Specific Applications--Nulling; Specific Applications--Contact Bounce; Specific Applications--Checking Capacitors - Fluke 27 User Manual

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Users Manual
graph excels at displaying trends, or slowly changing
signals. In addition, autoranging on the Fluke 27 allows
monitoring the signal change through changing ranges.
Many diagnostic routines using the bar graph require
practice. The operator is looking for good or bad signal
patterns that occur over some span of time. Capacitance
checks and noisy resistance measurements create such
patterns. Therefore, familiarity with analog bar graph
response and movement is necessary to accurately
interpret a signal pattern. Compare the bar graph
response when making measurements on a known-good
unit to the bar graph response when making
measurements on a faulty unit.

Specific Applications--Nulling

The Fluke 27 bar graph is ideal for nulling adjustments.
As an adjustment approaches zero, fewer bar graph
segments are displayed, then no bar graph segments are
displayed. The - annunciator flickers when the input level
is within 10 counts to zero. The flickering null indication is
displayed every time the input approaches zero or swings
from one polarity to the other. The operator merely
watches for the - annunicator indication, then reverses
the direction of the adjustment when the polarity sign is
displayed. In one or two passes, a near-zero input level is
possible, then the digital display can be used for exact
zero adjustment.
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Specific Applications--Contact Bounce

When subject to vibration, relay contacts may begin to
bounce open. Checking for this intermittent problem is a
routine troubleshooting measure associated with many
types of equipment, including computers. Since the
bounce problem will worsen as the relay fatigues, early
diagnosis is important.
When the contact bounces open, its resistance value
changes momentarily from zero to infinity and back.
Ordinary hand-held DMMs take more than 300
milliseconds to update their displays--much too long to
detect a brief contact bounce. A traditional VOM needle
will move slightly at the instant of contact bounce, but the
inertia of the needle movement dampens the response.
The analog bar graph, however, will display at least one
segment the moment the contact opens. The bar graph
can detect contact bounce as brief as 0.2 milliseconds,
while most analog needle movements require a 3
millisecond opening before they will respond.

Specific Applications--Checking Capacitors

Volt-ohm meters are often used as simple capacitor
checkers. In the capacitor kick test, the needle of the
VOM in the resistance mode moves quickly from open
(infinite ohms) toward short (zero ohms) as the capacitor
is placed across the VOM input. The VOM battery