Measuring with an oscilloscope, Electrical Engineering

1. Introduction:

Theory:  The voltage measured across a load follows the Ohm's law which says that the current passing through a conductor between two points is directly proportional to the potential difference across the two points.

V α I   => V = I.R   (where R is proportionality constant)

R is known as resistance and it changes with material and temperature of conductor

Material Required: Oscilloscope, frequency generator, connecting wires, resistor, multimeter.

Description and Aim: To measure voltage using an oscilloscope and determine current and impedance by using Ohm's law.

For this exercise, the voltage drop across the resistance is measured. The measured voltage value is divided by resistance value to get the value of current.

I = V / R

2. Method:

Step 1) First, pick the resistor and place it on the circuit board. It is connected in series with the other circuit. To ensure that R2 and L2 does not short, the two scope probes and the ADD-INVERT method are used.

Step 2) Connect the channel 1 input to the side of the component with the greatest potential. The channel 2 input is connected to the other side. Connect ground clips on the oscilloscope probes to the common port of generator (black lead).

Step3) Invert channel 2 and switch vertical mode to ADD. The display on the oscilloscope shows the voltage drop across the component.

Step 4) Use oscilloscope to measure peak-to-peak voltage across R2, and calculate the total circuit current (I).

Step 5) Replace the two post connector across sensing resistor R2. Now use Add-Invert method to measure voltage drop across R1.

Step 6) Connect Channel 1 probe to left side of R1, and channel 2 probe to other side of R1.

Step 7) Set both channel vertical input attenuators to display 2.0 V/div. Select channel 2 INVERT function on the oscilloscope. Set vertical mode to ADD. Measure Vpk-pk across R1.

3. Results: (please correct them)

V = IR

I = V / R

I = V2/R2   = 200/ 10 = 20 mA (pk-pk)

4. Discussion:

Using the INVERT-ADD mode of oscilloscope measurement of current is done. Concept is measuring the maximum potential difference across the series path and subtracting the potential difference due to second resistance. Using current and potential difference we can find unknown impedance. Since resistance doesn't introduce any phase difference between current and voltage across it, this way is easier way to compute an accurate resistance value.

Posted Date: 3/2/2013 12:54:22 AM | Location : United States

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