Last N, First N - Lab 2 Report

ID: 1234567

Part Intro -  Introduction and background

0.1 AC-DC adapter

Consider the AC adapter for your laptop, or the charger for your phone, what is its electrical function? In other words, what does it do electrically?

Answer below here

0.2 A street pattern

Consider the street pattern below. The arrow indicates one-way traffic. If a street path has no arrow sign, the traffic can go either direction. Is the traffic in path BC one-way or two-way? See gif on the web page as well as the Lab guide app.

Answer below here

0.3 From street to circuit

If we build a circuit like the street pattern above, and for each one-way street sign, we use a LED, which, as we have already learned in Lab I, allows current to go one way but not the other. What will our circuit look like and how will it behave?

It turns out that this circuit is known as the “rectifier bridge,” which is most often used in typical AC-DC adapters. Diodes are generally used for this reason. LED or light-emitting diode is just a sub-category of all diodes. Of course there is no need for light emission in a rectifier circuit, where most efficient diodes are used. Here, we use LEDs only for the educational purpose, so that we can visualize the current flow process. Observe the Rectifier gif circuit on the web page and explain in plain English how an AC voltage - which reverses its polarity periodically, can have a unipolar output - which is necessary for DC.

Import["http://courses.egr.uh.edu/ECE/ECE3340/Class%20Notes2100/Lab_2/LEDRectifier_sm.gif", "Animation"]

Click on the left image below for link to a gif animation.

Answer below here

0.4 Wheatstone bridge

Instead of diodes (one-way street sign), if we use resistors, how do the currents in the various paths flow? To examine this question, we consider this circuit below, also known as the Wheatstone bridge. G stands for galvanometer: a current-sensitive device that allows a current flow through, detects the direction and responds to very small current.

i) is there a current flow between B and C, and which direction is the flow? (if “it depends,” explain “what” depends?)
ii) If the 4 resistors are known, can the question i) above be answered definitively?

Answer below here

0.5 Kirchhoff’s circuit laws

What circuit laws can we use to solve both circuits above: rectifier bridge and Wheatstone bridge?

Answer below here

Part A - Rectifier circuit

A.1 Build the circuit

Build the circuit below. 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.

Fig. 5

Answer below here

A.2 Run with positive source voltage

Choose a positive source voltage between 6-10 V (recommended: start at 6 V and go up slowly). 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, don’t worry if you are not sure). The gif on the web page 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). Take a picture and paste here, showing 3 LEDs lighting up.

Answer below here

A.3 KVL

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

Answer below here

A.4 KVL for reverse polarity

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).

Answer below here

A.5 Rectification demonstration

Option A: Observe the demonstration of rectification and write a description of your observation, interpretation, and understanding.

Option B: For those who are eager and adventurous, you can perform the experiment yourself for extra credit: 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 (or any AC waveform) 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. Slowly vary the frequency from 29.5 to 30.5 Hz. Observe the aliasing effect at 30 Hz.

Answer below here

Part B - LED Wheatstone circuit without bridge.

Fig. 6

B.1 Build the parallel circuit

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 using different colors is recommended so that you can identify the circuit meshes on the breadboard.

B.2 Position 1: low impedance potentiometer

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.

B.3  (KVL again: skip)

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.

B.4  Node B and C: case  (Subscript[v, C])>(Subscript[v, B])

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.

B.5  Node B and C: case  (Subscript[v, C])<(Subscript[v, B])

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

Part C - Bridged LED Wheatstone circuit

Fig. 7

C.1 Build the bridged Wheatstone circuit

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.

C.2 Skip substep 1 below, do only substep 2  (do only KCL, not KVL)

Turn the R4 potentiometer to 0 Ω position, (If are not sure which end is the zero, leave it in aproximately 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 Ω position.

1. skip - Do mesh voltage measurement for all three meshes. Verify KVL like Step A3 or B3 above. 
2. 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

C.3 Skip substep 1 below, do only substep 2 (do only KCL, not KVL)

Tune the R4 potentiometer to the maximum position (opposite to that in C2), you will see all LEDs 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) and the LED from B to C (orange in the below) are bright enough for you to see, except for LED 3, which can remain dark and is not important.
1. skip - Do mesh voltage measurement for all three meshes. Verify KVL like Step C.2 above.
2. 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

C.4 Balanced bridge

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.

Discussion, summary, and conclusion

Answer below here