ELEC 242 Lab

Experiment 1.2

Stroboscopic Tachometer



We now have a source of light (the LED) which we can control by varying the current through it. To build a stroboscope, we need a source of periodic, narrow pulses of current whose current we can control. The function generator is a source of periodic, square waves of voltage. However, with a bit of fiddling, we can make it work for our purpose.

We need to do two things: convert the output to current, and make the pulses narrower. We will use the internal resistance of the function generator to limit the current. We will use the duty cycle control to make the pulses narrower.

In Question 1 you considered what would happen if you were to connect an LED directly to an ideal voltage source. Although the power supply is very close to being an ideal voltage source, the function generator is not. According to its specifications it has an output impedance of 50 ohms. That means that it can be modeled as an ideal voltage source in series with a 50 ohm resistor. So if you were to connect the function generator directly to the LED you would have the following circuit:

This is essentially the same circuit we used in the previous experiment. This means we can use the function generator's internal resistance as a current limiting resistor and drive the LED with the desired current by setting an appropriate voltage.

For our stroboscope, we want the LED to be as bright as possible (without destroying it). According to the package, the maximum rated forward current for the LED is 50 mA.

Part 1: Building a Stroboscope

Step 1:

From your measurements in the previous experiment, estimate the forward voltage of the LED which corresponds to a current of 50 mA. Add to this the voltage drop across the 50 ohm internal resistance of the function generator at a current of 50 mA. This will be the peak open-circuit value of the function generator voltage. Call this value .

Step 2:

Connect your BNC T connector to CH1 of the scope.

Step 3:

Use a BNC patch cord to connect one end of the T to the function generator output. Connect a BNC clip lead to the other end of the T.

Step 4:

Set the function generator to produce a 10 Hz Square wave. Adjust the AMPLITUDE control so that the peak-to-peak (p-p) voltage is the twice the value you calculated above (i.e. the signal alternates between and ).

Step 5:

Connect the clip leads from the T connector to the LED. Connect the positive (red) clip to the cathode (shorter lead) and the negative (black) clip to the anode (longer lead) of the LED.
The LED should be flashing at 10 Hz.

Step 6:

Rotate the DUTY control of the function generator fully clockwise. The rate of flashing should slow down and the length of the flashes should be much shorter than the off times. If the LED is on longer than it is off, reverse the leads.
We now have a stroboscope which we can use to measure the speed of the motor. When we changed the duty cycle, we also changed the frequency of the waveform. However, the digital readout on the function generator correctly displays the new frequency, so all is well.

Part 2: Measuring motor speed

Step 1:

Get an 18 V motor and a strobe test disk from the cart. If they aren't already connected together, press the disk onto the shaft of the motor with the large end outward.

Step 2:

Set the DMM to DC Volts. Remove the probes from the DMM and replace them with your BNC-banana adapter. Make sure the ground bump is connected to the COMMON terminal of the DMM.

Step 3:

Plug a BNC clip lead into the adapter and connect the clips to the terminals of the motor.

Step 4:

Plug a black banana patch cord into the COMMON terminal of the power supply (blue binding post). Plug the other end into the left hand hole in the BNC-banana adapter. Plug a red banana patch cord into the 0 TO +20V terminal of the power supply. Plug the other end into the right hand hole in the BNC-banana adapter.

Step 5:

Because the diameter of the disk is greater than the width of the motor, you can't set it down or it will rub on the bench top. Either hold it firmly in your lab partner's hand, or wedge it between the top shelf of the bench and the power supply, with the disk hanging over the edge.

Step 6:

Turn on the power supply and slowly increase the voltage until the motor begins to turn. Note whether it's turning clockwise or counterclockwise. Increase the voltage to 20 V.

Step 7:

Shine the flashing LED on the disk and adjust the frequency until the disk appears to stop with only a single stripe. Increase the frequency until you find the highest frequency which will stop the disk with a single stripe. This is the frequency of rotation of the motor.

Step 8:

The frequency displayed on the function generator will be in Hz. Since motor speed is usually given in revolutions per minute (RPM), multiply the frequency in Hz by 60 to get RPM. You should get a number between 5000 and 6000 RPM. If not, check your calculations and make sure you do not have an aliased rate.

Step 9:

Slightly increase the frequency of the function generator and verify that the image appears to rotate slowly in the opposite direction from the actual rotation of the disk. Slightly decrease the frequency and see that it rotates slowly in the same direction.

Step 10:

Reduce the voltage in 2 V steps until the motor stops running. At each step, measure the speed of the motor and record the voltage and the speed.

Step 11:

Plot the motor speed vs. voltage. You should get a straight line which crosses the x-axis at around 1 or 2 volts.

Step 12:

Compute the slope of this line. How?

Step 13:

Reverse the wires from the power supply to the motor. Verify that the motor runs in the opposite direction.

Question 2:

How effective is the LED stroboscope as a tool for measuring speed? Suggest ways in which it could be improved.