Oscilloscope Isolation Issues

shaorc

Oscilloscope Isolation Issues [Copy link]

 

[Ask if you don't understand] I have never understood the isolation of oscilloscopes, so I would like to ask for help. For example, I accidentally clipped the small clip of the non-isolated oscilloscope probe on the Vcc pin of the 2844 chip in the main circuit of the 220 volt power supply. The other end of the probe was not connected to anything, and smoke was seen on the board. Later, I found out that the oscilloscope was not isolated. How did the smoke come from this situation? My idea is that the 220 volt power supply has a positive pole and a power ground, and the oscilloscope also has its own ground inside. This ground is the potential of the small clip on the probe. What is the isolated oscilloscope usually isolated from?

cruelfox

The probe clip of the oscilloscope is connected to the ground of the oscilloscope power cord. Then, if the ground of your board is also connected to the ground of the power cord, the Vcc will be connected to the ground by this clip, burning the power supply.

maychang

"My idea is that the 220V power supply has a positive pole and a power ground." That's not true. 220V is an alternating current, with only a live wire, a neutral wire, and a ground wire, but no "positive pole".

maychang

"What does the isolated oscilloscope we usually talk about isolate from what?" It isolates the oscilloscope case (which is also the ground clamp of the oscilloscope probe) from the 220V AC neutral wire (the AC mains neutral wire is connected to the ground at the distribution transformer) and the ground.

topwon

In fact, it is very simple. Just don't use the oscilloscope with a three-pin power cord with a ground wire. Then be careful not to touch the exposed metal parts on the oscilloscope, such as the ground clamp, the outer ring of the BNC terminal, etc., because these places are floating and may be charged. It depends on where you connect the ground clamp in the circuit, such as the live wire, the neutral wire, or V+.

bioger

In fact, it is recommended that you separate the ground of the oscilloscope during testing and use two power supplies. This will avoid many unexpected events during testing.

topwon

The safest way is to add an isolation transformer before the AC 220V input of the DUT. This way, you don't have to worry about anything. As long as people don't touch two places with high voltage difference at the same time, they won't be electrocuted. Using an oscilloscope will be no problem. Using a three-pin plug will also be fine. At this time, the point where the ground clip is located is the potential of the earth.

shaorc

maychang posted on 2018-1-19 11:52 "What is the isolation oscilloscope that we usually talk about? " It is to isolate the oscilloscope housing (which is also the ground wire clip of the oscilloscope probe) from the 220V AC...

Thank you

shaorc

maychang posted on 2018-1-19 11:52 "What is the isolation oscilloscope that we usually talk about?" It is to isolate the oscilloscope housing (which is also the oscilloscope probe ground clip) from the 220V AC...

[1] Since the AC mains neutral line is connected to the earth at the distribution transformer, why do we say that the 220V neutral line should also be isolated from the earth? [2] If it is not isolated, it is easy to connect the GND of the circuit board and the ground of the probe (i.e. the earth), because there is a voltage difference between the two, which will cause a short circuit? Is this understanding correct?

maychang

shaorc posted on 2018-1-19 16:37 [1] Since the neutral line of AC mains is connected to the earth at the distribution transformer, why do we still say that the 220V neutral line should be isolated from the earth? [2] If not...

[1] Since the neutral line of AC mains is connected to the earth at the distribution transformer, why do we still say that the 220V neutral line should be isolated from the earth? I did not say to isolate the 220V neutral line from the earth. What I said on the 4th floor was to isolate the oscilloscope housing (which is also the oscilloscope probe ground clip) from the earth.

maychang

shaorc posted on 2018-1-19 16:37 【1】Since the neutral line of the AC mains is connected to the earth at the distribution transformer, why do we also say that the 220V neutral line should be isolated from the earth? 【2】If not...

【2】If not isolated, it is easy to connect the GND of the circuit board and the ground of the probe (i.e. the earth) because there is a voltage difference between the two, which will cause a short circuit? Is this correct? It is not "easy". If not isolated, as long as there is a voltage between any point on the circuit board and the earth (whether AC or DC), it will definitely cause a short circuit through the oscilloscope probe ground clip (connected to the earth).

wsdymg

It is recommended that the OP first understand the principles of TT/TN-C/TN-CS in the power supply system.

topwon

I made a simple schematic diagram for easy understanding. If the ground pin in the oscilloscope's power cord is connected to the earth (the hexagonal star), then when the probe's ground clip is connected to a place with high voltage relative to the earth, such as the live wire in the figure, the live wire high voltage will be short-circuited to the earth by the oscilloscope's ground clip, which will burn the components on the circuit. A relatively simple solution is to disconnect the hexagonal star. I will make a special power cord, remove the middle ground pin on the plug, and replace the oscilloscope with this power cord when testing the primary circuit of the isolated power supply or the non-isolated circuit. In addition, sometimes when it is necessary to measure the voltage drop across the resistor, this power cord will also be used when the ground clip is not at ground potential.

shaorc

topwon posted on 2018-1-20 09:41 I made a simple schematic diagram for easy understanding. If the ground pin in the oscilloscope's power cord is connected to the earth (where the hexagon is), then when the ground wire of the probe...

Thank you so much, I need this diagram to understand, thank you thank you

shaorc

maychang posted on 2018-1-19 17:53 【2】If it is not isolated, it is easy to connect the GND of the circuit board and the ground of the probe (i.e. the earth), because there is a voltage difference between the two, which will cause a short circuit? Is this understanding correct...

Thank you, thank you

topwon

By the way, I also changed the schematic diagram to use a 1:1 isolation transformer. The advantage of this is that the oscilloscope casing and ground wire can be connected to the ground, which guarantees the safety of the operator. The disadvantage is that the isolation transformer is more expensive and takes up space (the specific choice is related to the power of the product).

PowerAnts

The most economical way: use pliers to twist off the PE on the three-pin plug of the oscilloscope, turning it into two pins (only L and N) The most advanced way: buy an isolation probe

iffi123

topwon posted on 2018-1-20 11:19 By the way, I also changed the schematic diagram of using a 1:1 isolation transformer. The advantage of this is that the oscilloscope case and ground wire can be connected to the ground, which is convenient for the operator...