The Agilent E3631A has three power supplies, a +6 V supply capable of
delivering 5A, and two supplies of +25 and -25 V capable of delivering
1A each. The 
(ground) output is
the reference ground and is connected to the ground of the building. Under
normal use (for safety reasons) it is important to connect the COM (common) terminal of the +25 V supplies, and the (-) terminal of the +6 V supply to the (ground) reference.
- Looking now at the control keys: The Output ON/OFF key turns the output ON or OFF.
- To Set the Output Voltage:
- Press the +6, +25, -25 keys to select the power supply to be set.
- Press Voltage/Current key so that the Volt Display is active.
- Use the circular control knob to set the output voltage. Use the arrow keys for selecting the resolution.
- To Set the Maximum Output Current:
- The Display Limit key lets you select the maximum current that the power supply can deliver (up to 5A for the 6V and 1A for the +25V supplies). This is basically your current protection feature.
- Press Voltage/Current the key so that the Current Display is active.
- Use the circular control knob and the resolution keys to set this limit (if needed).
- Practice. Set each output to 3.7 volts with current limit at 0.100 amps.
- To Read the Output Voltage or Output Current:
- The Voltage/Current key also shows the output voltage and the output current of the power supply.
- To measure the output current of the supply, make sure that the Display Limit key is not active.
Warm up the oscilloscope, function generator, and the DC supply.
Set up the circuit in Figure 1 below using the + and COM terminal of the +25 volt output terminal of the DC supply for VS. So, the + side of VS is the + side of the +25 terminals and the - side of VS is the COM side of the +25 terminals. Set VS to 8 Volts. Set the current limit to 0.100 Amps.
Figure 1.
Let
R1 = 20K
R2 = 33K
R3 = 47K
Calculate V1, V2, V3, V4, and V5. Measure each of the voltages using channel 1 of the oscilloscope. (Press Auto Scale for easy scope measurements.) Note that these voltages are all DC values. So, be sure that the channel 1 coupling is set to DC. You should see only a straight horizontal line on the display of the scope. This line will be above the horizontal axis for channel 1. The distance above the axis times the vertical scale is the DC value of the voltage. If the image is very "fuzzy" try setting the channel 1 vertical scale (dial just above the 1 button) to a larger value like 2.00V/ or press the Single button. Record your measurements. Repeat these measurements using channel 2. Record these measurements. Do channels 1 and 2 give exactly the same measurements? Note that you could very accurately measure the voltages using Quick Measure and the average value measurement option. Compare your measured values to your calculated values from your preliminary report and determine the percent error using:
%ERR = [(measured value - calculate value)/(calculated value)] X 100
Next we will be measure two voltages simultaneously and
have the math mode feature of the scope display their difference.
Connect the negative (black) terminals of both channel 1 and 2 to the COM terminal
of the DC supply. (Note that COM is NOT the ground () terminal.)
To measure V3 connect the positive (red) terminal of channel
1 to the + polarity node of V3 and connect the positive (red) terminal of channel 2 to the - polarity node of V3. Now
press the Math button and select option 1 - 2. Turn off
channels 1 and 2 (press the channel 1 and 2 buttons twice each.) The image on the display is now V3. Prove that this
must be true using Kirchoff's voltage law. Remember that you are able to
adjust the vertical scale of the math mode image. (See experiment 1.)
Adjust the math mode vertical scale so that you may get an accurate
measurement. You will notice that there is no horizontal axis marker at the
left edge of the display. You can create a horizontal axis using the
cursor option. Press the cursor button. Now choose the
X Y button to get the Y (horizontal) cursors. Select the Y1
button and then turn the indicated dial knob to set the Y1 cursor
to read 0.00 volts. The position of this cursor is now the location of the horizontal axis.
You can now measure the value of the voltage with respect to the location
of the Y1 cursor. To reposition the horizontal axis press the math button one time
and select the settings option. Now select the offset option
and then turn the indicated dial. Adjusting the offset will allow you
to position the horizontal axis (the Y1 cursor.) Use this method to move the axis down to the first line above the bottom of the display.
Now select the scale option and adjust the scale to 2.00V/. You may need to reposition the axis again as explained above. You should now be able to get a very accurate measurement.
Use the differential measuring method to measure all of the voltages in Figure 1 including VS. Record your measurement. Compare these measurements to your calculated values.
Leave the circuit set up as it is. Get another black cable and use it to connect
the ground terminal () of the DC supply to the COM terminal of the +25 volt
output of the DC supply. Doing this will have no effect on the circuit.
However, this will cause a problem when measuring voltages with the scope.
Repeat all of the measurements of the previous two sections. How has the accuracy of your measurements been affected.
The negative side of the scope is connected to earth ground through the chassis of the scope. So whenever a voltage measurement is made with the scope, the measurement is being made with respect to earth ground. There is no getting around that fact! Therefore if a circuit under investigation has a node connected to earth ground, then the negative side of the scope (the BLACK lead) must be connected to that node. If the negative side of the scope is connected elsewhere, a "short circuit" will be created and all voltage (and current) values in the circuit will change!
A source, instrument, or circuit that has no connection to earth ground is said to be "floating." When the ground terminal of the DC supply is not being used, the supply is floating, as it was in the initial part of this experiment. For a circuit that is floating the negative side of the scope may be connected to any node of the circuit without upsetting any voltage or current values. A short circuit can cause a disaster to a circuit and its components. So, if you are not sure about the ground situation for a circuit then use the differential measuring technique when measuring voltages with the scope.
Remove the Agilent DC supply from the circuit and replace it with the
Agilent function
generator as the voltage source VS. Be sure to use the black
terminal of the function generator as the - side of VS.
Set VS =
5 cos(3000pit) volts. (Don't forget to set the function generator into the HIGH Z output mode. (See experiment 1.) Be sure that the DC offset is set to zero. Calculate V1 through V5. Using the differential measurement technique, measure and record Vpeak-to-peak for all of the voltages. Repeat all of the
measurements using the direct measurement technique Calculate the %ERR
of each of the measured voltages with respect to the calculated values.