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AC Current Probe for Oscilloscopes
A current probe with a low frequency cutoff of 44 Hz and the calibrated sensitivity is 50 millivolts per amp.
Formulae are given to enable customization.

The current probe transformer and termination is housed in a plastic box.
A hole in the box and an insulating sleeve allows a wire with the current
to be sensed to be passed through the center of the toroid core.

I needed several current probes when designing the deflection circuits and high voltage supply for a computer display monitor, and the lab in which I was consulting only had one current probe, which I shared with the other four engineers on the project.  We did not really have a budget for test equipment on that project. Designing the deflection circuit included designing several inductors, a current transformer among them. It was merely a small diversion to design and put together a couple of simple current probes so I could, for example. simultaneously watch the base current and the collector current of switching transistors.

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.

Waveforms from a 187 KHz transmitter circuit. The lower trance
is 200 milliamps per division, measured with this current probe.

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. 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.

 This probe only measures AC current.

The Circuit

Current from the single loop of wire passing though the hole in
the toroid induces current in the secondary, which creates a voltage drop.

The current probe is a current transformer. A single loop of wire passes through the center of the toroid core, inducing current in the secondary. A 2.7 Ohm resistor is the termination resistor and a 1K pot is used to calibrate the output voltage as a function of input current.

The resistance of the wire in the secondary
affects the probe's low frequency response.

The current induced in the secondary is inversely proportional to the number of turns in the secondary. My probe has a 42 turn secondary, so the current in the secondary is 1/42 that of the primary, which means there are 24 milliamps of secondary current for every amp of input current. The illustration above is a low frequency analytical model of the probe's circuit. The secondary current causes a voltage drop across the 2.7 Ohm sense resistor, and this voltage is further divided by the 1K potentiometer.

The sensitivity of the probe in volts out per amps input is:  

which in this case 1/42 x 2.7 = 64 millivolts per amp. The 1K pot is used to adjust this sensitivity downward to 50 millivolts per amp.

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 secondary of one probe was wound with #30 wire magnet wire and the secondary of the other was wound with #30 Kynar insulated wire wrapping wire. A larger wire diameter would mean better low frequency response, but at the would have also induced more high frequency losses because of eddy currents resulting from the flux gradient from the core across the conductor. The resistance of #30 copper wire is 0.104 Ohms per foot, which is 340 microohms per millimeter. One turn of wire is 34 millimeters long. Therefore, the resistance of the secondary is:


To this, you have to add the value of the 2.7 Ohm termination resistance, so total resistance is:


which is equal to 3.185 Ohms.

The other value needed to compute the low frequency corner is inductance.

Where L is inductance, AL is the inductance index of the core, and N is the number of turns in the secondary. I used a Ferroxcube 846T500 toroid cores for these probes.  A test winding of 7 turns gave an inductance of 319 microhenries., so I calculated the AL of these cores to be about 6.51 microhenries per turn squared.

This coil has 42 turns on it, so the inductance is:

With a total resistance of 2.7 Ohms for the termination + 0.485 Ohms for the wire = 3.185 Ohms, and an inductance of 11.5 millihenries, the single pole high pass RL filter value can be calculated:



After assembly of the toroid inductor and the circuit board it was time for calibration.

The calibration fixture was a FET switch, a 10 Ohm power resistor, a large decoupling capacitor pulse generator and a bench power supply. Using an oscilloscope, I adjusted the gate drive and bench supply voltage so that 10 volts was switched across the 10 Ohm resistor. This provided a 1 amp peak-to-peak square wave through the wire going through the current probe. I adjusted the 1K pot to get 50 millivolts peak-to-peak on the scope. Calibration was complete.


I cut a piece of pre punched fiberglass circuit board to an appropriate size and drilled a hole in the spot over which I mounted the toroid. I made matching holes in the plastic box and the plastic cover.

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 the transformer. The other probe used the barrel from a BIC ballpoint pen as the insulted sleeve, but this was rigid and not very forgiving of mechanical misalignment of the holes, thus the use of vinyl in this one.

The shielded cable with the BNC connector on it was fastened to the circuit board, the vinyl sleeve was installed, and the board was fastened into the box with a few dabs of hot melt glue. After calibration, the cover was fastened into place with the screws and the probe was ready to go.

Plastic boxes are very nice for this sort of use. They are easy to cut holes in and there is no worry about the box shorting to the circuit under test, as might happen with a metal box.

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First posted in August, 2005

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