HP 428B Instruction Manual

Errata

Title & Document Type: 428B Clip-On DC Milliammeter
Manual Part Number: 00428-90003
Serial Prefixes: 995
Revision Date: Circa November 1970

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Hewlett-Packard's former test and measurement, semiconductor products and chemical analysis
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Changes to this Manual

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above).

If you wish to print on 11x17 or larger just print the page and scale it to fit the paper size.

About this Manual

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®

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OCR errors may exist and as such the user of this document should take care and use common
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Page intentionally left blank.

OPERATING AND SERVICE MANUAL

-hp- Part No. 00428-90003

MODEL 428B

CLIP-ON DC MILLIAMMETER

Serials Prefixed: 995- and above

Appendix A, Manual Backdating Changes

adapts this manual to instruments with

earlier serial prefixes.

Copyright Hewlett-Packard

Company 1970 P.O. Box 301,

Loveland, Colorado, 80537 U.S.A.

Printed: NOV 1970

Model 428B

TABLE OF CONTENTS

1-1. INTRODUCTION 2
1-7. INSTRUMENT AND MANUAL IDENTIFICATION.

2

INSTALLATION 3

2-1. UNPACKING AND MECHANICAL INSPECTION.

3

2-4. OPERATION CHECK. 3
2-6. INSTALLATION. 3

2-8. POWER REQUIREMENTS. 3
2-10. OPERATION ON 115 OR 230 VOLTS. 3

2-13. THREE CONDUCTOR POWER CABLE. 3
2-15. RACK MOUNT MODEL. 3

2-17. PREPARATION FOR STORAGE AND SHIPMENT.

3

2·20. STORAGE. 3

OPERATING INSTRUCTIONS 5

3-1. INTRODUCTION. 5
3-3 OPERATING PRECAUTIONS. 5

3-4. OPERATING CONSIDERATIONS. 5
3-5. INTERCHANGING PROBE HEADS. 5

3-7. EFFECT OF MEASUREMENT ON CIRCUIT. 5
3-12. EFFECT OF CIRCUIT ON MEASUREMENT. 5

3-19. Magnetic Fields. 6
3-26. OPERATING PRACTICES. 6

3-27. MECHANICAL OPERATION OF PROBE. 6
3-29. DEGAUSSING OF PROBE HEAD. 6

3-33. ELECTRICAL ZERO SET. 6
3-37. POLARITY OF CURRENT. 6

3-39. INCREASING THE ABSOLUTE SENSITIVITY 7
3-41. CURRENT CHECK LOOPS.

3-44. NULLING CURRENTS.
3-46. USE OF OUTPUT JACK.

3-48. With Oscilloscope.
3-50. With Recorder.

THEORY OF OPERATION

4-1. INTRODUCTION.
4-3. THEORY OF OPERATION.

4-9. CURRENT PROBE.
4-19. 20 kHz OSCILLATOR.

4-23. HEAD-DRIVE AMPLIFIER.
4-25. DETECTOR GATE AMPLIFIER.

4-28. 40 kHz INPUT/AMPLIFIER CIRCUIT. 11
4-30. SYNCHRONOUS DETECTOR AND FILTER (C24).

4-36. DC AMPLIFIER. 12
4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT. 12

