Exercise 2: Measuring More Than One A.C. Voltage

Setting Up the CRO

• The point of this exercise is to measure the A.C. voltage drop across the resistors R2, R3 and R4 in the diagram below.
• Let's start by connecting the circuit shown below. If you have any trouble with this please refer to the A.C. Circuit Connection Guide. Or the instrucions for the A.C. Supply.

R1 = 20 ohms R2=220 ohms R3 = 47 ohms R4 = 100 ohms

• Now that the circuit has been connected the CRO must be prepared to make accurate measurements by:
  1. Adjusting the trace focus and intensity.
  2. Ensure that the CRO controls are in the Calibrated Positions.
  3. Adjust the Horizontal and Vertical Trace levels on the CRO.
  4. Both CRO inputs must be D.C. Coupled.This may seem odd but the CRO should only be A.C Coupled in special circumstances.
• If you require more information regarding steps 1 to 4, please refer to the Explanation of Basic CRO Operation.
• Is our CRO set up correctly?

Connecting the CRO to the Circuit

• Now that the CRO has been set up correctly connect the CRO to the circuit to take readings across resistors R2, R3 and R4.
• If we use similar connections to the last exercise we would simply connect one channel of the CRO across each resistor in the circuit separately taking 3 different readings. Well we are not going to do it that way this time.
• The challenge now is to measure the required voltages using two input channels on the CRO and taking one measurement from each of these channels.
• Try and work out how to make these connections on your own before checking the answer.
• Is our circuit connected properly?

• The final step is to adjust the Vertical Gainand Time Base of the input and read the amplitude and frequency of the voltage from the display.
• Do we have the right answer?.

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Revised: Friday, 17 July 1998