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Labwork:

For this Lab and like all others, circuit simulation apps allow understanding as well as to check lab work. The accuracy of app calculations should be better than a few percents, if not <= one percent most of the time. The discrepancy between calculation and measurement is usually due to circuit elements' manufacturing variation compared to model calibration. Hence, if you see a discrepancy more than 10%, it should be a warning, perhaps some circuit elements degrade or not what you think they are (a most common mistake is wrong-value resistance; second most often is degraded solid state devices, e. g. diodes, transistors, or IC chips). If you see 20% or more, that is definitely a red flag, often indicating wrong circuit wiring, accidental shorts, parasitic connections, or perhaps you are not measuring the right thing. Always report relative discrepancy (error) in your Lab reports. Don't worry if a discrepancy is less than 5%. Do some checking if it is above 5%.

1.  Part A: LED rectifier circuit

Fig. 5
    

- Step A1: Build the circuit above. You can use other LED colors as you have/wish, but the default color scheme as shown is recommended so that you can identify the circuit meshes on the breadboard.  If you use a different color scheme, make sure selecting the correct LED colors in the App associated with this circuit.

- Step A2: Chose a positive source voltage between 7-8 V (recommended 8 V). You can go a bit higher or lower depending on your LEDs, but the key criterion is that the reverse voltage between B-D and C-A should be no more than 5.5 V. The gif above shows different source voltages. Too low voltage that results in LED biases below the contact potentials will essentially shut off the circuit. Hence, you want enough current flow in the bridge (but don't burn the bridge).

- Step A3: Once you decide on the source voltage, measure the current. Then, measure the voltage around the mesh as shown in Fig. 5. There are 6 elements of the mesh, including the source. Add their voltage up, which should be near zero. Determine how accurate the measurement is by using this formula:

  (Eq. A.1)
and report all the results in the lab notebook.

As a practical approach to apply Eq. A.1 above, use this Mathematica code:

v= {v1, v2, v3, v4, ... } ;   (* where v1, v2,... are the voltages. Type in your voltage measurements, in curly brackets, separated by comma *)

KVLtest = Total[v]/Sqrt[ v . v]

Shift+Enter to have the code executed (below is the screen shot of an example).

- Step A4: Use the same voltage magnitude, but reverse the polarity. Measure the current and the mesh voltages like Step 3 above (i. e. repeat Step 3 but with negative source voltage).

- Step A5 (extra credit): For those who are eager and adventurous, you can use the signal generator to generate a square wave with +V and -V approximately the same as what you choose above. Set the frequency at a few Hz. Connect the square-wave signal to the circuit, you can see alternate LED blinking indicating the cycle of rectification. This is what happens in all the AC-DC rectifiers around us. Make sure you show scope trace (we'll help you on this) to get extra credit. 

2.  Part B: LED Wheatstone circuit without bridge.
   

Fig. 6

- Step B1:Build the circuit in Fig. 6 above. One item you will use for the first time in this course is the potentiometer (pot for short) for resistor R4. It is a variable resistor, and the one in the kit can vary from 0 to 10 kOhm. For the LEDs, you can use any LED color as you have/wish, but the default color scheme as shown is recommended so that you can identify the circuit meshes on the breadboard.  If you use a different color scheme, make sure selecting the correct LED colors in the App associated with this circuit.

- Step B2: First, set the potentiometer to its low R limit, which should result in 0 Ohm for R4. You can verify by simply seeing bright output of LED 3. If you turn the pot all the way to the opposite end, the LED should be totally off.

- Step B3: Measure the mesh voltages for mesh 1 and mesh 2 as shown. Verify KVL by adding all together and determine the accuracy with the formula in Eq. A.1 Part A above.

- Step B4: Measure the voltages of nodes A, B, C, D relative to the minus terminal of the source voltage (minus is NOT the same as negative). Verify that node C is higher than node B.

- Step B5: Turn the pot all the way to the other end, then repeat steps B3 and B4. Verify that now that node B is higher than node C and LED 3 is practically off.
  

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