Miscellaneous Diode Applications

Discrete Semiconductor Devices and Circuits

• Question 1

 Don’t just sit there! Build something!!

Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
2. Draw the schematic diagram for the circuit to be analyzed.
3. Carefully build this circuit on a breadboard or other convenient medium.
4. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
5. Mathematically analyze the circuit, solving for all voltage and current values.
6. Carefully measure all voltages and currents, to verify the accuracy of your analysis.
7. If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.

When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.

As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

• Question 2

A technician builds her own audio test set for use in troubleshooting audio electronic circuitry. The test set is essentially a sensitive detector, allowing low-power audio signals to be heard:

What purpose do the two diodes serve in this circuit? Hint: if you remove the diodes from the circuit, you will not be able to hear the difference in most cases!

• Question 3

When the pushbutton switch is actuated in this circuit, the solenoid valve energizes:

The only problem with this simple circuit is that the switch contacts suffer from extensive arcing caused each time the solenoid is de-energized. One way to combat this arcing, though, is to connect an ordinary rectifying diode in parallel with the solenoid like this:

Explain what causes the excessive arcing at the switch contacts, and how the presence of a diode in the circuit completely eliminates it.

• Question 4

What will an ammeter (with an input resistance of 0.5 Ω) register when connected in parallel with the diode in this circuit?

Usually, ammeters are connected in series with the component whose current is to be measured. However, in this case a parallel connection is acceptable. Explain why, and determine the ammeter’s current reading in this circuit.

• Question 5

Suppose a very important piece of electronic equipment (nuclear reactor shutdown controls, for instance) needed to be supplied with uninterruptible DC power. For reliability’s sake, this circuit gets its power from three (redundant) DC voltage sources:

The only problem with this scenario is the possibility of one of these power sources internally short-circuiting. Describe what would happen if one of the three DC power sources developed an internal short-circuit, and explain how this problem could be avoided by placing diodes in the circuit.

• Question 6

Using a commutating diode (sometimes called a free-wheeling diode) to eliminate switch contact arcing for inductive loads in a DC circuit works well, but it has an unfortunate side-effect:

With a diode in place, the release time for the solenoid increases measurably. In other words, it takes longer for the solenoid to completely de-magnetize after the switch contacts open, than if there is no diode in the circuit.

Explain why this is, and also propose a solution for the minimizing the solenoid’s release time.