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Fluke 27 [17/30] Specific applications nulling

Analog Bar Graph Application

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.

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

Fluke 27 [17/30] Specific applications nulling

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