Application Note

Application Note
The elusive electrical intermittent
is one of the most difficult prob-
lems for anyone to troubleshoot.
The difficulty lies primarily in the
timing. Being able to look at the
symptoms while the problem is
present simplifies the trou-
bleshooting process considerably.
However, the old adage “A
watched pot never boils” seems
to be applicable to electrical
intermittents as well. Being pres-
ent, as well as being able to take
an electrical measurement while
the problem exists, are the two
biggest challenges in tracking
down an intermittent.
There are a number of test
tools that can help make the
intermittent troubleshooting
process a little easier. These tools
range from complex signal ana-
lyzers and storage oscilloscopes
to handheld digital multimeters
(DMM). Of course, you may not
have these tools available or the
location of the problem makes it
difficult to bring a large analyzer
to the problem site. A DMM may
be able to tell you a lot about an
intermittent without having to go
back to the shop and haul that
storage scope to the job site. This
article will describe how to use a
DMM as an intermittent trou-
bleshooting tool.
DMM features for hunting
intermittents
Couple the basic measurement
features of a DMM (ac volts, dc
volts and resistance) with some
form of measurement recording
ability, and you have a tool for
detecting the symptoms of inter-
mittents. Not too long ago, you
could buy a voltage or current
measuring tool that was built
around a mechanical strip chart
recorder. Just place the input on
a voltage, or clamp a current
transformer around a conductor
and the recorder would make
progressive marks on a strip of
paper fed under the marking
pen. The maximum length of the
recording was determined by the
amount of paper that could be
placed on a roll of paper. This
same strip chart recorder tech-
nique has been incorporated into
some of today’s DMMs.
Fluke’s 170 and 180 Series
DMMs have a feature called MIN
MAX AVG Recording Mode. Just
like the strip chart recorder, the
DMM takes a reading of the input
at regular intervals. But instead
Detecting intermittents
with a DMM
From the Fluke Digital Library @ www.fluke.com/library
of saving each reading, it com-
pares the reading to two previ-
ously stored values to determine
if it is higher than previous high-
est reading or lower than the
previous lowest reading. If it is,
the new reading replaces the old
value stored in high or low read-
ing register. After allowing the
recording process to continue for
a period of time, you can recall
these registers to the display and
see the highest and lowest read-
ing taken during the recording
time. As an added bonus, these
DMMs will also compute and
store the average value of all
readings taken during a given
time period.