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
10:1 resistor divider probe with a 500 Ohm input impedance.
Close-up of Wire Wrapped on Current Probe
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 3. Setting the TG-EFT at 60 Volts for the Measurements
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
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 4. Measured Pulse using Four Turns of Wire Around Current Probe
(Vertical scale = 10 Volts/div, Horizontal scale = 50 ns/div)
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 5. Measured Pulse Using Four Turns of Wire Around Current Probe - Expanded
(Vertical scale = 10 Volts/div, Horizontal scale = 2 ns/div)
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 6. Test Setup For Measuring Pulse Induced by Current Probe into a Single Turn
Figure 7. Close-up of Current and 1163 Resistive Divider Scope Probe
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
the peak in Figure 9 for the single turn case.
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.
Figure 8. Measured Pulse using One Turn Around Current Probe
(Vertical scale = 2 Volts/div, Horizontal scale = 50 ns/div)
Measured Pulse Using One Turn Around Current Probe - Expanded
(Vertical scale = 2 Volts/div, Horizontal scale = 2 ns/div)
The F-33-1 current probe contains just a
magnetic core surrounded by a coil and handles the pulses from the
TG-EFT well. Some current probes, such as the Fischer F-65 and F-33-2
probes, contain resistive networks to modify the current probe
frequency and amplitude response. Probes such as these with internal
resistive networks should not be used to inject pulses as the required
high voltage pulses applied to the current probe may damage the