How to test current waveform with oscilloscope
Oscilloscopes are the most commonly used instrument by most electronic engineers. When people think of oscilloscopes, they immediately think of test voltage. Of course, many oscilloscopes can also do rough spectrum analysis, etc., but many oscilloscopes are very concerned about one indicator that electronic engineers are concerned about - - Current cannot be tested. In some analysis and verification, not only the voltage needs to be tested, but sometimes the current needs to be tested. Currently, some high-end oscilloscopes can test the current, but they need to purchase an active current probe separately. When the word active is mentioned, it means The price is quite high, yes, the cost of purchasing an active current probe can be almost as much as buying some brands of mid-range oscilloscopes, so this is not a "rich" equipment that ordinary small companies can afford.
When it comes to current testing, some people may say, can't a multimeter just measure it? Of course, a multimeter can measure the current at a certain moment, but there are several problems: 1. Because the response speed of the multimeter is slow (usually in the order of hundreds of mS) ;2. The multimeter cannot record long-term test results. Better meters can record the maximum and minimum values, etc.; 3. The most important thing is that the multimeter cannot see the process of current change. Many times what we want to see is the change process. Not just the results, for example, we want to know when the transistor overcurrent damage is most likely to occur instead of just seeing the transistor smoking.
Is it impossible to use an oscilloscope to see the changing process of current without an expensive current probe? In fact, we can still find a solution by changing our thinking. The method is actually very simple, that is, I=V/R, which we learned in middle school physics. I am crying. ?Note that V is not the voltage at a certain point, but the potential difference between two points. This is the key, and it is also where some beginners tend to fall into misunderstandings. If you use the voltage change at a certain point to predict the change in current, you will often make mistakes. Yes, we can see this from the example test later.
specific method:
The specific method of this method is: use two probes to measure the voltages V1 and V2 at both ends of a resistance (it can even be a section of line, of course, provided that the resistance of this section of line is large enough to produce a suitable potential difference at both ends), Then use the calculation function of the oscilloscope to calculate △V=V1-V2 in real time, and I=△V/R. As long as the environment does not change drastically, we can think that R is unchanged, so I changes with △V It changes linearly, so the change in △V reflects the change in current. Let's use an example to verify whether this method is feasible.
Example verification:
The oscilloscope tests the voltage and current changes between the drain and the source of a MOS tube on a PCB at the moment of power-on. The brown waveform is the source voltage Vs, the purple waveform is the drain voltage Vd, and the yellow waveform is smaller. The coarse waveform is the drain-source voltage △Vsd =Vs-Vd calculated through the oscilloscope's calculation function (in this example, channel C1 measures Vs, and channel C2 measures Vd, so the specific calculation settings are as shown in Figure 2 C1-C2); The green waveform is the drain-source current Isd measured with an active current probe. From the comparison of the waveforms of Isd and △Vsd, it can be seen that their change processes are very close; measured with an active current probe The Isd peak value is about 3.6A; the calculated △Vsd peak value is about 0.43V, and the line resistance measured with a multimeter is about 0.15?, so the current peak value obtained by the potential difference method is about 0.43V/0.15? = 2.87A, which is different from the results of the active current probe test. Of course, this is related to the on-resistance of the MOS tube in different states, the error of the oscilloscope, passive probe, and multimeter, etc., but use this method to test the current we are most concerned about. The change process is completely feasible. By observing the change of current, we can roughly know when the damage of the MOS tube is most likely to occur, thus providing a basis for taking correct measures.
Seeing this, experienced engineers may ask a question: How to solve the common mode rejection ratio CMRR when using ordinary probes for testing? This problem does exist, but as we mentioned before, the main purpose of this method is to allow us to Seeing the changing process of current, under the influence of various factors, the accuracy of the specific current value tested by this method is definitely not as accurate as that of a specialized active current probe (if this free method can completely solve the problem of tens of thousands of dollars) Active current probes will no longer be sold in the future. Of course, if you happen to read this article and solve a previous unsolved case one day by analyzing changes in current, you might as well persuade your boss to drink two bottles less and buy A current probe^_^); and to solve CMRR, you need to use an active differential probe. The price of this stuff is comparable to that of a current probe. In this case, we will not achieve our goal of not spending money^_ ^; However, Vs-Vd has the advantage of eliminating some of the interference on the signal.
