ESI 250-DA - Understanding and Resolving Sliding Balance

2,3.7
SLIDING
BALANCE
WHAT
IT
IS
CHANGING
THE
L
DIAL
CHANGING
THE
Q
DIAL
When
lossy
inductors
are
measured
the nulling
problem
known
as slid-
ing
balance
will
occur
if
the
usual
procedure
of
adjusting
first
one
con-
trol
then
the
other
is
followed.
As shown in the
above
drawings,
neither
the
Q
dial
nor
the L
dial
will
move
the
bridge
vector
toward
a null
directly
or
quickly.
When
Q
is
high
there
is
no sliding
balance
because
the
effects
of
Q
and
L
are
perpendicular to
each
other and
nulling is
easy.
WHAT
TO
DO
ABOUT
IT
WHEN
USING
ESI
250-DA
BRIDGE
This
problem
is
not
too
serious
if the
value
of
Q
is
greater
than
one. For
lower
values
of
Q,
an
external
D-Q
rheostat
may be
necessary.
For
low
Q,
the
L
series
circuit
should
be
used.
The
following
approach
should
be
used
if
balancing
is
difficult.
By
a
somewhat
different
nulling
technique
rapid
accurate
low
Q
measure-
ments can
be
accomplished.
First
set
the
Q
dial
to
its
maximum
position,
then
set L
for a
detector
minimum.
With
Q
set
high,
L
will
move
the
impedance
vector in
the L
direction
only.
The
bridge
null
detector
signal
is
almost
exactly
pro-
portional
to
the
difference
between
the
unknown
impedance
and
the im-
pedance
indicated
by
the
bridge
dial.
The
correct
L
setting
will
bring
the
impedances
closer
together
and
give
minimum
signal.
Next
set the
Q
dial
for
a
minimum
on
the
detector.
This
should
give
a
good start
on
finding
L
and
Q.
Move
L
a
small
amount
then set
Q
for a
minimum
and
observe
whether
this
minimum
is
better
or
worse
than
the
previous
one.
If
it
is
better
move
L
another
small
amount
in the
same
direction.
If it
is
worse try
the
other
direction.
Continue
moving
L
in
small
steps
in
one
^i^
2
^ection
only
while
setting
Q
for
minimum
detector
signal
until the
detector
signal
reaches
a
true
null.
Moving
L
further
will
make the
null
worse
again.
The
null
found
this
way
is
correct.
A
little
practice
will
allow
you
to
make
these
measurements
rapidly
even
when
Q
is in
the
order
of
0,1.
When
almost
nulled
it
is
often
easier to
move
the
Q
dial
"one
wire
at a
time"
while
null
seeking
with
the
center
L dial.
Be
careful with
the
L
series
circuit.
The
bridge
will
also
indicate
a null at
a setting
of
Q
=
L
=
O
because
at
this
setting
the
generator
is
shorted.
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250-DA
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