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Labwork (continued):

3.  Part C: Bridged LED Wheatstone circuit.
   

Fig. 7
        

- Step C1: With the source voltage off or 0 V, insert the bidirectional LED in the circuit you built in Part B. If you have a real bidirectional LED just insert it in and don't worry about its polarity. After you start Step C2 below, if you set the pot to zero such that LED 3 is bright, you will see it either amber or green, depending on the current direction).

The app default is that the LED is green for current flow from C to B. So, if you see amber color, just flip it around. However, it doesn't matter much if you choose amber or green when node C is higher than B. You can still do all the measurements below, just make sure you report that in Lab Notebook.

If you use a pair of LEDs, you can pick two different colors of your preference, as long as you make a note in your Lab Notebook which color indicates which direction.

 - Step C2:Turn the R4 potentiometer to 0 Ohm position, (If you are not sure which end is the zero, leave it in approximately in the middle and continue). Slowly turn up the voltage from 0 to between 11-15 V: as long as the LEDs are bright enough for you to see, but do not burn them. Then, turn the pot one way or another while observing LED 3 (the blue one in Fig. 7): if it is brightest, that is 0 Ohm position.

        i. Do mesh voltage measurement for all three meshes. Verify KVL like Step A3 or B3 above.
        ii. Measure current i2 (break open R5 and C), i3 (break open C and R6), and iCB (the current through LED C-B) to verify KCL: i2=i3 + iCB

- Step C3: Tune the R4 potentiometer to the maximum position (opposite to that in C2), you will see all LEDs become dimmer. If they are too dim, you can increase the source voltage a few volts as necessary, just enough so that LEDs 1 and 2 (red and yellow in Fig. 7 above - they can be of difference colors on your own circuit, your choice) and the LED from B to C (orange) are bright enough for you to see, except for LED 3, which can remain dark and is not important.
       i. Do mesh voltage measurement for all three meshes. Verify KVL like Step C.2 above.
       ii. Measure current iAB (break open R2 and B), iBD (break open B and R3), and iBC (the current through LED B-C) to verify KCL: iAB=iBD+ iBC

Fig. 8  Tune the R4 potentiometer to observe LED reversal current increase.

- Step C4: Monitor the voltage between B and C, tune your R4 pot until BC voltage is closest to zero that you can get (it doesn't matter positive or negative, as long as its absolute magnitude is closest to 0). Pull out the bidirectional LED or the LED pair between B anc C, will that make any change to the rest of the circuit? Cite evidences to support your claim. (no changes mean no change of any node voltage or mesh current. You don't need both. Just measure either voltage or current is sufficient to prove change or no change). Visually, all the remaining LEDs will have no visible change. (Hint: if the voltage difference is zero between 2 nodes, whatever passive device you put in there does not change anything to the circuit condition).

4.  Part D: Discussion, summary, and conclusion
   

Report in your Lab Notebook what you think of your study of KVL and KCL of this Lab. Report what you think of each circuit, Circuit 1 (rectifier) and Circuit 2 (LED Wheatstone bridge).  Do you think LED is a good indicator of current flow? What else do you learn (personally) by doing this Lab?



END
  

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