Let's figure out how this commercially available motor controller (schematic from http://www.diygokarts.com/vb/showthread.php?t=3263&highlight=schematic) works.  Some changes to the schematic were made to correct errors.

The LM339 is a quad comparator.  Assume that each section (labeled 1-4)  behaves in the same way as the LM311 from lab.

 There seems to be a 100k resistor missing between "+" input of comparator 1 and ground.  This is included below.

Problem A) What does the portion of the circuit shown below do?  In particular, sketch the capacitor voltage.  The capacitance is 680 pF.  You saw a similar circuit in lab.


Using the solution to Problem A) we can simplify the schematic.

Problem B) What does the portion of the circuit shown below do?  Assume a 1W zener (19mA max), so that we want at least 1.9mA flowing through the zener at all times.  You also know that the resistor can dissipate a maximum of 1W before overheating.   For this problem assume that the voltage across the zener is always 5.2V (ignore any small signal resistance).

  1. How much current goes through the Zener when ILOAD=0?
  2. What is the maximum value for ILOAD before the power supplies fails?
  3. For this maximum current compare the power dissipated in the load ("useful power") to the power dissipated in the 500Ω resistor ("wasted power") and calculate the efficiency, η=useful power/total power..


Using the solution to Problem B) we can simplify the schematic.

Problem C) What does the portion of the circuit shown below do (we are ignoring comparators 3 and 4).  The device labeled "Hall" is a throttle control (that uses a Hall-effect sensor) - the output of the throttle is between 1V (minimum) and 4V (maximum). 

(Note: we can neglect comparators 3 and 4 by assuming they are inactive (i.e., their outputs are open circuits).  When they become active, their outputs go low - we'll deal with that eventuality in a few weeks)

  1. Sketch the output of the comparator if the "Brake" switch is open and the output of the hall effect sensor is:
    1. 1V
    2. 2 V
    3. 3V
    4. 4V
  2. Sketch the output of the comparator in the four cases above if the Brake switch is closed.

Using the solution to problem C, we can further simplify the schematic with a PWM block controlled by the throttle with an "inhibit:" input that will set the PWM output to zero,.  The "inhibit" is activated by the "Brake" switch.

Problem D) The battery voltage is nominally 24V but will drop as the batteries wear down, or if the load on the batteries is too large.  The circuit is designed to set the PWM output to 0 if the battery voltage drops too low.  At what battery voltage will the PWM be inhibited.  You may consider only the circuitry shown below.


Using the solution to problem D, we can further simplify the schematic with a PWM block controlled by the throttle with an "inhibit:" input that will set the PWM output to zero,.  The "inhibit" is activated by the "Brake" switch.

Problem E) The two bipolar transistors, Q3 and Q4 act as a unity gain amplifier (a buffer).  We can redraw the rightmost part of the circuit as shown (for the moment replacing J1 and J2 with wires.

a) Assume, for the moment, that the 33Ω resistors have no effect on the circuit.  How will the motor behave as the hall effect throttle goes from 1 to 4 volts?

b) What purpose do the diodes serve?

The 33Ω resistors seem problematic. They will slow down the turn on and turn off of Q1 and Q2 which decreases the efficiency of the circuit (When the transistors are fully on, they act (nearly) as short circuits and dissipate little power; when they are fully off they act as open circuits and dissipate no power; when they are partly on they have current through them and voltage across them and can dissipate significant power).  One reason to use the resistor is to decrease power dissipated in the buffer, another is to decrease the maximum current coming from the buffer.  Consider the circuit shown:

c) Find an expression for the power dissipated in the resistor as the capacitor (which models the gate of the transistor) charges from 0 to Vdd and show that the total energy dissipated (i.e., the integral of the power from t=0 to t=∞ is independent of R.

d) Use the above result to explain how adding the 33Ω resistor decreases the power dissipated in the buffer when it turns on the transistor. Note: it also limits the maximum current into the gate of the transistor.


From the solution to Problem E we see that the circuit shown (above) is simply a drive circuit for a PWM for the motor.

Problem F) In the schematic below I have removed the parallel elements (diode and transistor/resistor) and replaced the motor by its electrical equivalent (a back emf (Eb) a resistor and a capacitor).  I measured a motor in the lab and it had an inductance of about 2mH and a resistance of 2Ω, so τ=L/R=1mS.  The period of the PWM in this circuit is T=1/12.8kHz=0.078mS.  Let's ignore the back emf for now (it is relatively constant), so set Eb=0.  When the transistor is on, we have 24 volts across the motor, when it is off we have approximately zero volts across the motor (1 diode drop, because the diode is on).

a) If the duty cycle is 50%, what is the DC current through the motor?

b) Approximately how large is the ripple in this current?

c) Repeat a and b for 10% duty cycle.

d) Repeat a and b if Eb=10V.