For ELE 202 Circuits I

This testing procedure should allow to to identify the configuration and obtain the values.

**The Ohmmeter Test**. Place the ohmmeter across the terminals of the black box. If the meter starts off low but climbs quickly to read**OL**you know that this is an**RC-series**box. Skip ahead to the RC-series testing section. If the meter gives you a reading and stays there, scratch RC-series from your list. You are now down to 3 choices.**The Phase Shift Test**. Hook up a test circuit like this:- Choose R
_{s}to be small, about 20 ohms. It is just a sampling resistance and is used for measuring current in the circuit, according to: - Turn on the scope & function generator, and
**MAKE SURE ALL THE CONTROLS ARE SET PROPERLY**i.e., all the knobs on the scope are turned over to CAL, the function generator gives a clean sine output with no DC offset, both channel A and channel B read zero at no input, etc. - Set the function generator for about a 1kHz sine wave at about 1V
_{p-p}. The actual voltage you choose isn't that important, just that blasting the box with 20V_{p-p}will only load down the generator and possibly mess with your results. - Display both channels on the scope and trigger off Channel A, set both channels to AC coupling. Adjust the time divisions to display the largest full wave of channel A. Play with the frequency until you get a good separation between the channel A and channel B waveforms.

_{A}or V_{B}is the leading waveform. Remember ELI the ICE man! If voltage leads current then it is a circuit involving an inductor. If current leads voltage, it is a circuit involving a capacitor.V

_{A}= channel A = voltage across test circuit

V_{B}= channel B = current through test circuit### Example

If V

_{B}leads V_{A}you know the circuit is**RC-parallel**- skip right to the RC-parallel testing section. If not, you know the circuit contains inductors. You are now down to two choices.- Choose R
**Inductor Test**. The only possibilities left are a series inductor or a parallel inductor.- Case A: Parallel Inductor (RL-parallel)
- Set frequency to zero. At zero hertz, inductor is a short circuit (X
_{L}= 0), and therefore the box is a short circuit. Channel A waveform is the same as channel B waveform, i.e.|V

_{A}| = |V_{B}|and the waveforms will be in phase.

- Set frequency to a high value, such as 20kHz. The impedance of the inductor will be high, nearing an open circuit (X
_{L}-> infinity). Channel A and channel B are in phase, but|V

_{A}| > |V_{B}|.

- Set frequency to zero. At zero hertz, inductor is a short circuit (X
- Case B: Series Inductor (RL-series)
- Set frequency to zero. The inductor will become a short and only the resistor remains.
|V

_{A}| > |V_{B}|and the waveforms are in phase because it is s resistive circuit.

- Set frequency to a high value, such as 20kHz. The inductor will be open, thus I = 0.
V

_{B}= 0 (no current)

V_{A}= function generator voltage

- Set frequency to zero. The inductor will become a short and only the resistor remains.

- Case A: Parallel Inductor (RL-parallel)

**To find the voltage (total voltage across test circuit)**:

You know:

- V
_{A}from oscilloscope - I from V
_{B}/ R_{s} - R
_{s}

**To find phase angle**: count the divisions of 1 full waveform. Count the divisions between the leading & lagging waveforms, and the phase angle is then:

phase angle = # of divisions between leading and lagging wave x 360 degrees ----------------------------------------------- # of divisions in one full waveform

You will now have a value

Thus you can calculate R and solve for C and you are done.

Now solve the capacitor expression for C and you are done.

Solve the inductor expression for L and you are done.

Now solve the inductor expression for L and you are done.

- Set input voltage to approximately 1 V
_{p-p}. - Use the meter to record frequency - if you use meter for measuring voltages, remember it gives values in RMS!
- Use a sine wave signal
- Do not adjust the AMPLITUDE of the input signal after you have set the input voltage.
- All black wires go to the same place, which forms the circuit ground.
- Remember how to calculate using admittances in polar and rectangular forms.
- Highest frequency used should be around 30kHz.
- If V
_{B}leads V_{A}remember your phase angle will be negative. - Remember ELI the ICE man.
- And uh, TURN THE EQUIPMENT ON - duh?