ESI 250-DA - DC Inductance Measurement Power and Supply Considerations

2.
3.
4.
2
THINGS
TO
WATCH
OUT FOR
(cont.
ii)
Most
line
operated dc
power
supplies
have
high
leakage
capacitance to
the
ac
line
and
enough
dc
leakage
to be
objectionable.
An
insulated, isolated
battery
is the
recommended
power
supply.
If
a
variable
supply is needed the circuit
shown
below
can
be
used.
This
circuit
has
a
maximum
output
of one watt so it
will not
exceed
the
rating
of any
bridge
element.
With
the
parallel
inductance
circuit
the
standard
capacitor
blocks the dc
from
all of
the
bridge
arms
except
the
inductor
being measured and
the
range
resistor.
As a
result
the
maximum
allowable
current
will be
limited
by
the
range
resistor.
This
current
limit
is
shown
in Section
2.
3.4.
3.
C.
WITH
DC
SUPPLIED
THROUGH
THE
DETECTOR
TERMINALS
Supplying
the dc
signal
through
the
detector
terminals
allows the
use of
higher
current
in
measuring
inductors
above
1,2
millihenrys
,
but
the
dc current
will
be
influenced
by the
main
dial
DEKASTAT
setting.
The
generator
circuit
must
be
isolated by a
capacitor. This
will require
an
external
generator
or the
connection
recommended in
2.3,4,
2-B,
A
grounded
power
supply can
be
used,
since
one
of the
detector
terminals is
grounded.
The
problems
resulting
from
stray ac
pickup
either
from the
ripple of
a
dc
supply,
capacitive
coupling
to
power
lines or the
bridge
generator, stray
field
pickup
by
the
isolating
inductor
(if
used),
or
inducti%re coupling to
the unknown
inductor,
are
much
more
serious
with
this connection
because
of the direct
coupling
to
the
detector.
In
this
circuit
the
current
goes
through the
unknown
inductor
and the main dial
DEKASTAT.
It
must
be
limited
by the
values given
in Section
2.
3.4.4. If
more
current
is
needed the
range
can be
increased by
one setting and
the first
DEKASTAT
dial
turned
to
zero.
This
will
decrease
the DEKASTAT
resistance
and
increase
its
current
rating.
Because
of the high
resolution of
the main
dial
DEKASTAT
accurate
measurements
are
possible
even
when
the
first dial is
not
used. By
setting
the first
two
dials
to
zero
higher
current is possible but
only
100
divisions
of
resolution
are
left.
LMsI
I j
250-DA
7/59