What are the unsafe operations when using an oscilloscope?

I. Introduction

How to use instruments correctly is a necessary requirement for every engineer, especially oscilloscopes. Many people do not pay attention to restrictions such as isolation, resulting in incidents such as exploding probes. So what are the unsafe operations when using oscilloscopes?

2. Floating ground measurement due to unsafe operations

Some engineers have this habit: when measuring high-voltage signals, they habitually disconnect the protective ground of the power plug and use ordinary passive probes to directly conduct high-voltage floating measurements. In fact, there are still dangers in doing so. Examples of common phenomena: You feel an electric shock when you touch the oscilloscope shell. Check: 1. Whether the power ground of the oscilloscope is artificially disconnected or has poor contact; 2. Change the power strip; 3. The ground of the building where it is located is not connected properly. Reason: Y capacitor is a capacitor connected across the live wire, ground wire, neutral wire and ground wire of the power supply. As shown in Figure 1, it mainly plays the role of filter protection and suppresses common mode interference. It is a safety capacitor. Failure will not cause electric shock and will not endanger personal safety. When the protective ground of the power plug is disconnected, the 220V voltage is divided by the Y capacitor, and the intermediate 110V voltage is directly applied to the metal shell of the oscilloscope. When a person touches the electrified area, an electric shock will occur like being stuck by a needle. Although it is not It will endanger personal safety, but it is also a dangerous operation.

Figure 1 Power circuit schematic diagram

3. Unsafe operation: pick up the probe and connect the wires for measurement

Examples of common phenomena: tripping/the circuit board under test is burned out/the oscilloscope or probe is burned out. Check: 1. Whether the ground of the signal being measured and the ground of the calibration signal of the oscilloscope are the same; 2. Whether a single passive source is used directly when measuring the mains power. Probe takes measurements. Before analyzing the reasons, first understand what mains power is and the composition of the power supply line. The specification of my country's mains electricity (residential electricity) is AC 220V@50Hz. The power supply line, that is, the wires in the three-plug power supply, are composed of live wires, neutral wires and ground wires, as shown in Figure 2. l Live line (L): also called phase line, provided by the power station or substation, voltage 220V, human body contact may be dangerous; l Neutral line (N): provides a loop for the live line, grounded at the power station or substation end; because it is remote The end is grounded, so the potential at the user end of a residential building is not necessarily zero, and may carry weak electricity, but it is relatively safe; Ground wire (E): a zero potential reference point, connected to the earth at the user end of a residential building, zero voltage, absolutely safe.

Figure 2 Three-wire relationship Reason: In power system testing, it is often required to measure the relative voltage difference between the live wire and the live wire, or the live wire and the zero (neutral) wire in the three-phase power supply. However, all channels of an ordinary digital oscilloscope share the same ground and are not isolated from each other. All signals applied to and provided by the oscilloscope have a common connection point, which is usually the oscilloscope chassis. Connect the probe ground to a on the test point. If a single-ended probe is used for measurement at this time, the ground wire of the single-ended probe is directly connected to the power supply line, and the result will be a short circuit.

4. Standardize operations

1. Self-check the "ground" before wiring measurement. How do we judge whether the ground on the probe can be directly connected to the ground of the board under test before wiring? It's very simple, three steps to implement self-test: Preparation tools: tested board, multimeter, oscilloscope (1) Preparation: turn on the multimeter, select the maximum AC range; (2) Wiring: connect the oscilloscope and the tested board to the power supply respectively. Power on; one end of the multimeter's probe is connected to the ground of the oscilloscope's calibration signal, and the other end is connected to the ground of the board under test. The simple wiring diagram is shown in Figure 3; (3) Measurement: Power on each and measure the AC value. If the measured value is not 0 at this time, it means there is a voltage difference between the board under test and the ground of the oscilloscope. The schematic diagram is shown in Figure 4. Then you cannot directly connect the ground clip of a single passive probe to the ground on the board under test.

Figure 3 Wiring diagram for measuring differential pressure

Figure 4 When there is a voltage difference, the two "grounds" cannot be directly connected. 1. "AB" pseudo-differential measurement of mains power uses ordinary passive probes to measure the mains power using the "AB" method. The negative terminals of the two-channel probes should be connected. (ground) are connected to the power ground wire, the probe probe (positive end) of one channel is connected to the neutral wire, and the probe probe (positive end) of the other channel is connected to the live wire (as shown on the left side of Figure 5), then the probe probe (positive end) of the two channels The measured difference is the mains waveform. However, this method will have measurement errors. This method can be used when the signal is a low-frequency signal and the signal amplitude is large enough to exceed any worrying noise.  

Figure 5 Recommended wiring diagram for measuring mains power 1. Best measurement method: Use a high-voltage differential probe to measure. Using a high-voltage differential probe for measurement is a safe and accurate method. The best solution for floating measurement and mains power measurement (the wiring diagram is shown on the right side of Figure 5) is to use a differential probe with a high common mode rejection ratio, because there is no grounding problem at both input ends, and the two input signals The differential operation is completed in the probe front-end amplifier, and the signal transmitted to the oscilloscope channel is the differential voltage, which can achieve safe measurement.

Figure 6 High voltage differential probe

5. Finally

Although high-voltage differential probes are more expensive than ordinary passive probes, they can ensure more accurate and safer measurement results when measuring high-voltage signals. They are recommended for mains and floating measurements.