iET LOM-4000 Troubleshooting guide

TEL: [phone] • [phone] • FAX: [phone]
www.ietlabs.com
IET LABS, INC.
534 Main Street, Westbury, NY 11590
Standards• Decades• Strobes• Sound Level Meters • Bridges
LOM-4000 SERIES
Digital Ohmmeter
User and Service Manual
♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
Copyright © 2005 IET Labs, Inc.
LOM-4000 im February, 2005

iii
WARRANTY
We warrant that this product is free from defects in material and workmanship and, when properly used, will
perform in accordance with applicable IET specifications. If within one year after original shipment, it is found
not to meet this standard, it will be repaired or, at the option of IET, replaced at no charge when returned to IET.
Changes in this product not approved by IET or application of voltages or currents greater than those allowed by
the specifications shall void this warranty. IET shall not be liable for any indirect, special, or consequential
damages, even if notice has been given to the possibility of such damages.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUD-
ING BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTIBILITY OR FITNESS FOR
ANY PARTICULAR PURPOSE.

vii
WARNING
OBSERVE ALL SAFETY RULES
WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.
Dangerous voltages may be present inside this instrument. Do not open the case
Refer servicing to qulified personnel
HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT
WHENEVER HAZARDOUS VOLTAGES (> 45 V) ARE USED, TAKE ALL MEASURES TO
AVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS.
USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARE
CONDUCTORS WHEN USING THIS INSTRUMENT.
Use extreme caution when working with bare conductors or bus bars.
WHEN WORKING WITH HIGH VOLTAGES, POST WARNING SIGNS AND
KEEP UNREQUIRED PERSONNEL SAFELY AWAY.
CAUTION
DO NOT APPLY ANY VOLTAGES OR CURRENTS TO THE TERMINALS OF THIS
INSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON
THE FRONT PANEL OR THE OPERATING GUIDE LABEL.

Contents
Chapter 1
INTRODUCTION ..................................................................................................................................... 1
1.1 General Description ........................................................................................................................ 1
1.2 Theory of Operation ........................................................................................................................ 1
1.3 Case Design .................................................................................................................................... 1
1.4 Available Accessories ..................................................................................................................... 1
Chapter2
SPECIFICATIONS .................................................................................................................................... 2
2.1 General: ........................................................................................................................................... 2
2.2 Measurement: ................................................................................................................................. 2
Chapter 3
OPERATION .............................................................................................................................................. 3
3.1 Initial Inspection ............................................................................................................................. 5
3.2. Safety ............................................................................................................................................ 5
3.3 Features ......................................................................................................................................... 5
3.3.1 Measurement Polarity ........................................................................................................... 5
3.3.2 Range Selection: ................................................................................................................... 5
3.3.3 Error & Status Lamps .......................................................................................................... 5
3.4 Making Measurements ................................................................................................................. 5
3.4.1 Ohmmeter Connections ........................................................................................................ 5
3.4.2 Start-up ................................................................................................................................. 6
3.4.3 Measurement ......................................................................................................................... 6
3.4.4 Current Mode ......................................................................................................................... 7
3.4.5 Over-Range .......................................................................................................................... 7
3.4.6 Open-Circuit Lead ................................................................................................................ 8
3.4.7 Low Battery .......................................................................................................................... 8
3.4.8 Connections........................................................................................................................... 8
3.5 Protection ....................................................................................................................................... 8
3.6 Temperature Compensation (Model LOM-4001 Only) .................................................................. 8
3.7 Temperature Measurement (Model LOM-4001 Only .................................................................... 8
Chapter 4
CALIBRATION ......................................................................................................................................... 9
4.1 Calibration Process ......................................................................................................................... 9
4.2 Equipment Required ........................................................................................................................ 9
4.3 Preparation ...................................................................................................................................... 9
4.4 Calibration Using MTS2 Standard .................................................................................................. 9
4.4.2 Entering a password, and Calibration ..................................................................................... 9
4.5 Calibration Using Discrete Resistance Standards ........................................................................... 9
Chapter 5
MAINTENANCE..................................................................................................................................... 11
5.1 General ......................................................................................................................................... 11

