An AC tachometer produces a sinusoidal signal with a frequency and amplitude both proportional to rotational rate.  While rotational rate information is encoded both into frequency and amplitude, it is often necessary to have a signal that is a DC signal voltage proportional to speed.  The diode rectifier circuit - shown below - is one way to convert an AC input signal (from the tachometer) to a DC voltage.  You need to investigate using this circuit.

• Connect the circuit.
• Use a 1.0mf capacitor.
• The resistor (R) should be 10kW.
• The input voltage to the motor should be from the power supply using the 0-6v range.
• Observe the waveform on an oscilloscope when the tachometer generates the input voltage.
• Compare the waveforms you observe with the waveforms in the rectifier simulator.
• You may want to note that the ratio of time constant to period of the sinusoidal input really determines the shape of the response.  In other words, as long as that ratio is constant (or equivalently, the product of time constant and frequency is constant) the graph of the output voltage (across the capacitor) will look the same (after adjusting the time scale).
• You should get four (4) cycles per revolution of the tachometer.
• Note the range of signal frequency at the voltage limits.  Determine the lowest and highest frequencies for the allowed voltage.
• Calibrate the tachometer, giving an expression for both the frequency and the output voltage (AC peak value) as a function or ratational rate.

• Adjust the voltage applied to the motor to get as close to 100 Hz as you can.
• Observe the waveform of the output signal on the oscilloscope.  Get a picture of that waveform using Benchlink.  Be sure that you are observing the tachometer signal without the diode rectifier circuit attached.
• Use the automatic measurement capability of the oscilloscope to determine the frequency as you adjust the voltage applied to the motor.
• Add the circuit above to the tachometer signal.
• Observe both the tachometer signal (the output of the tachometer, which is the same as the input signal to the diode rectifier) and the output of the diode rectifier voltage (the voltage across the capacitor in the circuit above).  You can do that by putting one signal into channel 1 and the other signal into channel 2.  Make sure that both channels are "ON", and that they have the same vertical sensitivity.  Adjust the zero (ground) levels (shown by the ground symbol on the right side of the scope graph) so that both signals have the reference at the same point on the scope trace (in the middle of the scope graph - vertically).
• Record what happens to the tachometer output when the rectifier circuit is attached.  You should observe a change in the tachometer voltage when the rectifier circuit is attached.  That effect occurs because the rectifier circuit draws current from the tachometer, and the tachometer voltage drops when current is drawn.  (Remember that your automobile lights dim when you have them on and start the starter motor.  Same idea.  Same effect.)  That effect is referred to as loading the voltage source (the tachometer in this case).
• Compare the output waveform with the input sinusoid.
• You should notice that the output voltage of the diode rectifier tends to follow the input voltage (the tachometer voltage) when the diode is conducting.  The difference between the output voltage and the tachometer voltage is due to the threshold voltage in the diode - about 0.7 volts.
• Get a picture of both signals - using Benchlink.
• Now, calculate the amount of decay you would expect in the output signal (due to the time-constant decay) assuming that the decay took a full cycle of the sine wave.  (The frequency of the tachometer signal should have been set to 100 Hz above.)  (Note that the decay does not go on for the full cycle, but this should provide a limit calculation for the amount of decay that does occur.  The problem of finding the exact decay is a miserable mathematical problem.)  Compare that decay with what you actually observe.
• NOTE:  Decay can be the percentage that the voltage "droops" from the peak to the time when the diode again begins to conduct (each cycle).
• Repeat your measurements so that you have data for 30, 60, 100, 200 and 300 Hz.  (To get 30 Hz, you may need to speed up the motor, then lower the speed.)  Click here for a hand-in sheet.