Measuring 220V mains with an oscilloscope is the most prone to damage
1. Tripping during measurement;
2. The user is accidentally electrocuted;
3. Burn the oscilloscope during measurement;
Today we are going to talk about why these problems arise and how to avoid them.
The power supply relationship between 220V mains and oscilloscope:
220V mains usually has three lines that are directly related to us: live wire, neutral wire, and ground wire.
Live wire (L): also known as phase wire, provided by the power station or substation, the voltage to the ground is 220V, and there is a risk of electric shock in human contact;
Neutral (n): provides the circuit for the live wire, which is grounded at the end of the power station or substation; Due to the remote grounding, the potential of the user side of the residential building is not necessarily zero, and it may be weakly charged. That's why we have an electric shock when we touch the neutral line;
Ground wire (E): Connect the ground wire at the user end of the residential building, and the reliable ground wire is equal to the potential of the ground under our feet, which is absolutely safe.
Why is it easy for an oscilloscope to measure 220V mains power? The root of the problem lies in the 220V power supply of the oscilloscope, as shown in the figure below
Let's take a look at how an oscilloscope is powered. The power supply of an oscilloscope is nothing more than the following:
v Direct power supply: the most common power supply method, the advantage is low cost; The wiring relationship between its power supply circuit and the 220V mains is as follows:
2. Battery-powered: high cost, but mobile measurement and floating measurement can be realized;
3. Power adapter power supply: high cost, generally using battery power supply design, its power supply circuit and 220V mains wiring relationship is as follows:
Confirm the measurement range
When measuring the mains, confirm that the voltage range of the probe and oscilloscope meets the measurement requirements.
For example, the standard probe is the TO1104C smart oscilloscope, and the maximum safe working voltage is 600VPK; The oscilloscope has a maximum vertical gear of 10V grids, and there are 10 grids in the vertical direction, that is, the maximum voltage measurement range is 100V peak, and the maximum voltage peak value measured by the probe is 1000V in 10 gears, so from the measurement range, it is to meet the requirements of measuring 220V mains.
The vertical direction of the oscilloscope represents the voltage value.
Wrong way to measure:
Ordinary oscilloscopes are not isolated, and the metal end of the housing and the negative end of the probe (ground) are connected to the ground wire. As shown in the figure, when measuring the neutral and live wires directly with an oscilloscope, the neutral or live wires will indirectly short circuit (equal to the red dotted line in the figure), which is very dangerous. As a result, oscilloscopes cannot directly measure municipal electricity.
Wrong way to measure:
Measurement methods that are not recommended:
Floating measurements are risky and can lead to electric shock accidents if not done properly. At the same time, this is an unsafe and inaccurate method, because the oscilloscope and the earth parasitic capacitor can cause the signal to ring, causing the signal to be distorted.
"Floating" measurements can be achieved by manually disconnecting the oscilloscope power cord (powered by a two-pin power plug or by an oscilloscope battery) or by using an isolation transformer.
Floating measurements (not recommended).
When measuring on the ground, the metal end of the oscilloscope housing has the same level as the probe ground. When measuring a municipal power supply, the enclosure may have a voltage of 220V. As soon as the human body comes into contact with the oscilloscope housing, the current flows through the human body into the earth, which is very dangerous!
Recommended Measurements:
Considering the safety angle of a common oscilloscope, it is absolutely safe to measure the mains with a high-voltage differential probe.
High voltage differential probe measurements.
A differential probe is an oscilloscope probe designed using the principle of differential amplification, which can convert two floating signals between them into a ground signal. Using a high-voltage differential probe is the best recommended way to do it, as it is safe and convenient.
Finally
An oscilloscope is an instrument used to measure the shape of alternating current or pulsed current waves, and periodic physical processes that can be turned into electrical effects can be observed with an oscilloscope. Therefore, it is very important to learn how to use an oscilloscope correctly and safely. Especially when measuring some high-voltage signals, it is necessary to find out the relationship between the various signal lines in order to properly connect the signal under test to ensure safe measurement.
The above content is compiled and shared by PRBTEK, Xi'an Puke Electronic Technology Co., Ltd. is committed to the research and development, production and sales of oscilloscope test accessories, covering products including current probes, differential probes, high-voltage probes, passive probes, power ripple probes, flexible current probes, near-field probes, logic probes, power probes and optical probes. It aims to provide users with high-quality probe accessories and create a well-known brand of probe accessories.