Figures
Figure 3.2 Combined current and potential probes (Kelvin clips) ....................................................................... 6
Figure 3.3 Connections for resistance measurements with various DUT configurations .................................... 7
Figure 3.4 Connection Diagram for Pt100 plug (viewed from the rear) .............................................................. 8

1
Introduction
LOM-4000Series
Chapter 1
INTRODUCTION
1.3 Case Design
The case is ruggedly constructed from ABS plastic.
The front panel is a reverse-printed polycarbonate
overlay with clear and unambiguous text. The batter-
ies are housed in a removable cassette for quick and
easy replacement.
1.4 Available Accessories
Supplied:
1 Set of test leads
1 Set of alkaline batteries
1 Operation Manual
Optional Lead Sets:
HSO1-P Duplex Handspikes with 2.5-meter
leads, for Kelvin measurements of
plates or buss bars
HSO2-P Similar to HSO1-P but with 2.5-
meter and 15-meter leads
LSO3-P Large Kelvin clips with 3-meter leads
– accept cables and bars up to 38
mm in diameter
LSO4-P Similar to LSO3 but with 3-meter
and 15-meter leads
Other Optional Accessories:
CO1 1 Meter Cable-Test Fixture – Wood Base
CO2 1 Meter Cable-Test Fixture – Metal Base
MTS2 Calibration Standard
For most efficient operation, a spare battery
cassette (BP-02) is recommended.
1.1 General Description
The LOM-4000 and LOM-4001 are accurate bench/
portable handheld Digital Milli-ohmmeters for the mea-
surement of resistance in the range 10 µΩto 4000 Ω.
They utilize the four-terminal (Kelvin) resistance-mea-
surement method to eliminate the effects of lead re-
sistance. The measured values are displayed on a 4-
digit, 4000-count LED display; there is also an over-
flow indicator. The LOM-4001 allows temperature
compensation of readings and temperature measure-
ment if the user connects a 100-ohm platinum-resis-
tance thermometer (PRT).
Simple push-button or auto-range selection of the
range required makes the LOM-4000 Series easy to
use. Error and status warnings are illuminated when
appropriate. The unit will withstand accidental appli-
cation of line voltage to the measuring terminals.
1.2Theory of Operation
The measurement is true 4-terminal, using the Kelvin
principle. A stable current is produced across the
resistance to be measured via the current (I) termi-
nals, and the voltage drop across the unknown resis-
tance Rx is measured at the voltage (U) terminals.
This potential drop is then compared against the volt-
age drop across internal standards. The ratio of these
is then converted to the resistance value of Rx and
displayed in ohms on the digital display. Use of spe-
cially selected components assures high accuracy and
long-term stability .

2Specification
LOM-4000Series
2.1 General:
Digital Display:
4 digit, 0.6" high LCD, 4000 count with
automatic decimal point and polarity
indication; additional annunciator
lamps.
Operating temperature range:
0 to +40°C, with maximum relative
humidity of 80%
Storage temperature:
-20°C to +50°C
Power:
Five “AA” alkaline batteries supplied.
Rechargeable batteries optional.
Battery Life:
~12 hours
Dimensions:
250 x 130 x 55 mm
(9.9” x 5.1” x 2.2”)
Weight:
0.8 kg (1.5 lb)
Chapter 2
SPECIFICATIONS
2.2 Measurement:
Resistance:
True four-terminal measurement with fixed
dc measuring currents
Measuring time:
Approximately 20 ms for normal measurement,
40 ms for average (AVE) mode
Polarity:
Selectable positive or negative current flow by
manually reversing test leads, plus an average
mode which automatically displays the average
value of the positive- and negative-polarity
measurements.
Temperature Measurement:
-200°C to +800°C, with user-supplied
Pt100 sensor
RESISTANCE
Range Resolution Current Accuracy Temperature Coefficient
4kΩ1Ω100 µA ±(0.05% Rdg +0.02% FS) 30 ppm Rdg +1 ppm FS
400Ω100mΩ1 mA ±(0.05% Rdg +0.02% FS) 30 ppm Rdg +1 ppm FS
40Ω10mΩ10 mA ±(0.05% Rdg +0.02% FS) 30 ppm Rdg +1 ppm FS
4Ω1 mΩ10 mA ±(0.05% Rdg +0.03% FS) 30 ppm Rdg +4 ppm FS
400mΩ100µΩ 10 mA ±(0.05% Rdg +0.05% FS) 30 ppm Rdg +25 ppm FS
40mΩ10µΩ 100 mA ±(0.05% Rdg +0.1% FS) 30 ppm Rdg +25 ppm FS
TEMPERATURE
Range Resolution Accuracy
-50...+800 °C 0.1°C ±0.2°C

