pulse width modulated (PWM) controller for high current

Posted on Feb 5, 2014

The second 555 timer was configured as a monostable circuit, commonly referred to as a one shot since an output pulse only occurs if there is a trigger on the input. When this timer is triggered, the potentiometer in the RC timing network allows the output duty cycle to vary, which determines how long each output pulse is high. As the output high

pulse width modulated (PWM) controller for high current
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pulse gets shorter, less current will flow in the motor causing the motor to slow down. Figures 2 and 3 show a high and low duty cycle pulse for the circuit, respectively, captured on an oscilloscope. The top graph of each figure is the output pulse, while the bottom graph is the input into the monostable timer. If the duty cycle is brought down to zero percent, there will no longer be a high part of each pulse, therefore no current will flow. The capacitor value was chosen to be 0. 039uF. I wanted a minimum RPM of 500 for the motor, so the potentiometer was chosen accordingly. Equation 3 shows how the value of the potentiometer was chosen. The maximum resistance was calculated as 46k ohms, therefore, I used a standard value 50k precision audio potentiometer. This would assure that I could achieve a low RPM on the motor. Position of the potentiometer can be adjusted using the rotary to linear action of the servo system of the radio. For this design, a linear slide potentiometer was chosen to accommodate this rotary to linear action of the servo system. As mentioned, the motor can draw several amps, requiring a robust transistor to provide the current control in the circuit. Based on their current handling capabilities, TIP120 BJT transistors were chosen as the output stage to drive the motor. Two TIP120 ’s were wired in parallel to distribute the current load and keep either transistor from running too hot. Also, space is...

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