
Model 428B
SECTION III
OPERATING INSTRUCTIONS
3-1. INTRODUCTION.
3-2. This section contains instructions and information
necessary for operation of the Model 428B clip-on
milliammeter.
3-3 OPERATING PRECAUTIONS.
CAUTION
a.
BEFORE APPLYING OPERATING POWER TO
THE 428B, VERIFY THAT THE LINE VOLTAGE
SWITCH ON THE REAR PANEL INDICATES THE
LINE VOLTAGE TO BE USED AND THAT THE
INSTRUMENT IS PROPER L Y FUSED.
b.
THE PROBE IS INSULATED TO WITHSTAND 300
VOLTS MAXIMUM. DO NOT USE THIS PROBE ON
A BARE WIRE WHICH IS MORE THAN 300 VOLTS
PEAK ABOVE GROUND.
c.
DO NOT USE THE 428B PROBE IN THE
PRESENCE OF STRONG RF FIELDS.
d.
DO NOT EXPOSE THE 428B PROBE TO
TEMPERATURES EXCEEDING 130° F (55°C). DO
NOT LAY THE PROBE ON TOP OF THE 428B
CABINET (OR ANY OTHER HOT SURFACE).
PROBE UNBALANCE AND EVENTUAL DAMAGE
WILL RESULT.
e.
DO NOT DROP THE PROBE OR RELEASE THE
FLANGES ABRUPTLY SO THAT THE JAWS SNAP
TOGETHER.
f.
DO NOT OPERATE THE DEGAUSSER FOR MORE
THAN THREE MINUTES CONTINUOUSLY.
g.
BECAUSE THE 428B IS COOLED BY
CONVECTION" PLACE THE 428B WHERE AIR
CAN CIRCULATE FREELY THROUGH THE
INSTRUMENT.
h.
DO NOT USE THE 428B TO MEASURE DC IN A
WIRE WHICH CARRIES MORE AC THAN
FULL-SCALE READING ON THE METER.
3-4. OPERATING CONSIDERATIONS.
3-5. INTERCHANGING PROBE HEADS.
3-6. Each probe is calibrated at the factory with a
particular instrument and carries the serial number of that
instrument
(
serial
number
appears
on
probe
connector
) (NOTE: if your buying one with the probe,
make sure you verify this. The numbers are scribed
with a vibrating pen. Not very HP)
. If a probe has to be
replaced, a realignment and recalibration of the
instrument
is
necessary
(see
also
Section
V
Maintenance).
3-7. EFFECT OF MEASUREMENT ON CIRCUIT.
3-8. Reflected Impedance.
3-9. The probe will add a small inductance to the
circuit of less than 0.5 microhenries due to the
magnetic core and magnetic shield. This makes it
ideal for measuring current in very low impedance
paths such as ground loops where other instruments
would disturb the circuit.
3-10. Induced Voltage.
3-11. The gating signal, driving the core in and out of
saturation, will induce a voltage in the wire carrying
the dc current. This induced voltage is less than 15
millivolts peak. If more than one loop is passed
through the probe the induced voltage will be
multiplied by the number of loops.
3-12. EFFECT OF CIRCUIT ON MEASUREMENT.
3-13. Circuit Impedance.
3-14. The impedance of the circuit being measured
has practically no effect on the dc current
measurement. A shorted loop inserted along with a
wire carrying dc current will decrease the reading by
only 0.2% of full scale.
3-15. AC Fields & Superimposed AC Current.
3 -16. The instrument is designed to allow a high
amount of ac ripple in the dc being measured. The
presence of ac whose peak value equals full-scale
reading (limited to 4 amperes peak on 10-ampere
range) will cause less than 2% error in the dc reading.
Examples of such high ac currents are found in the
input of dc filter sections of power supplies.
3-17. Ac currents having frequency components of 40
kHz or harmonics thereof will cause error, as such
signals will interfere with the 40 kHz output signal of
the probe. The meter will indicate a beat reading if the
interfering frequency is within approximately 15 cycles
of 40 kHz or its harmonics. Although this situation is
very improbable, accurate dc current readings can be
obtained by shifting the frequency of the external ac
signal slightly.
3-18. The instrument as well as the pro be
head .should not be used in strong ac stray fields.
Such fields may exist in the vicinity of open core
power transformers, or large dc filter chokes, etc.
5
Содержание 428B
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Страница 39: ...Model 428B Figure 7 5 Power Supply 32 ...
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Страница 45: ...Model 428B Figure A 2 Backdating Schematics for 428B 38 ...
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