Don't Probe HV with Single-ended Passive Probes!

Figure 1. A simple 120 Vrms switch-mode power supply
has a +/- 170 V peak and a 340 V pk-pk, difficult for
most single-ended passive probes to ground safely.

The high-impedance passive probes distributed with oscilloscopes of every major brand are sturdy, reliable and accurate within their specification limits, but they’re not intended for all applications.  This is especially true when measuring switch-mode power devices or other (relatively) high-voltage systems. These applications require probes that are both rated for their high voltage levels and isolated from ground as a reference voltage.  

High-impedance passive probes generally have maximum voltage limits of about 500 V and are ground referenced—meaning, one side of the probe is connected physically to earth ground through the oscilloscope.  If you’re measuring a single-phase 120 V line input to a power supply or inverter, you should be careful when connecting the probe ground to power neutral, which may not always be at ground level.  Using a differential probe, which is not ground referenced, eliminates this concern.

Three-Phase Power Calculations

Figure 1: Three-phase power calculations
entail summing of the individual
phases's power calculations

Until now, our discussions of power calculations have encompassed only single-phase systems with one voltage and one current. Now we'll turn to three-phase systems, which can be thought of as a collection of three single-phase systems. In general, three-phase power calculations are a simple summing of the individual phase power calculations and should be balanced across all three phases.

Power Calculations for Distorted Waveforms

Figure 1: The sum of many sine waves, of varying
amplitudes and frequencies, comprises the rough-
looking square wave shown in red

Our last post covered basic power calculations for pure sine waves, which are useful only up to a point in that pure sine waves are rather rare in the real world. Almost any real-world waveform carries some amount of distortion. Because distorted voltage and current waveforms comprise multiple frequencies, the relatively simple techniques used to measure power for pure, single-frequency sine waves no longer apply.

Back to Basics: Three-Phase Sinusoidal Voltages

Figure 1: Three-phase AC voltages
consist of three voltage vectors

In a previous post, we briefly covered the basics of single- and three-phase AC power systems. Single-phase systems, as we've noted, comprise a single voltage vector with a magnitude (in VAC) and a phase angle. Of course, a three-phase voltage consists of three voltage vectors and three phase angles. This installment will go on to describe three-phase AC voltages in similarly brief fashion.

Back to Basics: Fundamentals of AC Line Power (Part II)

Figure 1: AC line voltage is a single-phase
vector that rotates at a given frequency

Having reviewed a broad definition of power, how it is generated and distributed, and how motors consume almost half of all generated power, we will now turn to a more detailed discussion of just what it is that we call "power." When we discuss "power," we're typically referring to what comes out of a wall socket: AC line, or sinusoidal, power.

Back to Basics: The Fundamentals of Power

Figure 1: 45% of worldwide power
consumption fuels motors

Ask any number of engineers to define "power" and you'll get any number of answers, and none of them would necessarily be wrong. Sometimes it's just a matter of perspective.

Reducing Errors in Switch-Mode Power Supply Measurements

Figure 1: Skew between voltage and current probes

results in power measurement errors

Almost all portable electronic devices, and lots of non-portables, come with switch-mode power supplies. These range from common "wall warts" to the larger brick-sized supplies that power a laptop. We've taken a look at the typical measurements one might make on a switching power supply and at why single-ended measurement techniques should take a back seat to differential approaches. Now, let's see what steps we can take to ensure that our measurements on power supplies are accurate.

Zero in on Power Analysis

Given the emphasis on "green" initiatives, in which anything and everything is touted as "energy efficient," there's been lots of chatter about low-power design. Naturally, much attention is focused on switched-mode power supplies, power devices, and power-conversion circuitry of all kinds. This is where a lot of power efficiency is either lost or gained, depending on how carefully you approach the design task. You know, a milliohm here, a milliohm there, and pretty soon you're talking about real voltage drops that are going to affect the performance of a power-distribution system.