PHY 2049L
Lab 4

RC Measurements

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Updated: 1/1/2009


The BK Precision multimeter we use has an internal resistance of 10 M ohm according to the owner's manual. This information from the multimeter's user manual has also been confirmed by a direct measurement at the 20 V (DC) setting we will use for this experiment.

Connectors 1-2 on the "small" box connect to the 5 micro F capacitor that is our unknown C. All of the ones I have checked fall within +/- 0.1 of the nominal 5.0 muF value.

Connectors 3-4 on the "small" box connect to the unknown R you are to insert in parallel with the multimeter. All have values of about 5 M ohm, ranging from 4.7 to 5.6 M ohm. You will need to turn the box over and read the color code on the resistor to determine the nominal value of the one you used. All of the ones I have checked fall within 1 or 2 % of the nominal value, so you can use the color-coded value as "known".

Part I

This photo shows a measurement of the RC time constant in progress back when we were using the Keithley multimeters. The red lead, which was used to charge up the capacitor to its initial voltage, has been disconnected so capacitor is no longer connected to the power supply. It looks like the voltage has already fallen from 20.0 V to 13.2 V.

part 1

Look closely at the wiring. The only complete part of the circuits you can see is made by the long red and black cables that connect the capacitor with the 10 M ohm internal resistance of the voltmeter. The short black cable connects one end of the capacitor to the power supply, but this has no effect because the red (+) end of the power supply is not connected to anything.

Part II

This photo shows a different group performing the second part of this lab. Notice that they disconnected the red lead from the power supply rather than from the capacitor. This works the same way as in the example above.

part 2

The resistor in the box (located between 3 and 4) is connected in parallel to the capacitor and the internal resistance of the voltmeter. Can you trace the wiring to confirm this? It is a bit tricky, because one wire is below the data sheets on its way to the voltmeter, but you should be able to do it.


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