Oscilloscope Knowledge: Current Measurement and Current Probes

There are three common techniques for measuring current using an oscilloscope. The first is to use a series resistor (sometimes called a shunt) when measuring current. The second is to use a current transformer. The third is to use a current probe. All three methods require the current being measured to pass through the measurement sensor, making them somewhat invasive.

Current probes are the least invasive technique because they allow users to measure the current in a wire without having to unplug the wire. However, users need to understand some basic current probe principles to best take advantage of the latest probe designs.

This article will introduce various current measurement methods before introducing current probes and how to use them effectively.

Shunts

Shunts are typically designed into a circuit or associated test setup. The current is determined by measuring the voltage drop across the known resistance of the shunt. There is an engineering trade-off between the series resistance of the shunt and the sensitivity required for the current measurement.

The voltage drop must be reasonable without sacrificing circuit performance. Also, the power rating of the shunt must be large enough for the current being measured. An example of a shunt resistor is the Riedon RSA-10-100. This chassis-mount resistor has a resistance of 0.01 Ω, a tolerance of ±0.25%, and a power rating of 1 W. The resistor power rating sets the maximum current to 10 A, producing an output voltage of 100 mV. An oscilloscope is used to measure the voltage across the shunt. Most oscilloscopes can scale the voltage to an equivalent current (Figure 1).

Figure 1: Teledyne LeCroy HDO4104 oscilloscope channel settings showing the rescale settings to read out amperes vertically based on the shunt resistor value (Image source: Digi-Key Electronics). The

input channel settings of the Teledyne LeCroy HDO4104 oscilloscope are common to many instruments that allow rescaling of vertical data. The “Rescale” control allows the user to specify the units (amperes in this case) and the rescale factor in “Units/Volt.” For the RSA-10-100, the “Units/Volt” setting is the inverse of the resistor value, which is 1/0.01 = 100. The oscilloscope can also add or subtract offset current, which may be used in active sensors. After entering the “Rescale” setting, the oscilloscope input channel can be directly scaled vertically to read amperes, including measurement parameters and cursor readouts.

Shunt resistors respond to both AC and DC currents, and the bandwidth is limited by the internal inductance and capacitance of the resistor.

Coaxial or pulse shunts are an addition to standard flat shunt resistors. These devices arrange the shunt resistors in a cylindrical geometry to minimize their inductance. The voltage contacts are brought out to a coaxial connector suitable for the bandwidth.

Coaxial shunts are suitable for bandwidths up to hundreds of MHz, depending on the resistor value and maximum current rating.

The size of the coaxial shunt is proportional to the maximum current rating and is much larger than traditional flat shunts.

Current Transformers

Another possible approach is to magnetically sense the current passing through the wire. The simplest magnetic sensor is the current transformer (Figure 2).

Figure 2: Current transformers use a multi-turn secondary winding to sense current in a wire that passes through an opening in a ferrite core. (Image source: Digi-Key Electronics)

The wire carrying the current being measured (IMeas) passes through the center of the ferrite core that forms the transformer's primary winding. The magnetic flux induced in the core by the current is proportional to the current. N turns of the sense winding are used to sense the magnetic flux. The induced current in the secondary winding is proportional to the turns ratio (the number of turns in the primary winding to the number of turns in the secondary winding, 1/N in this case).

The secondary current is converted to a voltage by passing it through a resistor, most commonly the 50 W termination of an oscilloscope. If using the 50 W termination, set the input channel's Rescale setting to N/50 A/V. Current transformers are only suitable for AC signals. They cannot be used to measure DC current.

Note that the wire being measured must pass through the core. This requires unbundling the wire and passing it through the core. Some current transformers use a split core to allow insertion of the conductor being measured.

Current probes

Current probes are designed to provide a convenient way to measure current. They can be AC ​​coupled, using current transformer technology, or AC/DC coupled. In either case, the probe includes a split core geometry that allows for easy insertion of the current carrying conductor without having to disassemble it.

The Teledyne LeCroy CP030 is a typical example of an AC/DC current probe that can measure currents up to 30 A with a bandwidth of 50 MHz (Figure 3).

Figure 3: The Teledyne LeCroy CP030 is a 30 A, DC - 50 MHz current probe. It uses a ProBus interface and fully integrates with Teledyne LeCroy oscilloscopes. (Image source: Teledyne LeCroy)

The CP030 uses hybrid technology, incorporating both a Hall effect device for DC and low-frequency AC signals and a current transformer for high-frequency AC signals (Figure 4).

