A good part of understanding complex circuits is breaking it down into simpler parts.  As an example, throughout the semester we'll figure out how a 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.  Throughout the semester we'll solve individual parts.

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

There is a 100k resistor missing between "+" input of comparator 1 and ground on the schematic.  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 (only the resistor values have changed) in lab and it is also in E72 (P)review.

Problem B) What does the portion of the circuit shown below do?  Assume a 0.1W 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, rZ). Note: in class we usually put Vin on the left and Vout on the right, this shows the opposite. Also, the 50V marking near the capacitor is just its maximum rating, the voltage across the capacitor is just the battery voltage, 24V.

  1. How much current would go through the Zener when ILOAD=0? Is this a problem?
  2. What is the maximum value for ILOAD before the power supplies fails (i.e., before the output voltage drops below 5.2V)?
  3. For this maximum current calculate the power dissipated in the load ("useful power") as well as the power dissipated in the circuit the 500Ω resistor and zener ("wasted power") and calculate the efficiency, η=useful power/total power.