4-45. 40 kHz PHASE SHIFTER. 12
4-48. POWER SUPPLY. 12

7
7

7
7

7

9

9
9

9
11

11
11

11

MAINTENANCE 14 GENERAL INFORMATION 2

5-1. INTRODUCTION. 14
5-3. TEST EQUIPMENT REQUIRED. 14

5-5. IN-CABINET PERFORMANCE CHECKS. 14
5-7. CLEANING OF PROBE JAWS. 14

5-10. ELECTRICAL ZERO SET. 14
5-13. RANGE CHECK. 15

5-15. METER TRACKING. 15
5-17. OUTPUT CALIBRATION. 15

5-19. FREQUENCY RESPONSE. 15
5-21. AC OVERLOAD. 16

5-23. NOISE CHECK. 16
5-25. ADJUSTMENT PROCEDURE. 16

5-27. POWER SUPPLY. 16
5-29. MECHANICAL ZERO SET. 16

5-31. DC AMPLIFIER BALANCE. 16
5-34. ALIGNMENT. 17

5-35. OSCILLATOR BALANCE. 17
5-37. OSCILLATOR FREQUENCY. 17

5-39. OSCILLATOR LEVEL. 17
5-41. DETECTOR GATE. 17

5-43. TUNED AMPLIFIER. 17
5-44. Equipment Setup. 17

5-46. Input Alignment. 17
5-48. Interstage Alignment. 17

5-50. DETECTOR PHASE ADJUSTMENT. 17
5-53. Preliminary Adjustment. 18

5-54. Preset the controls as follows: 18
5-55. Drive Balance Adjustment. 18

5-59. TROUBLESHOOTING. 19
5-60. FRONT PANEL TROUBLESHOOTING. 19

5-63. DETAILED TROUBLESHOOTING. 19
5-64. Probe Check. 19

5-66. Power Supply Check. 19
5-71. Oscillator - Buffer Amp. Check. 21

5-80. Synchronous Detector. 21

REPLACEABLE PARTS 23

6-1. INTRODUCTION. 23
6-4. ORDERING INFORMATION. 23

6-6. NON-LISTED PARTS. 23

CIRCUIT DIAGRAMS 29

7-1. INTRODUCTION. 29
7-3. BLOCK DIAGRAM. 29

7-5. SCHEMATIC DIAGRAMS. 29
7-7. COMPONENT LOCATION DIAGRAMS. 29

MANUAL BACKDATING CHANGES 36

Model 428B

LIST OF TABLES

Table 1-1 Specifications 1
Table 5-1. Recommended Test Equipment. 13

LIST OF ILLUSTRATIONS

Figure 1-1. Model 428B Clip- On Milliammeter 2
Figure 3-1. Measurement Procedures 4
Figure 3-2. Polarity of Current. 6
Figure 3-3. Increasing The Absol ute Sensitivity. 7
Figure 4-1. Block Diagram 8
Figure 4-2. Simplified Block. 9
Figure 4-3. Magnetic Mechanical Analogy. 10

Figure 4-5. 428B Flux Gate. 10
Figure 4-4. Basic Flux Gate. 10
Figure 4-6. Waveforms. 10
Figure 4-7. Detector Bridge. 11
Figure 4-8. Negative Feedback 12
Figure 4-9. 90° Phase Shift 12
Figure 5-1. Cleaning Probe Jaws. 14
Figure 5-2. Electrical Zero Set. 14
Figure 5-3. Range Check. 15
Figure 5-4. AC Overload. 16
Figure 5-5. Oscillator Balance Probe. 17
Figure 5-6. Detector Phase Adjustment. 17

Table 6-1. Replaceable Parts 24

Figure 5-7. Detector Phase Display. 18
Figure 5-9. Detailed Troubleshooting Tree. 20
Figure 6-1. Parts Breakdown, current probe. 28
Figure 7-1. Block Diagram. 30
Figure 7-2. Component Locator For Circuit Board Part

No. 00428-66501 31

Figure 7-3. Front Panel Component Locator. 31
Figure 7-4 Rear Panel Component Locator. 31

Figure 7-5. Power Supply. 32
Figure 7-6. Block Diagram. 33
Figure 7-7. Component Locator for Circuit Board Part

No. 00428-66501 34
Figure 7-8. Front Panel Component Locator. 34
Figure 7-9 Rear Panel Component Locator. 34
Figure 7-10. Metering Circuit 35
Figure A-1. 428B Side Views. 37
Figure A-2 Backdating Schematics for 428B 38
Figure A-3 Backdating Schematics for 428B 39

Model 428B

428B SPECIFICATIONS

Table 1-1 Specifications

Current Range:

1 mA to 10 A full-scale, nine ranges.

Accuracy:

± 3% of full-scale ± 0.15 mA, from 0°C to 55°C.
(When instrument is calibrated to probe).

Probe Inductance:

Less than 0.5 µH.