3
Operation
LOM-4000Series
Chapter 3
OPERATION
Low Battery LED
Units Indication
4000
+
Function Keys
Display Value LED (4001 only)
Calibration
Display Mode LED’s (4001 only)

LOM-4000 Series
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5
Operation
LOM-4000Series
the AUTO key puts the instrument into the auto-range
mode.
3.3.3 Error & Status Lamps
The following LEDs will light to indicate the instru-
ment status:
CAL:
Instrument is in the calibration mode.
LOBAT:
Battery capacity is nearly depleted.
(The following LED’s are for LOM-4001 only)
Rx:
The display shows a resistance reading.
Rc:
The display shows a compensated
resistance reading.
°C:
The display shows a temperature reading.
3.4 Making Measurements
3.4.1 Ohmmeter Connections
The LOM-D4000/1 employ a four-wire method of
measurement, i.e. four connections to the resistor
under test are required. The unit is supplied with four
leads: two voltage leads to be placed across the ex-
act measurement points, and two current leads to be
placed “outside” the measurement points. The Kelvin
clips, shown in Figure 3.2, place voltage and current
connections at the same point.
a) Connect the red leads to the +I and +U terminals,
and the black leads to the –I and –U terminals.
b) Clip the test leads onto the resistor under test.
Cleanliness is important; if the DUT is not clean, clean
it with fine emery cloth or steel wool to remove ox-
ides.
3.1 Initial Inspection
After unpacking, inspect unit for shipping damage and
report any problem immediately to the carrier, retain-
ing packaging materials for inspection.
3.2. Safety
This unit has been designed and tested in accordance
with IEC Standard EN61010-1, entitled "Safety Re-
quirements for Electronic Measuring Apparatus", and
has been supplied in a safe condition. This instruc-
tion manual contains information and warnings which
should be followed by the user to ensure safe opera-
tion and to keep the unit in safe condition.
3.3 Features
3.3.1 Measurement Polarity
The user can reverse the measuring-current polarity
by reversing the test leads. This is particularly useful
when evaluating circuits with thermal emf, or circuits
where diode effects can influence the measurement.
For measurements where thermal emf can cause a
large zero-error, there is an automatic-average (AV E )
button. When it is pressed, the measuring current
will automatically be reversed and the average value
displayed, thus eliminating the need for external com-
putations. This averaging feature also doubles the
measurement time from 20 ms to 40 ms. The AVE
lamp will light to indicate that the current polarity is
changing.
3.3.2 Range Selection:
Any of the six measuring ranges can be selected
manually by simply pressing the desired RANGE UP
or DOWN buttons. Over-range is indicated by the
display reading “- - - -“.
An auto-range capability is also provided: pressing

6Operation
LOM-4000Series
Fig. 3.2 Combined current and potential probes
(Kelvin clips)
c) It is not always possible to use the combined
current and potential (Kelvin) clips, in which case test
leads with spade lugs—or a special test fixture - may
have to be made for a particular application.
d) Fig. 3.3 illustrates connections to various types of
test resistors
e) When measuring 4-terminal resistance standards,
or using them for calibration, the Kelvin clips may not
be usable because of terminal geometry. Make four
separate connections to the current and potential ter-
minals
3.4.2 Start-up
When the LOM-DO7e is first switched on, an inter-
nal self-test is automatically performed. The display
indicates +8888 followed by PASS, then performs
an automatic zero sequence and finally sets to the
4 kΩrange. The unit is now ready to make mea-
surements. The unit will shut off automatically after
several minutes of inactivity.
If the internal checks indicate an error, the display
will read FAIL. Contact our Service Department or
your IET Representative for assistance.
3.4.3 Measurement
Connect the resistance to be measured (Rx) to the
measuring terminals in accordance with the diagram
on the instrument panel and using the most appropri-
ate connection technique for the unknown (See Figs.
3.2 and 3.3). Select the range required or AUTO,
and the AV E measurement mode if desired (The AV E
button LED will light to indicate that the AV E func-
tions is active. To initialize the measurement, press
the MEAS button. Asingle measurement will be taken
and the value held on the display. To take continuous
readings, press and hold the MEAS button until a
long beep is heard; this will activate a continuous-
reading mode.

