Discussion: In the  December 2007 Technical Tidbit, current probes were shown to be a convenient method of injecting series voltage pulses in circuits for troubleshooting. However, the amount of injected voltage may not be enough, 50 to 100 Volts was demonstrated in the December article, to cause the desired response.  Figure 1 shows the test setup used to measure the injected voltage when a wire was wrapped through the current probe for multiple turns. This will increase the injected voltage at the expense of additional series source impedance in the circuit to be stressed. Figure 2 shows a close-up of the wire wrapped on the current probe with the wire passing through the current probe four times. The induced voltage was measured using an Agilent 1163a 10:1 resistor divider probe with a 500 Ohm input impedance.


A Fischer Custom Communications TG-EFT high voltage pulse generator was used to drive an F-33-1 current probe for this article. As shown in Figure 3, its output was set to just ~60 Volts to keep the scope from being overdriven.



Figure 4 shows the induced voltage peaking at about 28 Volts for a 60 Volt drive from the TG-EFT. This means that with a drive of 400 Volts from the TG-EFT, the maximum I have used with the F-33-1 current probe, injected voltages of around 200 Volts are possible. Additional turns could be added on the current probe to yield higher induced voltages. At some point, parasitic capacitance in the coil, its inductance, or physical size compared to the frequency components in the pulse will become a problem. I recommend checking the injected pulse with an oscilloscope to make sure you are getting the pulse that is desired.

Note in Figure 4 the fast rising edge and long tail which is the shape of the output pulse of the TG-EFT. The dip below zero Volts is an artifact of the current probe being AC coupled. The area under the curve must be the same above zero Volts as it is below zero Volts averaged over time.


Figure 5 shows the same waveform expanded out from 50 ns/div to 2 ns/div to show a rise of about four nanoseconds to the peak from the baseline. The peak of ~28 Volts is more clearly seen in this view.


Figure 6 shows a measurement using just one turn through the F-33-1 current probe, in this case the ground lead of the scope probe. This is similar to one of the measurements described in the December 2007 Technical Tidbit. A close-up of the current probe and scope probe appears in Figure 7.




Figure 8 shows the induced voltage into the 1163A probe, for a 60 Volt drive from the TG-EFT, peaking at about 7.7 Volts. Figure 9 expands the time scale from 50 ns/div to 2 ns/div where the peak at 7.7 volts can be seen more clearly. The rise from the baseline to peak is about three nanoseconds. This is faster than the rise of four nanoseconds of the four turn case because of lowered series inductance and less capacitance to the current probe body resulting in less filtering for the single turn case. These added effects of the four turn coil are responsible for both the longer risetime and flatter peak in Figure5 versus the peak in Figure 9 for the single turn case.

The ratio of the induced voltage between four turns and one turn is 28/7.7 = ~3.6 a little lower than the turns ratio of 4:1 because the input impedance of the probe represents more of a load on the four turn coil as well as the filtering effects described in the last paragraph.