http://www.ieee-pses.org/symposium
http://www.emc2004.org/
So the moral of the story is "A dump question from a beginner can some time
pull the experts to long discussions". Thank you all for contributing your
valuable suggestions that gave an in depth idea to the technicalities
involved.
Sincerely
K.Balasubramanian
Sr.Engineer, Hardware.
"Chris Maxwell"
<chris.maxwell@NetTes To: "EMC-PSTC"
<[email protected]>
t.com> cc:
Sent by: Subject: Resistors in ESD
Test Setups
owner-emc-pstc@listse
rv.ieee.org
07/08/2004 06:33 PM
http://www.ieee-pses.org/symposium http://www.emc2004.org/
All,
The horse is dead, beaten, turned into glue (probably a few McDonald's
hamburgers) and buried J
David Sterner and I have bounced a few emails back and forth (offline) on
the subject. We discussed OHSA, EN 61000-4-2 and some individual test
setups. Just to clarify for the benefit of the group.
Most ESD setups require the 940Kohm resistance from the coupling planes
(HCP or VCP) to ground.
-The resistors complete the circuit from ESD gun to GRP through the
coupling plane, allowing for potential equalization (charge bleeding)
between discharges. This makes repeatable test results.
-The high value of the resistors protects the test personnel in the
following instance: if the test tech touches a hazardous voltage in the
EUT with one hand and touch the coupling plane with the other hand; then
the high resistance to ground will limit the current through the tech's
chest cavity (the worst possible area for hazardous current to flow).
On the other hand, if the coupling plane was grounded, or bonded to ground
with a low resistance, there would be a dangerous current through the
tech's chest cavity. This is OSHA's peeve with grounded conductive
tabletops. David was more enlightened regarding the OSHA requirements, so
this is a tidbit that I didn't have.
-Having said all of that, if the tech touches a hazardous voltage or if the
coupling plane becomes charged, the resistors will not protect the tech
>from the hazardous voltage going to ground through his feet or through his
bottom to a metal chair? This is why you still have to use caution, even
with the resistors installed.
Although the initial email just asked about the power rating (which we
mostly agree to be ¼ Watt, up to 15KV).
Another point that I haven't seen is with regard to the length of the
resistors. A good rule of thumb is to require 1mm of air space to insulate
1KV of ESD. So, just make sure that your resistors are long enough from
terminal to terminal to insulate the expected ESD voltage of your test
setup.
For example, if you plan on going up to 15KV, and you use two 470Kohm
resistors; then each resistor is subjected to 7.5KV?.which would imply that
the resistor body should be 7.5mm (about ¼ inch) long in order to prevent
the ESD from jumping over. Shorter resistors could be potted (RTV,
varnish?whatever you have handy) to help out. This may also be part of the
reason that Dave C. from Micron mentioned the 2W resistors. They would be
plenty long enough to handle higher ESD voltages without flashover.
Chris Maxwell
Design Engineer
Nettest
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