Calculating Power with a Scope

Oscilloscopes measure power as instantaneous, real, apparent, or reactive. Fortunately, the Power Analysis software on the Teledyne LeCroy HDO 6000 oscilloscope can make it easy and fast to see how much power you’ve got without all the off-line mathematical computations.

Basically, every oscilloscope responds to voltage. With the right transducer (usually a current probe or resistive shunt), it can give you a reading of current. The oscilloscope displays the instantaneous function of the voltage you’re measuring. It also plots current against time. Multiply the two and you get instantaneous power.

Power Analysis Made Easier

The image above shows a basic line power measurement. The elements of a power measurement (instantaneous voltage, current, and power) are shown on an HDO 6000 oscilloscope equipped with the Power Analysis option. The instrument automatically computes and displays both real and apparent power.

Multiplying the instantaneous voltage (channel 1) by the current (channel 2) gives you instantaneous power, which is shown in the lower line power trace. The power waveform exhibits twice the frequency of the current or voltage, offset by a constant. This “DC offset” represents the average or real power, P (in Watts), being delivered to the load.


This real power is calculated automatically by the Teledyne LeCroy HDO 6000 oscilloscope, and represents the mean or average value of the instantaneous power waveform. In this example, the real power being delivered to the load is 25.11 W. It is displayed as the parameter “rpwr.” The apparent power, S, is the product of the effective (rms) current and the effective (rms) voltage. It is measured in Volt-Amps (VA). In the figure above, S =120.59 * 0.328 = 39.6 VA. The Teledyne LeCroy HDO 6000 oscilloscope computes this value automatically and displays it as the parameter “apwr.”  (Note: When dealing with resistive loads, the apparent power and the real (or average) power are always equal.)


The ratio of the real (or average) power to the apparent power is called the power factor. When the waveform is sinusoidal, the power factor turns out to be the cosine of the phase angle between the current and voltage waveforms. More generally, you can compute the power factor by calculating the ratio of real (or average) power to apparent power. Again, the Teledyne LeCroy HDO 6000 oscilloscope computes the power factor automatically and displays it using the parameter “pf”. Here, the power factor is 0.633.


The crest factor of a waveform is the ratio of the peak-to-peak value of the current to the “rms” value. It is displayed as “Icrest”. Reactive power, N, is calculated from both the real and the apparent power, and is measured in Volt Amperes Reactive or VAR. The equation is:  N = (S2 - P2)1/2 (Because most of the time, oscilloscope users are looking for both real power and the power factor, the Teledyne LeCroy HDO 6000 oscilloscope has not been set up to calculate reactive power automatically.)

 

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This is a brief review of power analysis and features available on Teledyne LeCroy HDO oscilloscope. A more thorough description, written by Teledyne Technical Marketing Engineer Art Pini, was published by EE Times and EDN last month.