ESI 250-DA - Capacitance Measurement with DC Applied

2.4.4
CAPACITANCE
MEASUREMENT
WITH
DC
2,
4.
4.1
WHEN
TO
USE
DC
Some
capacitors
need
a
polarizing
voltage.
For
example
,
electrolytic
and
tantalytic
capacitors
can
be
damaged
by
the
reverse
polarity
half-cycle
of
the
test
frequency.
Even
if
the
capacitors
are
not
damaged,
erroneous
read-
ings
may
result.
A
polarizing
voltage
is
not
always
needed
in
these
measurements.
If
the ac
voltage
is
kept at
a
very
low
value
(typically
below
one-half
volt),
the
capacitor
will
not
be
damaged,
and
the
readings
will
be
reasonably
accurate.
Because
of
the
high
sensitivity
of
the
Model
250-DA
bridge,
accurate
measure-
ments
can
be
made
with
a
very
low
oscillator
voltage.
When
operating
with
a
low
oscillator
voltage,
be
sure
that
the
detector
sensitivity
is
high.
2.4.4.
2
THINGS
TO
WATCH
OUT
FOR
The dc
should
be
supplied
through
the
generator
terminals.
When
this
is
done,
both
bridge
current
paths
include a
series
capacitor.
All of the dc
voltage
will
appear
across
both
the
unknown
capacitor
and the
standard
capacitor
.
Stray
ac
pickup
may
change
sensitive
components
being
measured or may
give
erroneous
null
detector
readings.
The
ac
impedance
of
the
external
dc
circuit
must be
less
than
200
ohms for
maximum
sensitivity.
If
D
is
too
high
to be
read
directly,
an
external
rheostat
such as the ESI
Model
DB-655
DEKASTAT®,
connected
to
the EXT
D-Q
terminals can
be
used
to
extend
the D
range.
D
can
be
calculated
by
the equation:
D
=
D
reading
where:
Dissipation
factor
of
unknown
capacitor
D
reading
of
bridge
dial
External
rheostat
resistance in
kilohms
Operating
frequency
in
kilocycles
If
the
dc
leakage
resistance
of
the
capacitor
being
measured is
much
greater
than
the
resistance
of
the
range
resistor
most of the dc input voltage
will
appear
across
the
unknown
capacitor.
If
the
dc
resistance
of the uxiknown
capacitor
is
known,
the
voltage
across
it
will
be shown by
the
following
equation:
reading
D
D
^kD
^c
lelslil
250-DA
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