7
Operation
LOM-4000Series
Fig. 3.3 Connections for resistance measurements with various DUT
configurations
Measurements will continue until the MEAS key is
pressed again. To preserve battery life, the unit will
automatically turn off after several minutes of inac-
tivity.
3.4.4 Current Mode
The current mode can be set to AV E . The AV E mode
may be selected for measurements that are not in-
ductive, in order to eliminate errors caused by ther-
mal emf in the measurement circuit or test leads. In
the AV E mode the LOM-D4000/1 takes readings
with the current flowing in both directions and dis-
plays the average of the two readings. The AVE
mode should not be used for measurement of in-
ductive circuits.
3.4.5 Over-Range
If the measured value exceeds the upper limit of the
range selected, the display will indicate “- - - -“.
Select a higher range. In AUTO mode, this display
indicates a value above the range of the instrument.

8Operation
LOM-4000Series
exact type used. Always replace this fuse with one
of the same current rating (1 A) and at least a 40,000
A interrupting capacity.
3.6 Temperature Compensation (Model
LOM-4001 Only)
Automatic temperature compensation is available on
the model LOM-4001. A Pt100 temperature sen-
sor should be connected to the DIN socket next to
the measuring terminals.
To activate this function press the DISP key the Rc
LED will light to show that the displayed value is the
compensated resistance. The Pt100 sensor will mea-
sure the ambient temperature and compensate the
reading for the temperature coefficient of the mea-
sured Rx.
Different temperature coefficients may be selected
by pressing the COMP key, Copper and aluminium
coefficients are preset; a user coefficient may also
be entered. Repeated pressing of the COMP key
cycles through these options. To set the user coeffi-
cients the arrow keys are used select and increment
or decrement the digits.
3.7 Temperature Measurement (Model
LOM-4001 Only
The model LOM-4001 can also be used as thermom-
eter using a Pt100 temperature probe inserted into
the socket next to the measuring terminals. Press the
DISP key until the °C LED lights. The LOM-4001
will now display the temperature measured by the
Pt100 sensor.
3.4.6 Open-Circuit Lead
If one of the test leads is open-circuited, or exceeds a
preset resistance, the display will indicate “OC“.
(The I terminals are checked for compliance volt-
age). Measurements cannot be made if this warning
message is displayed. This warning will also be dis-
played if the current-lead resistance is high.
3.4.7 Low Battery
The LOBAT LED will light when the battery has
~10% charge remaining. When this occurs, a spare
battery cassette should be installed, or the existing
cassette should be resupplied with fresh--or recharged-
-batteries. Full measurement accuracy is maintained
even when the LOBAT LED is lit.
3.4.8 Connections
To make the most accurate possible measurements,
ensure that all test leads are in good condition, and
have less than 0.2 Ωresistance. Some spade lugs
and alligator clips - especially nickel-plated brass types
- can produce high thermal emf’s when heated. This
can sometimes cause problems , for example when
connecting to hot motor windings. Avoid such prob-
lems by using plain copper or brass connections and
keeping them clean and oxide-free.
3.5 Protection
This instrument is protected against voltages being
applied inadvertently to the terminals. A high-inter-
rupting-capacity fuse is installed internally in the cur-
rent circuit. The voltage terminals are not fused, and
will withstand up to 460 Volts without damage to the
.instrument.
To replace the protection fuse, the unit should be
opened by removing the six screws on the bottom of
the case,, but only after ac power and all input con-
nections are removed. The protection fuse is located
on the printed circuit board. This fuse has a 40,000-
to-200,000 A interrupting capacity, depending on the
+I
+U
-I
-U
GROUND
Fig. 3.4 Connection Diagram for Pt100 plug
(viewed from the rear)

