ESI 250-DA Instruction Manual page 22

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
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