Probe Induced Voltage:

Less than 15 mV peak (worst case at 20 kHz and
harmonics).

Output:

Variable linear output level with switch position
for calibrated 1 V into open circuit (corresponds

to full-scale deflection). 1.5 V Max. into open
circuit in uncalibrated position.

0.73 ± .01 V into 1 kΩin calibrated position.

Noise:

1 mA Range, < 15 mV rms across 1 kΩ.
3 mA Range, < 5 mV rms across 1 kΩ.

10 mA thru 10 A Ranges, <2 mV rms across 1
kΩ.

Frequency Range:

DC to 400 Hz (3 dB point).

AC Rejection:

Signals above 5 Hz with peak value less than
full-scale affect meter accuracy less than 2%.
(Except at 40 kHz carrier frequency and its

harmonics). On the 10 A range, ac peak value is
limited to 4 A.

Probe Insulation:

300 V Max.

AC Power:

115 or 230 V ±10%, 50 to 60 Hz, 71 W.

Operating Temperature:

- 20°C to + 55°C.

Cabinet Mount:

7½" wide, 11½" high, 14½" deep (190,5 x 292,1
x 368,3 mm).

Weight:

Cabinet Mount: Net 19 lbs (8,6 kg);
shipping 24 lbs (10,9 kg).

Rack Mount: Net 24 lbs (10,9 kg); shipping
35 lbs 15,9 kg).

Accessories Available:

-hp- Model 3528A Large Aperture Probe

-hp- Model 3529A Magnetometer Probe

-hp- Model 11035A Output Cable

-hp- Model 10110A Output Adapter

Dimensions:

Rack Mount

1

Model 428B

SECTION I

GENERAL INFORMATION

1-1. INTRODUCTION

1-2. The -hp- Model 428B Clip-On Milliammeter measures
the magnetic field, which exists around the wire carrying dc

current. Operating the instrument is simple. After zero
setting, the two jaws of the probe are clamped around wire

(arrow on probe head indicates direction of conventional
current flow) and the meter will indicate the current.

1-3. There are nine current ranges starting from 1 mA to 10
amp full-scale deflection. The sensitivity can be increased

even further by looping the wire several times through the
opening in the probe. The current indication is virtually

insensitive to superimposed ac signals and the series
loading of the circuit is less than 0.5 pH. A large amount of
feedback provides great stability.

1-4. OTHER PROBE HEADS.
1-5. Other probe heads are available to extend the

usefulness of your Clip-On DC Milliammeter. Write to the
nearest Sales and Service Office (listed in Appendix C) for

further information. At the time of publication of this manual,
the following accessory probe heads were available:

a. -hp-Model 3528A Large Aperture (2-1/2

inch probe head).

b. -hp- Model 3529A Magnetometer (1

gauss = 1 amp).

c. -hp- Model C11-3529A Magnetometer

(1 gauss = 1 mA).

1-6. Write to the nearest Sales and Service Office
(listed in Appendix C) stating your complete

requirements for information concerning special
applications.

1-7. INSTRUMENT AND MANUAL

IDENTIFICATION.

1-8. Hewlett-Packard uses a two-section serial
number. If the first section (serial prefix) of the serial

number on your instrument does not agree with
those on the title page of this manual, change

sheets supplied with the manual will define the
differences between your instrument and the Model

428B described in this manual. Some serial
numbers may have a letter separating the two

sections of the number. This letter indicates the
country in which the instrument was manufactured.

2

Figure 1-1. Model 428B Clip-On Milliammeter

Model 428B

SECTION II

INSTALLATION

2-1. UNPACKING AND MECHANICAL INSPECTION.

2-2. Inspect instrument for signs of damage incurred in
shipment. This instrument should be tested as soon as

it is received. If it fails to operate properly, or is
damaged in any way, a claim should be filed with the

carrier. A full report of the damage should be obtained
by the claim agent, and this report should be forwarded

to us. We will then advise you of the disposition to be
made of the equipment and arrange for repair or

replacement. Include model number and serial number
when referring to this instrument for any reason.

