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Figure 2. Current/Voltage Divider Circuit
[Q21] Compare the voltage between B and C to the result predicted for this voltage divider.
Then, adjust both R1 and R2 while keeping the sum of (R1+R2) approximately the same (to ensure that the
current remains below ≈ 1mA). Again, measure the voltage between points B and C and between points A and C,
and also record the resistance values. Do this for 5 different combinations of the resistances.
[Q22] Compare the voltage between B and C to the results predicted for these 5 voltage dividers.
3. Current Divider (15min: estimated amount of time):
Now replace the resistance substitution boxes with 10kΩ resistors, and use a resistance substitution box for R3,
the load resistor. You will need to use the breadboard to wire the circuit up in this manner. Measure the voltage
and current between points B and C and between points A and C, for values of R3 ranging from 1kΩ up to 1MΩ.
Remember to record the values of R3.
[Q23] Compare the voltage between B and C to the results predicted for these voltage dividers.
[Q24] Compare the current between B and C to the results predicted for these current dividers.
4. Thevenin Equivalent (30min: estimated amount of time):
Determine the Thevenin equivalent of a nine-volt battery. It is rated at 500 mA-hr and a typical current draw is
15 mA. (The obvious procedure for determining the Thevenin equivalent of a voltage source is to decrease the
value of the load resistance, R
load
, until the terminal voltage is 50% of the open-circuit voltage, V
th
. At this point
the internal resistance (R
th
) equals the external resistance (R
load
). This is sort of brutal, due to a relatively high
current draw from the battery. Very quickly this heats up the battery internally, causing R
th
to increase and V
th
to
drop.)
Develop a procedure to determine the Thevenin equivalent that is easier on the battery. Experimentally, use the
resistance substitution box as a load resistor in series with the battery. Use the Wavetek DMM to measure the
current through this load resistor, and use the Agilent DMM to measure voltages.
“Dial in” a load resistance so that the current flowing through it is ≈ 15 mA. To be safe and not draw too much
current, start with a high resistance like 1MΩ and decrease it.
[Q25] Make measurements to determine V
th
and R
th
of the battery. Report these values.
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