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.