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> The EMC
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An EMC troubleshooting kit - part 2 (ESD immunity)

Kenneth
Wyatt -June 20, 2012

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This is a continuation of a description of my EMC troubleshooting kit (see
past links below). In the next few installments, I’ll describe a few of the
tools I use to test immunity of a prototype product to various stimuli. Bear in
mind, immunity testing is a lot tougher to perform on the bench, so not all
tests will be accommodated. To really ensure immunity levels, the product under
test will have to be taken to a test lab.

So, what tests can be performed easily at the bench? These would include ESD,
radiated immunity and certain pulse phenomenon. Of all the immunity tests, it
seems my experience has been that ESD and radiated immunity tests are the most
likely to fail. In this installment, we’ll discuss various methods to assess
your circuitry or product against ESD phenomenon.

While you can’t beat an actual ESD simulator for pre-compliance testing,
there are a number of simple ESD generators that will work well to at least give
you a general feel for whether your circuitry is immune. My colleague, Doug
Smith (http://www.emcesd.com/), has developed some simple tools to
check ESD immunity. The simplest is a clear ZipLok® bag with several coins
inside. Shaking this near your circuit will result in several volts at rise
times of about 100 ps.

Figure 1 - Shaking a few coins in a ZipLok® bag will generate large
amounts of ESD with edges around 100 ps.

Another idea by Doug is what I call an “ESD Spark Gap Ruler”. This is
constructed by taping down a length of copper tape to a plastic foot-long ruler.
Take a metal straight edge and an X-Acto knife (or razor blade) and cut a couple
diagonals in the middle. By removing the upper and lower triangle pieces, you’re
left with a dipole antenna with a spark gap spaced very close together. The
objective is to get as narrow a space between the points as possible; but
without touching. I covered the tape with a layer of clear packing tape to
protect the small air gap. Next, holding the ruler on one end, take a small
styrofoam plate and rub it on your arm to create an electrostatic charge - then
wave it back and forth near the spark gap. The amount of ESD field generated by
this simple spark gap will surprise you. I’ve picked up the signal from 200 feet
away, using a sensitive commercial ESD detector. By holding this near your
circuitry or product under test, you’ll be able to quickly assess the ESD
immunity. This will generate several volts as measured with a UHF dipole antenna
connected to an oscilloscope.

Figure 3 - A “spark gap” ESD generator.

Other simple DIY ESD generators include this BBQ grill igniter. You can
purchase a replacement kit for a few dollars and the included wiring makes a
good loop antenna. Holding this near your circuit board or close to the I/O and
input ports to your product can indicate quickly if there are ESD issues. I’ve
measured several volts into a simple UHF dipole antenna connected to an
oscilloscope input. Edge speeds can be in the 50 to 200 ps range.

Figure 4 - A simple piezoelectric BBQ starter is the heart of this ESD
generator and can create large fields with 50 to 200 ps edges. The included wire
makes a good loop antenna.

It also occurred to me after making what I called a “shock box” in high
school, that a 12V power transformer connected to a 1.5V or 3V battery would
also make an quick and dirty ESD simulator. Depending on the transformer, you
can obtain 1,000 to 10,000 volts output at 2 to 5 ns edges by momentarily making
and breaking the battery connection. I would definitely NOT try connecting the
output of this directly to any circuit boards - just probe on the outside of
your shielded product enclosure just as you would with a real simulator. You
should be able to see a small arc as you hold the probe wire near to the chassis
enclosure or to the grounding shell of I/O connectors. The disadvantage of this
circuit is that the output voltage varies and the level is uncontrolled. While
it can give an indication of potential issues, DO NOT depend on this method for
accurate results! For that, you’ll need to purchase a calibrated ESD simulator.
Also, be careful not to short directly into I/O connector pins, as this is not
part of the ESD test standard.

Figure 5 - The circuit diagram and test setup for a simple DIY ESD
simulator based on a 12V power transformer connected backwards to generate a
high induced voltage when the battery connection is momentarily made and
broken.

One final idea for generating ESD fields includes a plastic dish with Hershey
Kisses. When shaken, the foil-covered candy generates a surprising amount of ESD
with measured edge speeds of 50 to 100 ps with associated radiated fields. Once
you’re finished with your product evaluation, you may then celebrate by eating
the candy!

Figure 6 - One last idea for generating ESD is this plastic dish with
Hershey Kisses! Measured edge speeds are 50 to 100 ps. After your testing, you
can have a treat!

The next installment will show you how to detect ESD pulses with low-cost DIY
tools. Stay tuned!

Related postings:

Troubleshooting Kit - Part 1a (Emissions)

Troubleshooting Kit - Part 1b (Emissions)

Troubleshooting Kit - Part 2 (ESD Immunity)

Troubleshooting Kit - Part 3 (Detecting ESD)

Troubleshooting Kit - Part 4 (Radiated Immunity)

Troubleshooting Kit - Part 5 (Summary & Parts List)

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