Download RC Time Constant Lab

Lab #5 - RC Time Constant
Copyright R Mahoney 2014
Name: ____________________
The purpose of this lab is to verify that a series RC circuit does in fact decay
exponentially.
To do this, we measure a resistance and capacitance (out of circuit), and then
we measure the voltage on the capacitor as a function of time when it is
decaying in series through the resistor. Is R times C measured (out of circuit)
the same as R times C determined by using the exponential formula?
1. Choose an R and C such that the product is something reasonable (like
80 sec). Remember, you are going to measure voltages across a decaying
capacitor pretty rapidly, so a time constant of 80 sec would allow you to make
about 9 measurements (at t = 0, and every 10 seconds) through 1 full time
constant (1 times RC).
R meas = _____
C meas = _____
RC meas = R meas x C meas = RC true = _____
2. Set up the following circuit:
B
A
Measure the voltage across the DC power supply, nominally set to 10 VDC.
(Notice that the capacitor is electrolytic, i.e., has a polarity or bias.)
V batt = V max = _____
3. With the switch connected to its battery (DC power supply) side (charging
side), put your voltmeter across the capacitor. Is the voltage = V max? If not,
write down the new V max.
V max = _____
Now connect the voltmeter with the plus lead on point A and the minus lead on
point B.
4. Counting "5, 4, 3, 2, 1, 0," flip the switch to the resistor side (discharging
side) and start your timer when you call out "0".
As the counter approaches 10 seconds, call out "8, 9, 10," and write, as best
you can, the voltmeter voltage as "10" is called out, in the voltage column
below.
Do this 7 more times.
Voltage, V
ln (Vmax / V)
V max
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1
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Time
RC exp
0
10
20
30
40
50
60
70
80
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5. For each measured V, calculate ln (V max / V), and write the results in the
ln (V max / V) column above.
6. Divide 10 by each calculated ln (V max / V), and put the results in the
RC exp column above
7. Average your 8 experimental RC values. RC exp, avg = ________
8. Calculate the % error between RC true and RC exp, avg
% error = ________ %