How to safely measure 220V mains electricity using an oscilloscope

The most common problems when measuring 220V mains electricity with an oscilloscope:

1. Tripping during measurement;

2. People accidentally get electric shock;

3. The oscilloscope burned during measurement

Today we will talk about why these problems occur and how to avoid them.

The power supply relationship between 220V mains and oscilloscope:

There are usually three wires directly related to us in the 220V mains electricity: live wire, neutral wire, and ground wire.

Live wire (L): Also called phase wire, provided by power station or substation, with a ground voltage of 220V. If touched by humans, there is a risk of electric shock.

Neutral wire (N): provides a loop for the live wire and is grounded at the power station or substation. Since it is remotely grounded, the potential at the user end of the residential building is not necessarily zero and may carry weak electricity, which is why we feel an electric shock when we touch the neutral wire.

Ground wire (E): The wire that connects to the earth at the user end of a residential building. A reliable ground wire has the same potential as the earth beneath our feet and is absolutely safe.

Why is it easy for an oscilloscope to have problems when measuring 220V mains electricity? The root of the problem lies in the 220V power supply of the oscilloscope, as shown in the following figure:

Let's take a look at the power supply methods of oscilloscopes. The power supply of oscilloscopes is nothing more than the following:

1. 220V direct power supply: The most common power supply method, the advantage is low cost; the connection relationship between its power supply circuit and 220V mains is as follows:

2. Battery powered: The cost is high, but it can realize mobile measurement and floating measurement;

3. Power supply by power adapter: The cost is relatively high, and it is generally designed to be powered by batteries. The connection relationship between its power supply circuit and 220V mains is as follows:

Measuring range confirmation

When measuring AC power, you should confirm whether the voltage range of the probe and oscilloscope meets the measurement requirements.

For example, the standard probe of the Macosin STO1104C intelligent oscilloscope is ×10 and the maximum safe working voltage is 600V pk; the maximum vertical scale of the oscilloscope is 10V/grid, and there are 10 grids in the vertical direction, that is, the maximum voltage measurement range is 100V peak-to-peak value. With the probe ×10 scale, the maximum peak-to-peak voltage measured is 1000V. Therefore, considering the range, it is sufficient to measure 220V AC power.

For example, the maximum vertical scale of the Macosin STO1104C intelligent oscilloscope is 10V/grid, and there are 10 grids in the vertical direction, that is, the maximum voltage measurement range is 100V peak-to-peak value. The peak-to-peak value of the AC power is about 620V. Therefore, considering the range alone, the AC power can be measured by using a probe with an attenuation ratio of more than ×10 and meeting the maximum safe working voltage.

The vertical direction of the oscilloscope represents the voltage value

Wrong measurement method

Ordinary oscilloscopes are not isolated. The metal end of the shell and the negative end (ground) of the probe are connected to the ground wire. As shown in the figure, when the oscilloscope is used to directly measure the neutral wire and the live wire, the neutral wire or the live wire will be indirectly short-circuited to the ground wire (equivalent to the red dotted line in the figure), which is very dangerous. Therefore, the oscilloscope cannot directly measure the mains.

Wrong measurement method

Measurement methods not recommended:

Floating measurement is dangerous and may cause electric shock if not done properly. It is also unsafe and inaccurate because the parasitic capacitance between the oscilloscope and the ground after floating will cause the signal to ring and cause signal distortion.

By manually disconnecting the oscilloscope's power ground wire (powered by a two-core power strip or the oscilloscope's battery) or using an isolation transformer to power the oscilloscope, the measurement loop can be disconnected and "floating" measurement can be achieved.

Floating measurement method (not recommended)

When measuring floating ground, the metal end of the oscilloscope housing and the ground end of the probe have the same voltage. When measuring mains power, the housing may have a voltage of 220V. Once the human body touches the oscilloscope housing, the current will flow through the human body into the ground, which is very dangerous!

Recommended measurement method:

From a safety perspective, it is absolutely safe to use a high-voltage differential probe to measure AC power with an ordinary oscilloscope.

High Voltage Differential Probe Measurements

Differential probes are designed based on the principle of differential amplification. They can convert floating signals between any two points into ground signals. Using high-voltage differential probes is the best recommended method, which is safe and convenient.

at last

An oscilloscope is an instrument used to measure the shape of alternating current or pulse current waves. Any periodic physical process that can be transformed into an electrical effect 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 understand the relationship between various signal lines in order to correctly connect the measured signal and ensure safe measurement.