2-3. Hewlett-Packard Company warrants each
instrument manufactured by them to be free from

defects in material and workmanship. Our liability under
this warranty is limited to servicing or adjusting any

instrument returned to the factory for that purpose and
to replace any defective parts thereof. Any damage to

the instrument upon receipt is due to the carrier. File a
claim with the carrier as instructed in the preceding
paragraph.

2-4. OPERATION CHECK.

2-5. This instrument should be checked as soon as it is
received to determine that its electrical characteristics

have not been damaged in shipment. Refer to the
In-Cabinet Performance Checks of Section V of this
manual.

2-6. INSTALLATION.

2-7. See Paragraph 3-3 before operating this
instrument.

2-8. POWER REQUIREMENTS.

2-9. Power requirements are given in Specifications
table at t he front of this manual.

2-10. OPERATION ON 115 OR 230 VOLTS.

2-11. This instrument may be used with either a 115
volt or 230 volt supply with a frequency of 50 to 60 cps,

single phase. This instrument is shipped from the
factory ready for operation from a 115 volt source

unless otherwise indicated.

2-12. To operate from a 230 volt source, the 115-230
switch on the rear apron must be flipped to 230. First

turn the instrument off and pull the power cable from the
socket. Place a pointed tool, such as the sharpened end

of a pencil, in the slot of the switch and pull down.
Replace the fuse with the one given in Table 6-1 for 230

volt operation.

2-13. THREE CONDUCTOR POWER CABLE.

2-14. The three-conductor power cable supplied with
the instrument is terminated in a polarized, three-prong

male connector recommended by the National Electrical
Manufacturers' Association (NEMA). The third

conductor grounds the instrument chassis for the
PROTECTION OF THE OPERATING PERSONNEL.

When using a three-prong to two-prong adapter ground
third lead (green wire) externally.

2-15. RACK MOUNT MODEL.

2-16. This instrument is available in a rack mount
version in addition to the cabinet model shown in this

manual. The rack mount version is identical electrically
and similar physically except that the degausser has

been moved to the front panel for greater convenience.

2-17. PREPARATION FOR STORAGE AND SHIPMENT.

2-18. The best method for packing this instrument is in
the original shipping carton with the original fillers

packed in the same manner as when received from the
factory. Therefore, when unpacking, note carefully the

method of packing and save the original packing
material for possible future reuse.

2-19. If the original packing material is not available, and
it is desired to package the instrument for storage or

shipment, first wrap the instrument in heavy kraft paper
to avoid scratching the paint. Then pack in a cardboard

carton with a bursting strength of at least 150 lb per
square inch. Pad the instrument on all sides with at least

2 inches of rubberized hair or at least 4 inches of tightly
packed excelsior.

2·20. STORAGE.

2-21. No special precautions are necessary in storage
except the usual protection against mechanical damage,

salt air, etc.

3

Model 428B

4

Figure 3-1. Measurement Procedures

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

Model 428B

3-19. Magnetic Fields.

3-20. If the jaws of the probe are incompletely closed,
the magnetic shielding and the magnetic circuit will have

an air gap. The result is that dc fields, not associated with
the dc current being measured, will cause a shift in the

meter reading.
3-21. However, there will be an indication of a strong

external dc field even with the jaws perfectly closed.
Usually zero setting with the ZERO control compensates

such residual readings for a particular probe location.
3-22. EARTH'S MAGNETIC FIELD. The earth's magnetic

field will affect the reading if the jaws of the probe are not
completely shielded (jaws partially open). The effect of this

field is relatively strong - comparable to deflection due to
about 500 mA of current. Complete closure of the jaws

can be checked by switching to the 1 mA range with no dc
current input. If the jaws mate properly, the zero set

should stay within 0.1 mA while rotating the probe head
with respect to the earth's magnetic field.

3-23. If the zero shift is greater, the mating surfaces of the
jaws need to be cleaned or the probe wiring may be open

(see Section V).
3-24. FIELDS OF PERMANENT MAGNETS. Meter

magnets have strong stray fields, which can cause shift in
the current indication. Such fields are detected by bringing

the closed probe in the area where the measurement is to
be made and observing the zero shift (1 mA range).

