1、 Floating survey of unsafe operation
Some engineers have such a habit: when they want to measure the high-voltage signal, they habitually disconnect the protective ground of the power plug, and use the ordinary passive probe to directly measure the high-voltage floating ground. In fact, it is harmful to do so.
Common phenomenon example: touch the oscilloscope shell to feel electric shock
Check: 1. Whether the oscilloscope power supply ground is artificially disconnected or in poor contact; 2. Change the arrangement; 3. The building is not well connected.
Cause: y capacitor is the capacitor that is connected across the live wire and ground wire of the power supply, and the zero wire and ground wire, as shown in Figure 1. It mainly plays the role of filtering protection and suppresses common mode interference. It belongs to the safety regulation capacitor, which will not cause electric shock and endanger personal safety after capacitor failure. When the protective ground of the power plug is disconnected, the 220 V voltage is divided by the Y capacitor, and the intermediate 110 V voltage is directly applied to the metal shell of the oscilloscope. When people touch the live area, they will have the same electric shock phenomenon of being pricked by a needle. Although it will not endanger personal safety, it is also a dangerous operation.
Figure 1 Schematic diagram of power supply circuit
2、 Unsafe operation of the pick up the probe on the wiring measurement
Examples of common phenomena: trip / burn out of circuit board under test / burn out of oscilloscope or probe
Check: 1. Whether the ground of the measured signal and the ground of the oscilloscope calibration signal share the same ground; 2. Whether a single passive probe is directly used to measure the power supply.
Before analyzing the causes, we should first understand what the utility is and the composition of the power supply line. The specification of domestic electricity (residential electricity) is AC [email protected] The power supply line, that is, the wire in the three power plugs, is composed of live wire, zero wire and ground wire, as shown in Figure 2.
L live line (L): also known as phase line, provided by power station or substation, voltage is 220 V, human contact will be dangerous;
L zero line (n): provide circuit for live line and ground it at the end of power station or substation; due to the remote grounding, the potential at the user end of residential building is not necessarily zero and may carry weak current, but it is relatively safe;
Ground wire (E): zero potential reference point, grounded at the user end of residential buildings, zero voltage, safe.
Figure 2 the relationship of three lines
Reason: in the test of power supply system, it is often required to measure the relative voltage difference between live line and live line, or between live line and zero (middle) line in three-phase power supply. However, all channels of 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 case. The probe ground wire is connected to a test point by using the third wire ground wire in the power line of AC power supply equipment. If the single ended probe is used for measurement, then the ground wire of the single ended probe is directly connected with the power supply line, and the consequence will be a short circuit.
3、 Standard operation
1. Self check the “ground” before wiring measurement
Before wiring, how can we judge whether the ground on the probe can be directly connected to the ground of the tested board? It’s very simple. There are three steps to realize self-test
Preparation tools: tested board, multimeter, oscilloscope
(1) Preparation: turn on the multimeter and select the AC file;
(2) Wiring: the oscilloscope and the board under test are connected to the power supply respectively, but the power is not available for the time being; one end of the probe of the multimeter is connected to the ground of the oscilloscope 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: respectively power on, measure AC value.
At this time, if the measured value is not 0, there is a pressure difference between the measured board and the ground of the oscilloscope. The schematic diagram is shown in Figure 4. The ground clamp of single passive probe can not be directly connected to the ground on the tested board.
Fig. 3 wiring diagram of measuring differential pressure
Fig. 4 when there is a pressure difference, the two “Earth” can not be directly connected
1. “A-B” Pseudo differential measurement of power supply
When using the common passive probe and applying the “A-B” method to measure the power supply, the negative end (ground) of the two channel probes should be connected to the power supply ground wire, the probe probe (positive end) of one channel should be connected to the neutral wire, and the probe probe (positive end) of the other channel should be connected to the grounding wire (as shown in the left of Figure 5), then the measurement difference between the two channels is the shape of the power supply.
But this method will have measurement error, when the signal is low frequency signal, the signal amplitude is large enough to exceed any worry about noise, this method can be adopted.
Figure 5 recommended wiring diagram for measuring utility power
1. Measurement method: high voltage differential probe is used
It is a safe and accurate method to use high pressure differential probe for measurement. The solution of floating ground measurement and power supply measurement (the wiring diagram is shown on the right in Figure 5) is to use a differential probe with high CMRR. Because there is no grounding problem at both input terminals, the differential operation of two input signals is completed in the front-end amplifier of the probe, and the signal transmitted to the oscilloscope channel is the differential voltage, which can realize safe measurement.
Zp1500d high-voltage differential probe is independently developed by our company. The probe can switch 5M and 100m bandwidth in standard 50 × and 500 × gear. The attenuation ratio and bandwidth gear can be flexibly used to help you realize the measurement of low-frequency signal and high-speed signal
Figure 6zp1500d high voltage differential probe
Although the high-voltage differential probe is more expensive than the ordinary passive probe, it can ensure that the measurement results are more accurate and safer when measuring the high-voltage signal. It is recommended to use the commercial power and floating ground measurement. As a common measuring instrument in daily life, oscilloscopes should follow the rules of safe use of electricity and conduct standard wiring operation to avoid accidents.
Editor in charge: PJ