Model 456A
Section
11
Pa ragraph 2-5 (Cont'd)
l
O
-L-
"]13
F
igure
2-3.
Cleaning Probe Jaws
alkyd ma terial which is very durable in normal use,
but is not made to w
i
thstand the shock of dropping.
3) Keep the probe jaws clean. If the
j
aws appear
dirty or if the low frequency response drops off, clean
the jaws as shown in f
i
gure
2-3
using carbon tetra
chloride and the brush provided or, in extreme cases,
a pencil eraser.
The probe jaws should mate perfectly with no air gap
between the ferrite pole pieces . Foreign matter (dirt
specks
,
wax from capacitors, sand particl es , pieces
of insulating tape etc.) may hold the pole pieces from
closing perfectly. Normally the only effect will be
to lower the gain at low frequencies (below 10 kc).
Even as small an air gap as 0.0005 inch will lower
the gain significantly at low frequencies
.
4) Always clip the probe around ins ulated wire . If
you must measure the current in a bare wire , fi rst
ins ulate the wire with tape or insulating tubing.
5)
Do
not expose the probe to high temperatures.
Expos ure to temperatures above 550C is not recom
mended .
B.
DIRECT CURREN
T IN
THE SIG NAL BEING
MEASURED. Direct current in the signal being
meas ured should preferably be kept below 1/2 ampere.
Direct current below that value will have no effect on
the measurement.
C . AL TERNA
TING
CURRENT FIELDS. The probe
is sh
i
elded against external ac magnetic and
electric fields. Extremely strong fields may cause
an erroneous reading
.
If a str
o
ng ac magnetic field
00395-1
is
suspected, rotate the empty probe head with the
jaws closed. If the reading changes, an ac field is
affecting the probe.
D. G
R
OUND LOOPS.
Wi th the ac supply avoid
ground loops in tes t s etups , such as those produced
between the thi rd prong on the ac cable and the ground
on
the indicating ins trument. An easy method of test
ing for ground loops is to clip the probe over the 456A
output cable . Any reading indicates the presence of
ground loops .
OTHER EQUIPMENT
r---- ..,
POWER�
I
LINE
I
I
,
I
I .':
I
I
'\
I
L�.
___
..J
'.
jt:f
__ _
GROUNO
8
US
"-
,-- SET
'UP
BEING TESTED -----.,
Figure 2-4. llius trating Developmen t of
Ground Loops
Referring to figure 2-4, note that the leakage cu rrent
lleakage causes a
VO
ltage drop ac ross the ground lead
resis tance
Rgnd'
This appears as a voltage, in ad
dition to the signal voltage, between the ac cu rrent
probe
Zl
and the indicating device Z2. An easy way
of minimizing this problem is to connect both ins tru
ments to the same ac receptac
l
e
.
Another method is
to use a banery-powered Model 456A .
2-3
