Sencore LC102 AUTO-Z Tech Tips

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How To Find Defective Coils With The LC102 AUTO-Z™
The increasing use of integrated circuits is
making coil applications more frequent.As
the circuits inside the IC become more
complex , they need more inductors to
make the circuits function correctly. Most
inductor values do not fit inside the IC .
Consequently , as I Cs become more
common , the number of inductors used will
also increase .
At the same ti me, more circuits are using
high frequency switched-mode power
supplies, sometimes called " switcher " or
"chopper" supplies. Inductors combined
with capacitors do a better job of filtering
high frequency ripple than capacitors
alone. This Tech Tip explains how the
LC102 can be used for testing these and
other coils. It will give you an
understanding of how and why both the
ringing test and the inductor value test
should be done.
What Goes Wrong With Coils
Inductors can develop three kinds of
failures: opens , shorts , and shorted turns .
An open , of course, breaks the circuit path
and stops the current. A dead short
replaces the inductance with a piece of
wire. A shorted turn reduces the quality
(0) of the coil at high frequencies .
A shorted or open coil will cause some
fairly obvious changes in ci rcuit operation ,
but let's see how a shorted turn affects
circuit operation.
How Shorted Turns Affect The
Circuit
To see why a shorted turn affects the
circuit , we need to look at what the coil did
when it was good. During times of
increasing current , the coil stores energy
in the form of an increasing magnetic field .
Then, during times of falling current , the
coil converts the collapsing magnetic fie ld
back into current. This smooths AC ripple ,
si nce the coil 's charge / discharge action
bridges the power supply ripple.
Shorted
Turn
EJ
Equivalent
Circuit
Fig. 1: A shorted turn absorbs power in the
coil, and acts like a secondary winding of a
transformer.
When the coil develops a sl1orted turn , it no
longer acts like a pure inductance. The
shorted winding acts like a small
secondary wind ing on a transformer . The
short absorbs part of the stored energy and
converts it to heat within the coil. This
reduces the amount of current available for
the load , which reduces its filtering
capabilities .
The shorted turn has more effect with high
frequency signals than with low frequency
signals. The net result is loss of high
frequency 0. Figure 2 shows how the
effectiveness of an RLC filter is affected by
O. When the coil is good , it is a high
impedance to the ripple frequency . At
lowered 0, it has less effect on the high
frequencies .
In a bandpass filter , such as in a rece iver's
fro nt end , the depth of the filter is reduced
by a coil with a shorted turn. In a
waveshaper, a shorted turn has a major
effect on the signal shape. In an oscillator,
the shorted turn prevents the circuit from
reaching oscillation.
Shorted Turns Are Tough To Find
Without A Z Meter
Before the Z Meter , technicians had only
two ways to find bad coils. They could
measure the inductance or the resistance
and compare either reading to normal
values . These methods often found shorted
or open coils . Shorted turns, however ,
often went undetected , so substitution was
still common.
%
Filter Effectiveness
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90 ~----"""---=""'-. - ---
80
70
60 -1----t-~
50
40
Coil With
Shorted Turn
30
20 - -
10 -
_J_
10Hz 100 Hz
Frequency
1 KHz 10 KHz
Fig. 2: A shorted turn in a power coil
causes a much lower effect on the ripple
signal.
Consider a coil with 500 turns of wire. If
the insulation on the tiny wire lets one turn
short to its neighbor , the coil's inductance
and resistance both drop by 1 / 500th of the
original values. If the coil had a 10 ohm
resistance , for example , the shorted turn
would only drop the resistance by 0.02
ohms. An ohmmeter does little to help find
this small change in value. A value tester is
also of little help , since the inductance
value has not dropped by a noticeable
amount either. But , this shorted turn has a
major effect on the circuit. As we will see
next , the LC102's Ringer test easily finds
this trouble .
#147
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