Introduction

Teledyne LeCroy offers a series of AC/DC sensitive current probes with maximum continuous current ranges to 500 A and bandwidths to 100 MHz. Figure 1 shows the whole family of current probes summarized in the table below.

Figure 1:

Teledyne LeCroy current probes (clockwise from upper left: CP031, CP150, CP500, AP015, CP030)

Note that all the current probes are fully integrated into the scope via the LeCroy ProBus interface. They receive power from the scope and produce waveforms in units of Amperes. They are fully controlled, including degaussing and autozero operations, from the input coupling menus.

While all these current probes have a wide sensitivity range, it is sometimes desirable to be able to increase the sensitivity of the probe. For measuring very small currents, the sensitivity of a current probe can be increased by wrapping multiple turns through the primary as shown in Figure 2.

Figure 2:

Increasing the sensitivity of the current probe

Because current probes follow rules governing transformers, the sensitivity will increase by a factor of the number of turns passing through the primary. Note that the insertion impedance will increase by the square of the number of turns. For example, wrapping 10 turns through the jaw opening of a current probe will increase the sensitivity by a factor of 10, and the insertion impedance by a factor of 100. Typically this is not a problem, because current levels this low will not generate very large voltage potentials across the insertion impedance. When using this technique, be sure to factor the transformer ratio into any on-screen scale factors and math functions.

By passing multiple conductors through the primary, as shown in Figure 3, the current probe will only measure the net current sum flowing through all of the conductors.

Figure 3:

Setup for measuring the net current in a pair of conductors

Currents of equal magnitude and opposite polarity will cancel. This technique can also be used to extend the DC or low-frequency AC current range without exceeding specified limits by subtracting an offset current with a second conductor that has a pure DC component of a known value. You can also cancel low-frequency, common components in this manner. The second conductor’s current can be increased by winding multiple turns.

When using current probes, be aware of some common characteristics shared by all such devices. All current probes have some type of shielding to minimize the pickup of electrostatic fields radiating from the test conductor. Optimizing the design of the shield for maximum rejection imposes some compromises in other probe parameters. Thus, different vendors’ probes have differing ability to reject fast dv/dt signals in the test conductor. Be aware that Teledyne LeCroy’s current probes have among the lowest voltage sensitivity in the industry. This means that wherever possible, probe the circuit under test on the low-voltage side of the circuit.

When that is not possible, a simple test, shown in Figure 4, can be used to quantify the electric field pickup in the actual circuit. Connect a short piece of wire to the test conductor. Do not terminate the opposite end. Place the jaw opening around the conductor and view the waveform. Because no current is flowing through the unterminated wire, any signal displayed in the waveform is due to the dv/dt coupling into the probe. Ideally, none of the voltage signal will be visible in the displayed waveform.

Figure 4:

A test for dV/dt sensitivity

Inductance and some resistance are added to the circuit from the loop added for probe attachment. The smaller jaw configurations of the Teledyne LeCroy probes reduce this impedance because they permit measurement using smaller test loops. A simple trick to reduce the loop inductance further is to twist the loop tightly near the probe reduces the inductance as shown in Figure 5.

Figure 5:

Reducing test-loop inductance by twisting the test loop

Whatever your current measurement needs, Teledyne LeCroy offers a range of probes with measurement capabilities from DC to 100 MHz and from milliAmps to 500-A.