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Tektronix P6022 Current Probe
When we think of an oscilloscope probe, the common 10X
voltage probe first springs to mind. After all, an oscilloscope displays voltage
versus time when in swept mode, doesn't it?
Well, yes, but suppose you wish to measure current, not
voltage. One way to measure current is to insert a resistor on the ground side
of the circuit under study and measure the voltage across the resistor. If it's
a 1 ohm resistor, then 1 mV deflection on the oscilloscope trace corresponds to
1 mA current.
But, breaking the circuit to insert a current sensing
resistor is inconvenient at best. Fortunately, "current probes" are available
from oscilloscope manufacturers. These probes are the current analog of the
voltage probe, permitting the trace to be interpreted in terms of mA or amperes
per division instead of millivolts or volts.
This page focuses upon one particular current probe, a
Tektronix P6022. A P6022 is frightfully expensive if purchased new (list price
currently $1,300) but occasionally shows up in the surplus market at more
reasonable prices.
The P6022 and similar vintage probes are AC only devices,
as they are based upon inductive (transformer) coupling to the circuit under
test. Newer designed current probes combine inductive coupling and Hall effect
coupling thereby extending the low frequency limit to DC. I also have one of
these devices, a Tektronix TCP202, that I use with my digital oscilloscope.
These are even more expensive (current list price $1,700) than AC-only current
probes and require either a separate DC power supply or an oscilloscope with a
powered center conductor. I won't further mention these more modern designs.
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Current probes use the same principle as snap-on ammeters; a
transformer wound on a toroidal core efficiently couples flux from a single wire
that passes through it. Hence if the core is mechanically split, it may be
opened, a wire inserted and then the core closed. The fixed windings form a
transformer secondary with the inserted wire as the primary. Properly
terminated, the secondary windings will have an induced voltage proportional to
the current through the primary winding. (Of course, you can use more than one
turn as the primary, thereby increasing the probe's sensitivity.)
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P6022 Current Probe with jaw open to receive wire
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Wire in place, jaw closed
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The probe's arrow and +/- signs indicate the conventional current flow
orientation corresponding to a positive voltage output. Conventional current
flow is in the positive to negative direction. (Yes, electrons flow in the
reverse direction. But circuit equations—developed before the electron was
discovered—assumed electrical current flowed from positive to negative.) In
other words, suppose the probe is clipped onto a circuit with a sine wave signal
generator and, say, a 10 ohm resistor. If we connect one channel of a
dual-channel oscilloscope to the voltage across the resistor, and if the current
probe is on the second channel, and we clip the current probe onto the resistor
lead such that the - sign in on the same side as the voltage probe ground lead
and the + side is on the same side as the voltage probe "hot" lead, the voltage
and current waveforms will be of the same polarity on the oscilloscope screen.
If we flip the current probe around, the current will appear inverted.
The photo below shows the correct orientation, with a 10 ohm
resistor on the output of a function generator set at approximately 1 MHz. Note
the oscilloscope voltage probe ground connection is on the - side of the
current probe and the + side of the current probe is nearest the "hot" side of
the voltage probe.
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Channel 1 (top trace) shows voltage across the 10 ohm
resistor; Channel 2 (lower trace) the current through the resistor with the
above test setup. Note that the voltage and current waveforms are in phase.
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Flipping the current probe 180 degrees produces the following result:
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Same test setup as above, but with the current probe
reversed. Note that the current waveform (bottom trace) and voltage waveform
(upper trace) are now 180 degrees out of phase.
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The P6022 is in 1 mA/mv mode for this test, so the current is 15.95 mA and the
voltage across the resistor is 152.0 mV (both peak-to-peak values). Hence, the
resistance is 152.0 mV / 15.95 mA = 9.52 ohms. Close enough to the nominal 10
ohm value (±5%) for our purpose. The accuracy could likely be improved by
calibrating both the oscilloscope and probes, but you see the idea.
And, of course, one can measure the phase angle between
the voltage and current traces thereby obtaining the complex impedance. I'll
leave this as an exercise to the interested reader. |
Incidentally, this discussion should alert you to an important aspect of the
current probe—it has no ground reference. The wire carrying the current to be
measured may have both ends at any potential to ground, unlike the case for a
simple voltage probe. Of course, there are safety limits related to the current
probe's insulation, but there are occasions when being able to measure current
without a ground reference is quite useful. One is when looking at power
supplies run from the AC mains, particularly if they are not transformer
isolated, such as in switching power supplies. Using a voltage probe in these
cases can be dangerous and may require careful thought.
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Another question that may occur to you is whether the current
probe reflects an impedance bump back into the wire. Yes, it does, but for many
applications the impedance bump is negligible.
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Impedance added by the P6022 Probe
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The P6022 has two sensitivity ranges. One is 1 mA/mV, i.e., a
current of 1 mA provides 1 mV output. The second is 10 mA/mv, where 10 mA is
required to produce 1 mV output. The user may select between the two sensitivity
options through a slide switch on the interface box:
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Here's a wider view of the business end of the P6022
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Tektronix provides the following performance specifications
for the P6022. |
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Using an HP3562A Dynamic Signal Analyzer, I swept a P6022
current probe over the range 100 Hz to 100 KHz, in both the 1 mA/mV and 10 mA/mV
ranges. The image below shows the 3 dB response point is about 6 KHz in the 1 mA/mV
position, well within Tektronix's 8.5 KHz specification.
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On the 10 mA/mV position, my probe shows the -3 dB point at 1.02 KHz, a bit
above Tektronix's specified 935 Hz point. This discrepancy may be caused
by something as simple as a bit of dirt on the jaw's mating surfaces.
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