Figure 1-9.Current probe used with electronic ammeter.
What is the primary advantage of using a current probe?
Current can be measured with an oscilloscope by shunting the input terminals with a low-value
resistor. The input terminals must then be connected in series with the circuit being tested. The value of
the resistor must be small enough not to interfere with the operation of the circuit under test. At the same
time, it must be large enough that the voltage developed will cause adequate deflection of the oscilloscope
trace. For example, if an oscilloscope with a vertical deflection sensitivity of 0.1volt rms per centimeter
(cm) is used in conjunction with a 10-ohm shunt resistor to measure a 25-milliamp current, the vertical
trace will be deflected 2.5 centimeters, as shown in the following example:
For current measurements the oscilloscope can be calibrated by connecting an ammeter in series with
the input terminals and the calibration signal source. An alternate method is to determine the value of the