PWM for 5v. motor with torque feedback ver. 1

PWM motor speed control, with torque control feedback.


Q6 is the mosfet motor driver.

Q5 delivers positive voltage to the gate of Q6, to cause Q6 to conduct harder so as to drive the motor speed faster.

Q4 and Q5 are the transistors that make up an 'Square Wave Oscillator',

The design of it is that Q5 is biased so that it is conducting as large of current as possible, so as to put the gate of Q6 at a lower voltage, so the motor is running at its lowest speed.

This is done by establishing a 10% duty cycle for Q5.
Then bias Q4 to give the highest duty cycle so the motor runs maximum speed.

Now a way needs to be established to manually vary the duty cycle so as to vary the speed of the motor.

This is done by Q7, it is biased into conduction through R12, then can be adjusted for minimum conduction through the manual control, R2.

R2 varies the duty cycle, as well as the frequency of the oscillator, which in turn varies the speed of the motor.

As R2 becomes smaller in adjustment, more neg. voltage is applied to Q7, turning it off more, so Q4 conducts less and its collector voltage rises more, which in turn causes Q5 base to become more positive, and Q5 conducts more, causing its collector to go more negative, this causes the duty cycle to change to a smaller value, and this is applied to the gate of Q6, turning it off more, so the motor slows down.

As the manual speed controle R2 becomes larger, then less neg. volt. at Q7 base so Q7 conducts more, making Q4 conduct more dropping its collector voltage, this in turn is fed to the base of Q5, causing its base voltage to drop, so Q5 conducts less, and its collector voltage rises through R6,
so now the duty cycle becomes larger, and this is being applied to the gate of Q6, causing Q6 to conduct harder, making the motor to run faster.

That is all there is to PWM with a manual speed controle.

However Q7 being used as a input from the manual speed controle, allows a feedback signal to be applied to it, so a automatic speed controle can be implemented, during a motor stalling situation.

This works as follows:

As the motor is running at its slowest speed, there is a voltage drop across R9, and this voltage at the top of R9, is being used as a feedback voltage to be compared to the voltage that will be developed across D4, so as the motor stalls its impedance goes down, making it more positive at the top of R9, when this gets possitive to cause Q8 to go into conduction, due to the voltage drop across D4, and the base emitter of Q8,
then Q8 will conduct current into the base of Q9, (PNP),

so now Q9 goes into conduction, as Q9 conducts, it applies a positive signal voltage to the gates of both Q11 and Q1 (mosfets), which in turn brings them into conduction.

Now for the moment assume that Q9 is NOT yet on.

Starting from the opto isolator and working back here is what is happening.

The LED in the opto isolator is turned on when Q3 is turned on, Q3 is acting like a variable resistance, for the LED current.

Q3 is turned on when its base is positive and its collector is more positive.
So tracing back from the collector, positive voltage is applied through D5 and R15.
But R15 is at a negative potential due to the conducting state of Q10, so as long as Q10 is conducting hard, Q3 remains off, and the opto.iso, is off also.

On top of that to ensure that Q3 has no leakage current to the opto iso, then a second inverter is coupled to its base which is Q2, so with Q2 conducting due to base current from R1, it causes the base of Q3 to be low, so now Q3 is completely out of the circuit, as long as both Q10 and Q2 are conducting.

Now when Q9 is turned on as before due to the motor stalling, Q9 will cause mosfets Q11 and Q1 to conduct making the base of Q10 go negative which allows its collector to rise positive through D5 and R15,
which makes the collector of Q3 be positive, also simultaneously the base of Q2 goes negative allowing its collector to rise positive through R4, turning on Q3.

So now Q3 is in full conduction, which feeds a positive signal the the LED in the opto.iso.,
which in turns makes the output transistor of the opto.iso. conduct positive signal, which in turn is fed back to the base of Q7, causing it to rise, which causes the oscillator to increase its duty cycle accordingly, making the motor speed up during this stalling period.

When the motor is out of the stall, Q8 drops out and the motor returns back to its original speed, set by R2.

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