9
Calibration
LOM-4000Series
Chapter 4
CALIBRATION
4.1 Calibration Process
This procedure describes the standard calibration
method for the LOM-4000/1 Micro-ohmmeter using
the IET MTS2 Calibration Standard, or a method us-
ing discrete resistance standards of the required val-
ues.
CAUTION
The LOM-4000/1 comes factory-calibrated to its full
accuracy, and any recalibration by the user will invali-
date this initial calibration. The user should therefore
be certain that only authorized and competent per-
sonnel are permitted access to the calibration process,
which is password-protected.
4.2 Equipment Required
IET Calibration Standard MTS2, or equivalent 4-ter-
minal standard/s.
4.3 Preparation
4.3.1 Place the instrument in a temperature-con-
trolled environment (20°C ±5°C) for a minimum of 2
hours. Ensure that the batteries are fresh or fully
charged.
4.4 Calibration Using MTS2 Standard
4.4.1 Connect each of the four LOM-4000/1 termi-
nals to the equivalent terminals on the calibration stan-
dard.
4.4.2 Entering a password, and Calibration
1. Turn the LOM-4000/1 on while holding down the
CAL key. The display will show CODE for a few
seconds and the 0000 with a flashing cursor under
the first zero. Enter the calibration code 9252 using
the UP and DOWN keys to select the digits and the
ZERO and AV E keys (with the left-and-right arrows)
to select the position. When done, press the MEAS
(OK) key. When the entry has been validated, the
instrument enters the calibration mode and the CAL
LED illuminates.
2. The display will now show 0.0, with the first digit
flashing. Set the MTS2 to zero and press OK. This
calibrates the zero on the first range. The display will
then show the full-scale reading on that range (4.000)
with the first digit flashing. Set the MTS2 to the same
value, and (if desired for better accuracy) enter the
exact calibrated value for that MTS2 setting using
the arrow keys as above. The press OK.
3. The calibration process then switches automati-
cally to the next range. Proceed as above until all
ranges are calibrated. Then press the CAL button to
exit the calibration mode.
4.5 Calibration Using Discrete
Resistance Standards
4.5.1
Discrete, 4-terminal resistance standards may be
used in place of the MTS2 Calibration Standard. The
standards must have an uncertainty better than 0.01%
if full calibration accuracy is to be achieved. The
following nominal standards are recommended: 4 kΩ
400 Ω, 40 Ω, 4 Ω, 400 mΩ, 40 mΩand 4 mΩ.

10 Calibration
LOM-4000Series
4.5.2
In all measurements the U + and - and the I + and
- terminals are connected to the equivalent terminals
(usually marked “Sense” and “Current”) on the four-
terminal standards used.
4.5.3
For the ZERO measurement, short the “Sense” ter-
minals of the standard connected to the instrument,
using the shorting clip supplied with the standard, or a
heavy wire. All four leads between the instrument
and the standard should remain connected. Remove
the short after the ZERO calibration is completed.
4.5.4
Use the procedure of 4.4.1 above to perform the
calibration using discrete standards. Replace the zero
setting and resistance values of the MTS2 with the
shorting procedure above and the discrete resistance
standards to be used.

11
Maintenance
LOM-4000Series
Chapter 5
MAINTENANCE
5.1 General
Normally no maintenance is required other than clean-
ing with a moist cloth. Avoid strong detergents or
solvents.
If a failure occurs, or if the unit cannot be calibrated,
it should be returned to IET or to an IET representa-
tive. Contact IET for an RMA number before return-
ing the unit directly to IET.
CAUTION: Replace only with the correct fuse
type shown below.
The input circuits are protected by a 1 A fuse located
in a holder on the printed circuit board. Open the case
by removing the six screws holding the two sections
together.. Examine the fuse and replace if necessary.
! CAUTION !
Replace only with the correct fuse type shown be-
low, or an equivalent part with the same or higher
interrupting capacity:
NFC 63210, Altech 1C10x38G1
or Cooper Bussman KTK-1