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Section
II
Pa ragraph 2-6
E.
EF FECT OF
PROBE
MEASURED.
The probe adds an inductance of
les s than
mic rohen ries to th e c ircuit under tes t .
0.05
With almost a l l conditions this small an inductance
will not affect the measurement.
Howev er, if the measurement is made in a very low
capaci tance and very high impedance c ircuit , with
high ac voltages, a nd at high frequencies , the shun t
capacitance of the probe to grou nd must be taken into
A
consideration.
capacitanc e is added from th e meas
ured w ire to the grounded s h ield inside the 456A probe
when the probe is clamped around the wire. This
capac itance typically has a value anywhere from 1 pf
to
5
pf, depending upon wire size, insula tion and
location w ith in the probe aperture . Th is capacitance
has two effects:
1) The meas u red wire is slightly "loaded" w ith a
capacitance to ground of around 3 pf.
2) Th e capacitive current which flows to groun d
th rough th is wire-to-probe capac itance can b e meas
ured by the probe, although it a lmost a lways adds
a negligihlc amount to th e reading of th e actual cur
rent in the wire .
To test if this effect shou ld be con sidered, solder one
end of a shorr, stiff, piec e of ins ulated wire to the
c ircuit at the point where the meas u rement is to be
made.
Leave the other end of this wire unconnected.
This wire will have VOltage on it but no current th rough
it. Clip th e probe over this wi re a nd read the mete r .
Rever se the direction that the probe is clipped on the
wire (point arrow on probe in other direction) and read
meter a ga in.
If th ere is no rea ding on the meter in
either of these positions th e probe capacitanc e has no
If th ere is a r eading proceed to the ne xt
effect.
para graph.
The s hunt capacitance in troduced by th e probe is 1 to
S
pf (typica lly 3 pf).
Assuming the worst case of
5 pf, a ma ximum error cu rrent of:
X
0.03 rna
volts
I
=
can flow through the 5 pf to ground. That is, a 1
v()l tage ()f 1 vol t impres sed ac ross this shunt capaci
tance will cause a reading of only 0.03 rna by the
456A.
However, at high er frequencies the effect will
inc rease proportiona lly.
Even w ith th is effect you may choose which side of the
circuit to measure the cu rrent .
Note in figure 2-5 the probe mea sures the current
flowing in the circuit on the same side as the wide
side of th e arrow on the probe.
O ccasionally a t h igh frequencies (above 15 me) a
greatly magnified voltage effect exists a t a pa rticular
frequency.
This effect is caused by a resonance be
tween excessive external ground l ead l engt h s and
stray capacities .
Th e following steps are s u gges ted
to el iminate tllis rype of problem:
2-4
ON
CiRCUIT
BEING
X
megacycles
me
See figure 2-5.
THE
PRO BE
AND
WIRE
HAS
AN EQUIVALENT
THIS CURRENT
MEASURED
-
/-
I
,
\
\
"
.... .. ..
----
-- <:...
.... ..
CURRENT IN
PRO BE
MEASURES
ME
THE SA
SIDE OF
OF ARROW,
F igure 2-5.
Illustrating Direc tional Effect
of Current Measurement
I) Ground oscilloscope or voltmeter to equipm ent
ground with as shorr leads as possibl e.
2) Connect special res istive ground lead (s upplied)
to th e closest ground to the conductor being meas u red.
456A cabinet
3) Space
a w ay
(steel
tables,
etc).
4 ) Grou nd 456A to oscilloscope or voltmeter with a
standa rd (non-resistive) short clip lead (not furnish ed).
2-6.
SPE CIAL MEASURING T ECHNI QUES .
Since the probe is effectiv ely a current transformer,
it ha s th e property tha t it will algebra ically sum the
ins ta n taneous value of the currents in two or more
conduc tors it may be clipped a round. This property
makes the probe a valuable and easily-applied rool
in appl ications in which it is desired to equalize or
b alance ac currents.
For exampl e , in th e class C
amplifier shown in figure 2- 6 it is possible to u s e
this summing property ro exa m ine the plate cu rrent
of
pulses exclusive
the cu rrent component flowing
through the capacity of the tube .
wil l al low you to mea s u re the true angle of conduc tion
of the class C amplifier .
The method used to obtain the bucking c urrent is indi
ca ted in figure 2-6.
The probe wa s clipped a round
the plate lead of the tube, but at th e same time a l ead
from a n exter na l va ria ble capacitor was connected to
th e pla te lead and pass ed th rough the probe as shown.
By suita bly adju sting the v a riable capac itor, a capa
citiv e current equa l ro but in opposite phase to the
capacitive cu rrent flowing at the plate can be a pplied
to th e probe.
Model 456A
BEING MEASURED
C I
RCU IT O F:
THIS CURRENT
I
!!QI
MEASURED
"-
\
5
I
TO
PF
/
-
RO
CIRCUIT ON
PRO BE
AS
SIDE
[rom groundedcondu
tors
This a rra ngement
00395-1
\
I
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