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Douglas C. Smith

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Technical Tidbit - May 2004
A Simple Spark Gap EMI Source

design contributed by
Edward Witte
Senior Electronics Engineer
Electronic Design Ware P/L
Tel: 61-3-9569 9208
Fax: 61-3-9504 3519

Spark gap EMI source

Figure 1.
Piezoelectric Oven Lighter Powered EMI Source
(contributed by Edward Witte for this article)

An easy to build and use EMI generator designed by Edward Witte is described. "Home brew" test equipment such as this can be very useful in troubleshooting equipment problems.

Discussion: Simple to build and use test equipment can be some of the most useful in troubleshooting equipment in the lab. Figure 1 shows an example of such an instrument. The design is related to the Static Field Powered EMI Source described in the June 2001 Technical Tidbit on this website. In this case, instead of static charge generating the spark, the voltage is provided by a piezoelectric oven lighter. Just squeezing the handle generates multiple sparks and the resultant EMI.

In Ed's original design, a 4 mm spark gap was used. If the spark gap is made smaller, the risetime of the radiated fields will become significantly faster. The fraction of a mm gap in the Static Field Powered EMI Source yields risetimes on the order of 300 ps. The radiated fields will have a damped oscillatory form with the frequency related to the length of the copper strips for both designs.

The Static Field Powered EMI Source from the June 2001 Technical Tidbit can produce hundreds of sparks quickly as a static charge (on a plastic cup, for instance) is passed back and forth over the spark gap while being held close to the copper foil. The automatic, Pulsed Field ESD Simulator (~440 kB pdf file) from Fischer Custom communications produces tens of impulses per second with ~200 ps risetimes. Ed finds that his EMI generator, shown in Figure 1, produces about 5 sparks per squeeze of the handle. After he reduced the spark gap from 4 mm to ~0.5 mm, each squeeze of the handle produced hundreds of sparks. Each of these instruments has its advantages.

Summary: This simple home built accessory can be a useful addition to a test lab, providing a low cost troubleshooting tool.  Many examples of simple test accessories, such as this one, can be found on this website.

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