Inside A Current Probe

[The Signal Path] had two Tektronix AC/DC current probes that didn’t work. Of course, that’s a great excuse to tear them open and try to get at least one working. You can see how it went in the video below. The symptoms differed between the two units, and along the way, the theory behind these probes needs some exploration.

The basic idea is simple, but, of course, the devil is in the details. A simple transformer doesn’t work well at high frequencies and won’t work at all at DC. The solution is to use a hall effect sensor to measure DC and also to feed it back to cancel coil saturation.

By switching transformer assemblies between the probes, he found that one worked and one did not. So the next step was to examine the assembly to see if anything was obviously wrong with it. There was also a plastic piece broken on both probes that needed replacing, but that didn’t seem like a difficult problem to surmount compared to the electronic problem.

Fortunately, having a good transformer allowed him to compare readings to determine any problems. The hall effect sensor appeared bad, but taking the module apart destroyed it. On the other hand, we get to see what’s inside. Unfortunately, something in the repair operation broke the working module. It would intermittently work with some pressure, but for the most part, it suddenly had the same symptom as the other module. [The Signal Path] speculates that the pressure of reassembly damaged the same connection to the hall effect.

It isn’t pretty, but you can make your own probe for considerably less. There are some attractive homebrew units, too.