3-25. FERROUS WIRE. Wires made out of magnetic
materials can cause a current reading of 2-3 mA without

any connection to the wire. This fact is important as leads
of most transistors are made out of magnetic material.

may be necessary after measuring current on the
1 thru 10 AMP RANGE.

3-32. Normally, it takes about 10 seconds to degauss
the probe when using the above method (see

Caution, Paragraph 3-3f).

3-33. ELECTRICAL ZERO SET.

3-34. If the instrument cannot be zero set electrically
(with ZERO control) there are two probable causes:

1) Incomplete closure of probe jaws, 2)
Magnetization of probe head.

3-35. Dust deposits on the lapped surfaces of the
probe jaws create an air gap. If the jaws are not

completely closed, the earth's magnetic field will
affect the reading. With the RANGE switch at 1 mA,

rotation of the closed probe should not vary the zero
set more than 0.1 mA. Cleaning of the jaws will

restore proper operation conditions (see Section V,
Cleaning of Probe Jaws).

3-36. Magnetic shields protect the probe head from
stray magnetic fields. However, excessive dc

currents (such as short circuit discharge currents
from electrolytic capacitors, etc.) will magnetize the

probe. For demagnetization of probe head, see
Paragraph 3-29, Degaussing of Probe Head.

3-37. POLARITY OF CURRENT.

3-38. The arrow on the probe head indicates the
direction of the conventional current flow for upscale

reading. Reversal of the current flow direction will
reverse the indication on the meter (see Figure 3-2).

3-26. OPERATING PRACTICES.
3-27. MECHANICAL OPERATION OF PROBE.

3-28. The probe jaws are opened by simply squeezing
together the two flanges on the probe body. An internal

spring returns the jaws to their proper position when the
flanges are released. (See Paragraph 3-3e.).

3-29. DEGAUSSING OF PROBE HEAD.

3-30. To demagnetize the probe, proceed as follows:
a. Insert probe into degausser at the rear of the

instrument (located on front panel of rackmount
models) with arrow on probe in same position as

arrow marked on chassis.

b. Depress degausser switch S3 to energize degausser.
c. Withdraw probe very slowly for the first few inches

while depressing the degausser switch until probe is
removed approximately one foot.

d. Zero instrument on 1 mA range with ZERO control

3-31. Under normal operating conditions, degaussing

Figure 3-2. Polarity of Current.

6

Model 428B

3-39. INCREASING THE ABSOLUTE SENSITIVITY

3-40. The sensitivity of the instrument can be increased by
looping the wire (carrying the dc current) several time

through the opening of the probe (see Figure 3-3). For
example, three turns increase the sensitivity three times.

With an increased sensitivity, however, the induced voltage
between the probe and the circuit under measurement will

increase also.

Figure 3-3. Increasing The Absolute Sensitivity.

3-41. CURRENT CHECK LOOPS.

3-42. In restricted situations such as printed circuit boards,
wire loops for the probe can be built into the circuit to allow

convenient current measurements with the Model 428B.
Here, currents can then be measured under operating

conditions with the same ease as voltage measurement.
3-43. Circuits can also be modified to accept an impromptu

loop for testing. As an example, to measure the collector
current of a transistor for troubleshooting purposes, the

collector lead can be removed from the board and a loop of
fine wire soldered between the collector lead and the board.

To measure current through a resistor, lift one lead and
install a series loop, clip the 428B probe around the loop and

measure current through the resistor. As an alternative, an
equivalent resistor with long leads can be installed to replace

the resistor in question.

3-44. NULLING CURRENTS.

3-45. The resolution of the 428B can be increased by nulling
one current against another and measuring the difference

between the two. To null the reading, clip the probe over
both wires at once with the wires so arranged that the

currents are going in opposite directions. The considerations
mentioned in Paragraph 3-39 also apply to current nulling.

For example, assume that a 0.6 A current source is to be
tested against a 0.4 A standard. The 0.6 A supply should be

looped twice through the probe jaws and the 0.4 A supply
should be looped three times through the jaws such that the

7

two currents oppose each other. It should be
remembered when making such a measurement,

that the absolute value of any deviations observed
have been multiplied. If, in the above example, the

0.6 A supply wavered by .01 A, the change would
be read as .02 A on the meter.

3-46. USE OF OUTPUT JACK.

