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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.
If you want to measure three-phase line voltages, you have a similar problem. Making line-to-neutral measurements must account for the neutral line possibly not being at ground in the presence of an unbalanced load, so measuring with a single-ended passive probe can be problematic.
Line-to-line measurements are a definite “no-no” for the ground referenced single-ended probe. These measurements require a high-voltage differential probe to prevent shorting one phase to ground.
Consider the simple 120 V input switch-mode power supply in Figure 1. The supply rectifies the input voltage resulting from power rails that are at the peak voltages of the AC mains, namely +170 V and -170 V. That’s a total of 340 V peak to peak.
The voltage levels may be within the range of a single-ended passive probe, but the primary circuits are not referenced to ground. Most of them use the negative rail (-170 V) as the reference. Attaching a passive probe ground clip to that line would likely result in a pretty bad short circuit, one that could take out the input rectifiers and protection circuits on your oscilloscope.
Some people try to get around this by “floating the scope”, meaning they cut the ground lead on the oscilloscope’s power cord. Now, connecting the probe’s ground to the primary reference does not cause a short; but the oscilloscope case is tied to -170 V. Ouch! Try explaining that accident to your company’s Safety Officer. (Actually, please DON’T try this at home/work. We take no responsibility for the results of your science experiment.)
When measuring power systems, a far better solution is to leave the oscilloscope connected to ground (as intended) and use a high-voltage differential probe, such as one of the HVD3000 series. The HVD3000 probes cover the fullest range of applications, from 120/240 V switch-mode power supplies through 600 V class and 5 kV class electrical apparatus. Each model has the best available gain accuracy, widest differential and offset voltage range, and superior common mode rejection ratio (CMRR).
Another issue to consider with single-ended passive probes is the frequency derating of the maximum voltage these probes can handle. Not all power devices operate at low frequencies. Some, like DC-DC converters, operate at switching frequencies up to 1 MHz or more.
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| Figure 2. Derating curve for a 500 Vrms single-ended passive probe. Maximum input voltage is limited as a function of frequency. |
High-voltage differential probes like the HV3102A, HVD3106A and the new HVD3220 have similar derating characteristics, but they start out with a much higher maximum voltage limit of 1 kV or 2 kV, leaving quite a bit more headroom for reaching the 1 MHz derating breakpoint.
See Also:
Probe Safety Demystified: Dynamic Range and Voltage Swing
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