Figure 4: Functional block diagram of the CP030 AC/DC current probe. (Image source: Digi-Key Electronics)

The CP030 uses a split ferrite core that allows for quick insertion of the conductor carrying the current being measured.

It incorporates a Hall effect sensor and a feedback winding in the gap of the core. The Hall effect sensor, when properly biased, produces an output current proportional to the magnetic flux in the core. This output is amplified and drives the feedback winding, which causes the core to go to a zero flux state. At this point, the current through the feedback winding is proportional to the magnetic flux due to the current carrying conductor.

The output termination converts this current to a voltage. Because the Hall effect sensor output decreases with increasing frequency, the feedback winding acts as a current transformer, allowing accurate measurement of the high frequency portion of the measured signal.

The CP030 has a sensitivity of 1 V/A. The probe transmits this information to the oscilloscope via the ProBus interface. The oscilloscope automatically scales the channel into which the probe is inserted to read amperes (Figure 5).

Figure 5: Teledyne LeCroy HDO 4104 oscilloscope channel setup with CP030 connected. The probe is automatically recognized and shown in the “Probe” input box. The correct scale is automatically entered into the “Units/V” input box, and the vertical units are set to “Amperes.” (Image source: Digi-Key Electronics)

In addition to sensing and scaling the probe output, the oscilloscope includes a dialog box with all the controls related to the probe (Figure 6).

Figure 6: The CP030 probe setup dialog box showing the probe degauss and autozero controls. (Image source: Digi-Key Electronics)

The dialog box includes degauss and autozero controls. The degauss feature removes any residual magnetic flux in the probe core by applying a degauss signal. Degaussing should be performed before any important measurements to ensure the best accuracy. The autozero control sets any offset in the probe output to zero volts when no current is flowing. Having these controls on the oscilloscope means that no space is taken up in the probe or interface box, making the probe smaller. The dialog box also identifies the probe and provides its key specifications.

Common Techniques to Improve Current Probe Effectiveness

When measuring low currents, current probe sensitivity can be improved by wrapping more turns of wire around the primary winding (Figure 7A).

Figure 7: Improving current probe sensitivity by wrapping more turns of wire around the probe core (A). Passing multiple wires through the probe core for differential measurements (B). (Image source: Teledyne LeCroy)

As with any transformer, the more turns that are passed through the probe core, the higher the probe sensitivity. In Figure 7, there are four turns of wire wrapped around the probe core, which increases the sensitivity by a factor of four. This factor must be manually entered into the probe “Rescale” setting. Note that the insertion impedance will also increase by the square of the number of turns. In this case, the impedance will increase by a factor of 16. Since the measurement is for low current levels, the voltage drop across this impedance is typically low and has minimal effect on the measurement.

If multiple wires are passed through the probe, the oscilloscope will read the net current (Figure 7B). This technique can be used to measure the differential current in two wires. Passing wires with equivalent DC currents in opposite directions can also be used to cancel large offset currents. This can extend the range of the current probe.

Using Third-Party Current Probes

Many manufacturers offer current probes for general-purpose oscilloscopes that do not use proprietary interfaces. These probes include power, degaussing controls, and controls to adjust the probe's DC offset. An example is the Cal Test Electronics CP6990O-NA AC/DC 40 A, 1.5 MHz stand-alone current probe (Figure 8).

Figure 8: Cal Test Electronics CP6990-NA stand-alone current probe and related accessories (Image source: Cal Test Electronics)

This current probe is battery powered and connects directly to the oscilloscope via the included BNC cable. The probe has dual sensitivity ranges, 1 V/A or 100 mV/A. Accordingly, the Teledyne LeCroy HDO4104 "Rescale" field enters 1 or 10 Units/Volt respectively. The Unit field should select "A". The vertical scale of the selected channel can now be calibrated in "Amps".

Summary

Oscilloscopes can use shunts, current transformers, or current probes to make current measurements. Regardless of the sensor used, the oscilloscope channels can be rescaled to read directly in current units.
Current probes are easy to connect and are therefore the easiest device to use. Current probes provided by oscilloscope manufacturers sense the probe and automatically adjust the scale of the current data.