We have experienced noise from variable frequency motor controllers used in
air conditioning systems. There may also be some of these controllers in
your factory controlling conveyer belts and other machinery.
Richard Woods
----------
From: Chris Maxwell [SMTP:chris.maxw...@nettest.com]
Sent: Thursday, September 06, 2001 11:38 AM
To: EMC-PSTC Internet Forum
Subject: Noise from flourescent light ballasts?
Hi all,
We have personally experienced an interesting phenomenon. We build
a
certain circuit that detects a 20Khz tone. This circuit is housed
in a
product that has an EMI spray coated case. One of our engineers
noticed
that our techs on the manufacturing floor were having a difficult
time
setting up units on the floor. The tone detect circuit kept getting
set
off for apparantly no reason.
However, back in the engineering lab, we have no problems.
So we did a little experiment. We took a unit out to the
manufacturing
floor and opened its case (which is how they work on them in
manufacturing).
We held it up near the lights. The tone detect circuit went crazy.
We put the unit down on the bench. No problem.
But, if you leave it opened up on the bench and hold your hand over
the
board, the circuit goes off again.
When we close up the case work, all of these problems go away.
So, our obvious solution is to make a modified casework with
"tweaker
holes" so that manufacturing personnel can tweak the amplifier pots
with
the casework closed. But.... I'm still curious. What's causing
the
interference?
I was wondering if flourescent light ballasts could be giving off an
emission in the KHz range. (Maybe that's why holding it to the
lights
sets it off.) But what about the hand waving? If I assume the
ballasts
are giving off emissions, can I also assume that the human body can
change the local field pattern?
Am I barking up the wrong tree? Maybe its the LAN cables in the
ceiling?
Anybody else have a similar experience or some insight into this?
Thanks,
Chris Maxwell | Design Engineer - Optical Division
email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315
797
8024
NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
web www.nettest.com | tel +1 315 797 4449 |
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