Clipper and Clamper Circuits

My first encounter with this application of diodes came when I was quite young, soldering together a kit multimeter. I was very confused why the meter movement had two diodes connected to it in parallel like this. All I knew about diodes at the time was that they acted as one-way valves for electricity. I did not understand that they had a substantial forward voltage drop, which is the key to understanding how they work in applications such as this. While this may seem to be a rather unorthodox use of diodes, it is actually rather common.

Incidentally, I highly recommend that students build such an audio test set for their own experimental purposes. Even with no amplifier, this instrument is amazingly sensitive. An inexpensive 120 volt/6 volt step-down power transformer works well as an impedance-matching transformer, and is insulated enough to provide a good margin of safety (electrical isolation) for most applications. An old microwave over power transformer works even better (when used in a step-down configuration), giving several thousand volts worth of isolation between primary and secondary windings.

The circuit even works to detect DC signals and AC signals with frequencies beyond the audio range. By making and breaking contact with the test probe(s), “scratching” sounds will be produced if a signal of sufficient magnitude is present. With my cheap “Radio Shack” closed-cup headphones, I am able to reliably detect DC currents of less than 0.1 μA with my detector! Your mileage may vary, depending on how good your hearing is, and how sensitive your headphones are.

I have used my own audio detector many times in lieu of an oscilloscope to detect distortion in audio circuits (very rough assessments, mind you, not precision at all) and even as a detector of DC voltage (detecting the photovoltaic output voltage of a regular LED). It may be used as a sensitive “null” instrument in both AC and DC bridge circuits (again, DC detection requires you to make and break contact with the circuit, listening for “clicking” or “scratching” sounds in the headphones).

Another fun thing to do with this detector is connect it to an open coil of wire and “listen” for AC magnetic fields. Place such a coil near a working computer hard drive, and you can hear the read/write head servos in action!

If it isn’t clear to you already, I am very enthusiastic about the potential of this circuit for student engagement and learning . . .

The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.

Ask your students why the waveform will be square rather than sinusoidal. Is it a perfect square-wave? Why or why not?