Model 456A
Section Il
Pa ragraphs 2- 2 to 2-5
SE CTION II
O P E R ATING I NST RUCTIONS
2-1 .
GEN ERAL.
The Mcxlel 456A must be used with an ac voltmeter
or an oscilloscope . Figure 2-2 shows a typical setup .
2-2.
OPE R AT I N G I N ST R U C TIO N S .
Connect the shielded plug of the Model 456 A OUTPUT
cable to a voltmeter or oscilloscope of appropriate
ra nge. The conversion ratio is millivolts out equals
milliamperes in. Thus the voltmeter should read in
millivolts the expec ted milliamperes being measured.
The current range of the Model 456A is less than
1 milliampere to
I
ampere. The
It;!!
Model 400D/H/L
Vacuum Tube Voltmeter is ideal for this measurement
as it covers the entire range and has a 3/4 inch spac
ing input jack.
WARNING
00
NOT PLUG THE OUTPUT
CABLE
OF
THE 456A INTO
A
SOURCE OF AC OR DC
VOL TAGE. Doing. so will burn out the tran
sistors and perhaps other components.
The spacing of the pins on the OUTPUT cable fits all
standard 3/4 inch connectors , such as on Hewlett
Packard voltmete rs , wave-analyzers , and oscillo
scopes (when used with
AC-76A
adapter) . See table
1-2 for a list of Hewlett-Packard instruments s uitable
for use with the Model 456A.
The
�
Mcxlel 400D has an additional output terminal
for viewing the Voltage being measured with an oscil
loscope .
This ma y be very useful , for example,
measuring the cu rrent a t which a transistor s tans
clipping.
The Model 456A may be used to measure very low
current, even below the noise level of the probe, if
some provision is made to filter out the noise. The
I/jJ
Mcxlel 302A Wave Analyzer is ideal for this purpose,
particularly if the BFO of the
302A
is used as the
s ignal source (see figure 2-1).
Tunsisler under lest
-
.
---
-.",
---_
/
-J.p-
��A
CURRENT
PROBE
I
/
-hp-
302A
WAVE ANALYZER
BFO OUTPUT
20'" -��C
Figure 2- 1 . Probe can be used with
It;!!
30
2
A
to measure distortion in current waveforms
or to measure small audio current, as
described in text
00395-1
In
this manner, currents even below 1 0 microamperes
ca n be measured.
2-3.
OPERAT I N G T HE PRO BE.
The probe jaws may be operated with one hand. While
holding the probe handle in the palm, squee ze the
flanges together with the finge rs . This action will
open the jaws. Move the probe to the v icinity of the
insulated wire being measu red and slowly releas e the
flanges until the jaws fit over the wire .
CAUTION
Do NOT let the jaws snap closed as they may
be
damaged .
Clip the probe over the wire with the arrow on the
probe in the direction of conventional current flow
for a positive-going output signal. This procedure is
important when using an oscilloscope but it ma y also
be important when using this probe to measure uns ym
metrical waveforms with half-wave rectifier type
meters . All Hewlett
·
- Packa rd meters listed in table
1 - 2 a re full-wave rectifier types so you will get the
same reading whichever way the probe is clipped over
the wire.
However, with half-wave type meters
the readi ngs may be different. This action is a limi
tation of the meter and not a fault of the Mcxlel 456A.
Refer to the Operating and Servicing Manual for the
meter or change to one of the recommended meters .
2-4 .
I N CREAS IN G T HE S ENSITIVITY.
Sensitiv ity of the Model 456A may be increased by
looping the wire ca rrying the current to be measured
th rough the jaws more than once. The reading on the
meter or oscilloscope will be multiplied by the number
of times that the cu rrent passes th rough the jaws . To
obtain the true reading divide the c urrent indication
by the number of turns enclos ed by the jaw s . The
series loading effect of the probe on the circuit being
measured is then multiplied by the square of the
number of turns. This effect may have to be taken
into account (see pa ragraph 2-50).
2-5.
MEASUREME N T P R E CAU TIO NS.
Bes t performance will be achi eved by observing the
following precautions:
A. P ROBE HANDLING
1) Do not snap the jaws closed by letting go of the
probe flanges abruptly. This action may damage the
shield or the jaws.
2) Do not drop the probe . The jaws a re made from an
2- 1
