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| 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.
