Oscilloscope AC Probe

Posted on Oct 3, 2012

The probe described here was designed to be used over the range of several tens of Hz to a couple MHz with currents from a few tens of milliamps to about 10 amps peak-to-peak. Formulae are give which will allow you to create a design suitable for your own applications. To use this probe, a wire is connected in series with the load that you want to see the current through, and that wire is passed through the hole in the probe. An oscilloscope is used to view and measure the current waveform, which is calibrated in terms of volts output per amp of input current.

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The sensitivty of the circuit can be multipled by using multiple turns in the sensing winding. For example, to get 5 times the sensitivity, pass the input wire through the hole in the toroid five times. Of cousre, it would be much better to design the probe to have the sensitivity required for your application, but this trick is ok if you don't have to do it too often. After scale factor, the low frequency corner frequency is the next most important consideration. At this frequency, sensitivity is -3 db (-30%) from what it was at significantly higher frequencies, and below this frequency the sensitivity is cut in half every time the frequency is halved, which in other words, is -6 db per octave. The low frequency corner is a function of the inductance of the secondary and the total resistance across the secondary. The total resistance is the sum of the resistance of the secondary winding (Rwire below) and the termination resistance. We will ignore the effects of the 1K pot and the connection to the oscilloscope. The toroid is held in place by a couple of plastic wire ties (Panduit brand tie-wraps to be precise) and inserted a length of vinyl insulation I stripped off of a piece of multiconductor cable though the hole in the toroid and the hole in the circuit board. The termination resistor and the calibration pot are mounted on the circuit board, their leads bent over and soldered to each other and the leads from...

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