3-47. The OUTPUT jack enables the 428B to be
used as a dc coupled: amplifier/I-E transducer

/isolator. The basic action of the 4 28B (considered
as an input/output device) is to sense the magnetic

field around a current carrying wire and deliver a
proportional voltage at the OUTPUT jack. The

value of the output can be varied by using the
OUTPUT LEVEL control to produce as much as 1

1/2 volts at 1 mA. While the 428B meter registers
average dc (ignoring ac), the output at the

OUTPUT jack contains both the dc and ac
components of the signal being measured.

3-48. With Oscilloscope.

3-49. To display the output of the 428B on an
oscilloscope:

a. If the oscilloscope is dc coupled, it can be

calibrated as in Paragraph 3-51.

b. Clip the probe around the wire which varies the

signal to be displayed.

c. Connect the oscilloscope input to the 428B

OUTPUT jack.

d. Adjust the 428B RANGE switch to the

appropriate range.

3-50. With Recorder.

3-51. To record the output of the 428B on a graphic
recorder:

a. Insure that the recorder's input impedance

exceeds 1400 ohms.

b. Connect the recorder input to the 428B

OUTPUT jack.

c. Zero the 428B on the 1 mA Range, turn

OUTPUT LEVEL to minimum output.

d. Zero the recorder.
e. Adjust the 428B ZERO control for full-scale on

the 428B meter.

f. Adjust the 428B OUTPUT LEVEL control for

full scale on the recorder.

g. Zero the 428B, switch to the appropriate range

and clamp the 428B probe around the wire
which carries the signal to be measured.

3-52. When recording current variations with the
428B, it should be borne in mind that the 428B

displays some long term zero drift. The 428B zero
drift normally amounts to about 300 µA (indicated)

per clay so periodic checks should be made to
determine whether or not the ZERO controls need

adjustment.

Model 428B

8

Figure 4-1. Block Diagram

Model 428B

SECTION IV

THEORY OF OPERATION

4-1. INTRODUCTION.

4-2. This section describes the overall operation of the Model 428B, the operating principle of the current probe and
the function of the different circuits of the instrument.

4-3. THEORY OF OPERATION.

4-4. The simplified block diagram of Figure 4-2 shows the basic operation of the Model 428B Clip-ON Milliammeter.
4-5. The probe clips around a wire carrying dc current and delivers a 40 kHz output signal which is proportional to the

dc current. For transducing the dc current into a 40 kHz signal, the probe requires a 20 kHz gating signal, as described
in detail under Paragraph 4-9, Current probe.

4-6. The 40 kHz output signal of the probe is amplified, detected and fed back as negative feedback current to the
probe head cancelling the effect of the measured dc current and thus reducing the 40 kHz output signal almost to zero.

The negative feedback current, being proportional to and magnetically almost equal to the dc current of the inserted
wire, is used to indicate the measured dc current.

4-7. The 20 kHz oscillator has two functions: First, it supplies a 20 kHz signal for driving the probe head, and also
provides a 40 kHz (second harmonic) signal for gating the 40 kHz Synchronous Detector.

4-8. Due- to slight unbalances, the probe head output contains a small 40 kHz signal, even with no dc current being
measured. A 40 kHz phase-shifter output cancels such residual 40 kHz signal (zero-set controls).

4-9. CURRENT PROBE.

4-10. The probe head is a specially designed second harmonic flux gate type of a magnetometer used to measure the
magnetic field around a wire carrying direct current.

4-11. The flux gate principle is easily understood by referring to the mechanical model shown in Figure 4-3.
4-12. Coil A (representing wire through probe), is energized with dc, producing a dc flux in the core. Armature is

rotating at a constant rate (F), gating the flux 2F times per second inducing a voltage of 2 F frequency in coil B. The
amplitude is determined by the dc in coil A.

9

Figure 4-2. Simplified Block.