Oscilloscope is a convenient self-test and probe compensation standard square wave, the general is 1 k Hz, we has the standard USB oscilloscope square wave, amplitude around 1.5 v, the default is a standard square wave, when necessary, can also be set to the PWM wave, frequency adjustable duty ratio,
Digress, I am in the process of technical support about the damage of mains problems with more than half are due to new curiosity, conveniently to the mains caused, so I have to emphasize that the oscilloscope is bringing square wave can be early adopters, novice don't begin to measure, video games, the old drivers are very careful, will not do this,
The effects of the new people most often asked question is:
Why do I always tripping oscilloscope measurement grid?
Why would I measure mains the oscilloscope burned?
First we need to understand the mains power supply circuit and principle of help to the safe and security measure,
* * * * line, zero line and ground
Our mains (residential electricity) specifications for ac 220 v @ 50 hz, power lines by the fire, the zero line and the ground, their relations are shown in figure 1.1,
Wire (L) : also called phase line, provided by the power plant or transformer substation, the RMS voltage 220 v, of the dangers of human exposure;
The zero line (N) : to provide wire loop, on power plant or transformer substation grounding; Because is the remote ground, in the residential building client potential is not zero, certainly may take weak current, but relatively safe;
Ground wire (E) : zero potential reference point, in the residential building client pick up the earth, and the zero voltage, absolute security,
Figure 1 line, zero line and the relationship between the ground
First to confirm the oscilloscope range:
LOTO most types of oscilloscopes, for example, the standard of the probe under x 1 rank range is + 5 v, under the X10 gear is 50 v + - range of individual models under x 1 rank to + 20 v, under the X10 gear is + - 200 - v.
But the range is not enough, because 220 v electric, which actually refers to its equivalent value, fengfeng value at about 600 v voltage,
With just 100:1 high voltage passive sensor is ok, is ok in theory, but in practice we are possible because the measurement method of using the wrong question,
Error of measurement: * * * *
On the desktop or laptop with a power adapter when using the USB oscilloscope LOTO, shell metal side with the negative terminals of the probe (ground) are connected to the ground of USB, USB ground wire connected to the computer to again, and then connected with the lug plate earth,
This case, when measured with oscillograph directly to the zero line and the line of fire, it indirectly the zero line or wire to ground short circuit (equivalent to figure in the red dotted line), is very dangerous, and it is also one of the most common method for measuring error,
Figure 2 error measuring method of
measurement method is not recommended to float:
As shown in figure 3, using the method of isolation transformer to a desktop computer USB power supply, the oscilloscope can achieve the purpose of disconnect measuring loopback, realize the "floating" measurement, using laptops connect LOTO oscilloscope, do not use the power adapter, and float to the oscilloscope,
Float to measure, the oscilloscope shell metal end and probe the grounding end with the same level, and with the high voltage being measured at one end of the nipple, so when measuring the utility, if neglect the oscilloscope on the wire clamp, makes the oscilloscope and computer shell with a 220 v voltage, once the body in contact with the oscilloscope shell, electric current passes through the body into the earth, very dangerous!
Figure 4 float to measure safety
does not recommend the resistance of the partial pressure measurement method:
Some engineers have been familiar with high voltage of 220 v mains by resistance partial pressure to a lower voltage and then measure, thus avoiding the risk of exposure to high voltage,
As shown in the above, the use of two resistance wire and ground 100 times between attenuation, and then use the oscilloscope to measure:
This kind of practice and the use of 100-1: the principle of passive pressure probe is the same:
From the figure above you can see, this kind of practice can be successful measurement is not recommended, because it have higher request for the operator, to be familiar with high voltage circuit to be tested and be careful not make a mistake, because once neglected to take line inside out, or not good, then it is likely that they will have disastrous consequences,
For ordinary oscilloscope based on security perspective, the use of "A - B" probe to the mains measurement method and pressure difference is very safe, respectively to introduce below,
"A - B" pseudo differential measurement
Using common passive probe "A - B" method was applied to measure the mains, in the use of appropriate voltage probe attenuation ratio will limit within the scope of oscilloscope range, can two channel should be the negative terminals of the probe (ground) are connected to the power ground wire, A channel probe probe (positive side) to zero, another channel probe probe (positive side) by wire (as shown in figure 5 left), the measurement difference between the two channels is the mains waveform,
It is also possible, but must pay attention to the two probes are not grounded, although you can to avoid the problem of re and prevent the reverse problem, but still need to pay attention to the range of not more than the oscilloscope, we recommend another method, that is described below with isolation difference measurement module,
Isolation difference measurement module
Isolated differential module is used to measure the mains, firewire and zero line test point and it doesn't matter, probe the internal magnetoelectric isolation ways to measure the earth and oscilloscope isolated, not cause short circuit problem,
LOTO oscilloscope kit isolation difference module does not like the traditional high voltage isolation probe 23000 yuan expensive, probably more than one hundred, but can be in the form of isolation difference measurement + - 800 - v high pressure,
IDM01 isolation difference module is the best way of recommendation, safe and convenient,
We took a two probe measured 220 v mains demonstration on video as follows:
https://www.bilibili.com/video/BV1qJ41137u1
Oscilloscope, as a kind of very important measuring instrument, we should correctly and safely use, especially in the measurement of some high voltage signal, must make clear the relationship between the various signal lines, to connect the measured signal correctly, guarantee the safe measure,