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Douglas C. Smith

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Technical Tidbit - June 2016
Measuring Current of a High Voltage Air Discharge

Measuring spark current

Figure 1.
Overview of Setup for Measuring a High Voltage Air Discharge

Abstract: Special precautions need to be taken when measuring the current of a high voltage discharge. A specially designed target that balances insulating a current probe from the high voltage with minimizing inductance of the discharge path is described. Construction details are given.

Figure 1 shows an overview of the target on the receiving end of multiple 50,000 Volt discharges, mounted on a brass plate connected to earth ground. In trying to measure the discharge current, I found it was difficult to prevent sparking over to the current probe during my early attempts at this measurement, a more elaborate test target was necessary. The parameters of interest were insulation of the probe from the current path and the inductance of the current path.

Figure 2 shows a close-up picture of the target. Its construction consists of a 6 inch/15 cm length #10 screw (you can see the screw head at the top of the target) inserted into a brass tube with a close fit. The purpose of the brass tube is to reduce the inductance of the current path due to its greater diameter and the lack of screw threads on its surface. Skin effect will cause high frequency to have to travel over the surface of screw threads, increasing the path inductance. Two large washers with a center hole for a #10 screw were used on both sides of the brass plate where the #10 screw passed through the plate. This strengthens the fastening of the brass tube to the brass plate by the #10 screw to avoid bending of the plate, screw, or the end of the brass tube during lab usage.

The brass tube was then covered with four layers of heat shrink tubing and then several plastic plumbing pieces fitted into each other were placed over the insulated brass tubing. The bottom large plastic piece spaces up the current probe above the metal plane, providing the required insulation against breakdown between the plane and the current probe. The tubular upper parts of the plastic pieces serve to center the current probe on the brass tube and to provide more insulation. This can be seen in the top view of the target with current probe in Figure 3 and in the side view of Figure 4. The current probe used was a Fischer Custom Communications F-65, which is ideally suited to these kind of measurements due to its wide bandwidth of 1 MHz to 1 GHz with flat frequency response.

Closeup of target

Figure 2.
Close-up of Target

Closeup of target with current probe

Figure 3. Top View of Current Probe Centered in the Target

Closeup of high voltage target

Figure 4. Side View of Current Probe in Target

This target worked well and allowed me to take a lot of data on high voltage discharges which will appear later this year in a published paper and on this website in support of a new method of ESD troubleshooting and reliably finding dangerous failure modes. The peak current of the discharges shown in Figure 1 are approximately 50 Amperes measured with a 500 MHz bandwidth.

Summary: Measuring the current of high voltage discharges can be tricky. Construction details of a target for such measurements were presented.

Links in this article:

  1. Fischer Custom Communications F-65 current probe

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