Knowing how to properly use instruments is essential for every engineer, especially oscilloscopes. Many people neglect isolation and other limitations, leading to incidents such as probe explosions. So, what are some unsafe practices when using an oscilloscope?
1. Unsafe Operation of Floating Ground Measurement
Some engineers have a habit of disconnecting the protective ground of the power plug when measuring high-voltage signals and using a regular passive probe to perform high-voltage floating ground measurements. However, this practice is actually harmful.
Example of a common phenomenon: Feeling an electric shock when touching the oscilloscope casing.
Check: 1. Whether the oscilloscope power ground is accidentally disconnected or has poor contact; 2. Replace the power strip; 3. Whether the building ground is not properly connected.
Reason: The Y capacitor is a capacitor connected between the live and ground wires, and between the neutral and ground wires of the power supply, as shown in Figure 1. Its main function is filtering and protection, and it suppresses common-mode interference. It is a safety capacitor; its failure will not cause electric shock or endanger personal safety. When the protective ground of the power plug is disconnected, the 220V voltage is divided by the Y capacitor, with the intermediate 110V voltage directly applied to the metal casing of the oscilloscope. If a person touches this live area, they will experience a needle prick-like electric shock. Although this will not endanger personal safety, it is still considered a dangerous operation.
Figure 1 Power supply circuit schematic
2. Unsafe operation: Connecting the probe and measuring immediately.
Examples of common phenomena: circuit breaker tripping/the circuit board under test is burned through/the oscilloscope or probe is burned out.
Check: 1. Whether the ground of the signal being measured is the same as the ground of the oscilloscope calibration signal; 2. Whether a single passive probe is used directly when measuring mains power.
Before analyzing the causes, let's understand what mains electricity is and the composition of power supply lines. In my country, the mains electricity (residential electricity) standard is 220V AC @ 50Hz. The power supply line, i.e., the wires in the three-prong plug, consists of a live wire, a neutral wire, and a ground wire, as shown in Figure 2.
Live wire (L): Also known as phase wire, it is supplied by a power station or substation, with a voltage of 220V. Contact with it is dangerous for humans.
Neutral wire (N): Provides a return path for the live wire and is grounded at the power station or substation end; because it is grounded remotely, the potential at the user end in the residential building may not be zero and may carry a weak current, but it is relatively safe;
Ground wire (E): Zero potential reference point, connected to the earth at the user end of the residential building, zero voltage, absolutely safe.
Figure 2 Relationship of the three lines
Cause: Power system testing often requires measuring the relative voltage difference between live wires in a three-phase power supply, or between a live wire and the neutral (or neutral) wire. However, ordinary digital oscilloscopes have all channels sharing a common ground, without isolation between them. All signals applied to and provided by the oscilloscope have a common connection point, which is usually connected to the test point via the oscilloscope chassis using the third wire (ground) in the AC power supply cable. If a single-ended probe is used in this case, the probe's ground wire will be directly connected to the power supply line, resulting in a short circuit.
3. Standardized Operation
1) Self-test for "ground" before wiring and measurement.
Before wiring, how do we determine if the ground on the probe can be directly connected to the ground of the board under test? It's simple, just three steps.
Implement self-check:
Tools needed: Board under test, multimeter, oscilloscope
Preparation: Turn on the multimeter and select the highest AC setting;
Connection B: Connect the oscilloscope and the board under test to their respective power supplies, but do not power them on yet; connect one end of the multimeter probe to the ground of the oscilloscope calibration signal, and the other end to the ground of the board under test. A simplified wiring diagram is shown in Figure 3;
C Measurement: Each component is powered on, and the AC value is measured.
If the measured value is not 0, it indicates that there is a voltage difference between the board under test and the oscilloscope ground, as shown in Figure 4. Therefore, a single passive probe ground clip cannot be directly used to connect to the ground of the board under test.
Figure 3. Wiring diagram for measuring differential pressure
Figure 4 shows that two "grounds" cannot be directly connected when a pressure difference exists.
2) "AB" pseudo-differential measurement of mains power
When using a common passive probe to measure mains power using the "AB" method, the negative terminals (ground) of both channels of the probe should be connected to the power supply ground wire, the probe probe (positive terminal) of one channel should be connected to the neutral wire, and the probe probe (positive terminal) of the other channel should be connected to the live wire (as shown in Figure 5 on the left). The measurement difference between the two channels is the mains power waveform.
However, this method may have measurement errors. It can be used when the signal is a low-frequency signal with a sufficiently large amplitude to overcome any worrying noise.
Figure 5 Recommended wiring diagram for measuring mains power
3) Optimal measurement method: using a high-voltage differential probe
Using a high-voltage differential probe is a safe and accurate method for measurement. The best solution for floating ground measurements and mains power measurements (wiring diagram shown in Figure 5 on the right) is to use a differential probe with a high common-mode rejection ratio. This is because there is no grounding issue at either input terminal, and the differential operation of the two input signals is completed in the amplifier at the front end of the probe. The signal transmitted to the oscilloscope channel is the differential voltage, which enables safe measurement.
Figure 6 High-voltage differential probe
4. Last
Although high-voltage differential probes are more expensive than ordinary passive probes, they ensure more accurate and safer measurement results when measuring high-voltage signals. They are recommended for both mains power and floating ground measurements.
