the voltage to be applied to the motor.Įach ADC and DAC within the design requires its own reference signal (typically a voltage). The ADC converts the voltage back to a digital value, which provides a digital representation of the analogue tachogenerator voltage. This voltage provides the input to an op-amp based signal conditioning circuit that modifies the tachogenerator voltage levels to the levels required by the n-bit ADC. The output from the amplifier provides the voltage and current required to turn the motor in either direction.Ī tachogenerator produces a DC voltage with a polarity determined by the direction of motor shaft rotation, and a magnitude determined by the speed of rotation of the motor shaft. The op-amp based signal conditioning circuit produces an output voltage that is in a range required by the power amplifier stage. The output voltage from the DAC is applied through an op-amp based signal conditioning circuit, and this provides the input to a class B amplifier. The digital output from the controller provides the data input to an n-bit DAC. One potential problem with this arrangement is that the motor speed varies with different loads connected to the motor output shaft, even when the applied voltage is constant.įigure 8.45. Varying the motor voltage will cause the motor to turn at a different speed. This is an example of an open-loop system where the voltage applied to the motor from the controller circuit causes the motor to turn. The two reverse-biased diodes are connected across the transistor collector-emitter nodes and are used to protect the transistors from the high voltages that could be produced from fast-changing currents in the inductive coils of the motor. Current flows from the common node to the negative power supply through the motor, and the motor turns in the other direction. When the input voltage (output voltage from the op-amp) is negative (with respect to the common node), the PNP transistor conducts. Current flows from the positive power supply to the common node through the motor, and the motor turns in one direction. When the input voltage (output voltage from the op-amp) is positive (with respect to the common node), the NPN transistor conducts. The class B amplifier uses one NPN and one PNP transistor. The class B amplifier drives the DC motor. The output of the op-amp drives a class B amplifier. The potentiometer output is buffered using an op-amp. Ī user sets the position of the potentiometer to produce a voltage that represents a required motor speed.The circuit is operated from a dual-rail power supply where +V S is positive power supply voltage and –V S is negative power supply voltage. If the circuit attempts to switch the transistor ON and OFF too fast, the transistor cannot react fast enough and the result will be incorrect circuit operation.įigure 8.43. ![]() When used as a switch (particularly applicable for motor control), the switching ON and OFF times also must be considered to ensure correct operation of the circuit in which the transistor is used. ![]()
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