IET Labs LOM-510A User manual

Operation Manual
MODEL LOM-510A
High Accuracy
Micro-Ohmmeter
User and Service Manual
♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
Copyright©2019IETLabs,Inc.
LOM510Aim/Dec.2019
www.ietlabs.com
Email: [email protected]
TEL: [phone] • [phone] • FAX: [phone]
IET LABS, INC.

♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
www.ietlabs.com
Email: [email protected]
TEL: [phone] • [phone] • FAX: [phone]
IET LABS, INC.

iii
LOM-510A Series
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.

iv
LOM-510A Series
Contents
WARRANTY ...........................................................................................iii
WARNING ............................................................................................... v
CAUTION................................................................................................ v
Chapter 1
INTRODUCTION ..........................................................................1
Chapter 2
SPECIFICATIONS.........................................................................2
2.1 Drive Specifications ............................................................... 2
2.2 Performance Specifications................................................... 2
Chapter 3
OPERATION...................................................................................3
3.1 General Operating Instructions .............................................. 3
3.2 Continuous DC Mode ............................................................ 3
3.3 Switched DC Mode ............................................................... 3
3.4. Pulsed Mode ......................................................................... 4
3.5 Dry Circuit Measurements .................................................... 4
Chapter 4
CIRCUIT DESCRIPTIONS..........................................................5
4.1 Power Supply......................................................................... 5
4.2Timing Circuitry ..................................................................... 5
4.3 Component Drive Circuitry................................................... 6
4.4 Measurement Circuitry .......................................................... 6
4.5 Display Circuitry .................................................................... 7
4.6 I/O Circuitry........................................................................... 7
Chapter 5
CALIBRATION..............................................................................8
Figures
Figure 1.1 LOM 510A ............................................................................. 1

v
LOM-510A Series
WARNING
OBSERVE ALL SAFETY RULES
‘WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.
ELECTRICAL SHOCK HAZARD. DO NOT OPEN CASE.
REFER SERVICING TO QUALIFIED PERSONNEL.
HIGH VOLTAGE MAY BE PRESENT WITH HIGH VOLTAGE OPTIONS.
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.
REMOVE POWER WHEN HANDLING UNIT.
POST WARNING SIGNS AND KEEP 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.

vi
LOM-510A Series

1
Introduction
LOM 510A Series
Chapter 1
Introduction
The Model 510A Micro-ohmmeter is a high perfor-
mance instrument, useful wherever there is a need for
measuring very low values of resistance. The highest
range of 200 ohms full scale on the Model 510A is
close to the lowest range on general purpose 4 1/2
digitmultimetersandohmmeters. Generally, the mea-
surement range of the Model 510A begins where the
general purpose instruments end.
The most significant features of the Model 510A are
the basic accuracy of .02% (even on the lowest scale)
and the three separate component drive modes. It’s
the combination of good basic accuracy and select-
able drive modes which makes the Model 510A such
ausefulandaccurateresistancemeasuring instrument.
The Switched DC Mode totally eliminates the effects
ofthermal EMF, averyimportantsource of error. The
Continuous DC Mode is required when measuring
the resistance of highly inductive components such
as transformers. The Pulsed Mode is important when
measuring heat sensitive components such as ther-
mistors.
In addition, the Model 510A can perform “dry cir-
cuit” measurements. This measurement mode is use-
fulwhen measuring the contact resistance oflow level
switches. Several MIL specs require “dry circuit”
measurements of certain switch types.
The Model 510A has a 25 pin connector mounted to
the rear panel which contains a set of parallel BCD
lineswhich are driven with thesame digitsasthefront
panel display. This allows the instrument to interface
to a companion limits comparator. In addition, the
connector contains signal lines to allow the unit to be
remotely triggered.
Figure 1.1 LOM 510A

2Specifications
LOM 510A Series
Chapter 2
Specifications
Pulsed Mode:
One cycle of Switched DC Mode. Enabled by
shorting two contacts on rear panel connector.
Dry Circuit:
Open circuit voltage limited to 50 millivolts max.
(AModel510A/20MV is availablewhichlim-
its the open circuit voltage to 20 millivolts
max.)
Speed:
667 milliseconds per reading.
Warm-up Time:
1 minute to rated accuracy
Connections to Unknown:
4 terminal plus shield
Rear panel connector:
25 pin “D” type connector; 5 BCD digits for
value, polarity indication, Pulse Mode con-
trol lines, Power Supply lines.
Display:
4 1/2 digit, .5 inch high, red LED display.
Display Overrange Indication:
Flashes for Switched DC and Continuous DC
Modes, blanks in Pulsed Mode.
Power Requirements:
105 - 135 VAC, 60 Hz, 30 Watts max.
Other voltages and frequencies available.
Dimensions:
9" wide x 2.25" high x 11.5" deep.
Weight:
6 lbs.
2.1 Drive Specifications
2.2 Performance Specifications
Accuracy, Switched DC:
+/-(0.02% of reading + 2 counts + 2 μΩ)
Accuracy, Continuous DC:
Typically: (0.04% of reading + 2 counts + 2 μΩ)
Accuracy, Pulsed Mode:
+/-(0.02% of reading + 4 counts + 2 μΩ)
Accuracy vs. Temperature:
Statedaccuracy applies over anambienttempera-
ture range of 18ºC to 27ºC. Error doubles
over the range of 10 degrees C to 40ºC.
AC Hz Noise Rejection:
80 dB for Switched DC and Pulsed Modes; 60
dB for continuous DC Mode.
Zero Adjust:
Front panel screwdriver adjustment, active only
in continuous DC Mode.
Adjustment range is +/- 150 counts (150 μV)
minimum.
Switched DC Mode:
Current switched on for 167 milliseconds, then
off for 500 milliseconds.
(duty cycle = 1/4)
Range
(Full Scale) Drive POWER
DC
POWER
SW DC
ENERGY
PULSED RESOLUTION
19.999 m1 A 20 mW 5.0 mW .003 mJ 1 µ
199.99 m0.1A 2.0 mW 0.5 mW 330 µJ 10 µ
1.9999 10 m A 200 µW 50 µW 33 µJ 100 µ
19.999 1 mA 20 µW 5.0 µW 3 µJ 1 m
199.99 100 µA 2 µW 0.50 µW 0.3 µJ 10 m

3
Operation
LOM 510A Series
Chapter 3
Operation
4. Select the proper resistance range by pushing
in the appropriate range button. Very often
the correct range can’t be determined in ad-
vance. In such cases start with the highest
range, working down to the lower ranges.
This keeps the stress on the unknown to a
minimum.
3.2 Continuous DC Mode
The Continuous DC Mode is the traditional method
of measuring the value of’a resistor. Aconstant cur-
rent is forced through the unknown and the voltage
developed across the device is measured. This volt-
age is directly proportional to the resistance of the
device. Unfortunately, this methoddoesn’t cancel one
important source of error, small voltage sources in
series with the measured resistance. These voltage
sources are mainly due to thermocouple effects and
unlesstheyarecarefullynulledoutwiththefrontpanel
zero control they can seriously degrade the accuracy
of the measurement. When attaching the test leads to
the unknown try not to warm the contact area with
your fingers.
The Continuous DC Mode is the proper mode only
when measuring highly inductive components such
as transformers, chokes and motor windings. Wire
wound resistors are not inductive enough to require
measurement in this mode.
3.3 Switched DC Mode
The Switched DC Mode first measures the voltage
acrosstheunknownwiththedrivecurrentoff. Itstores
thisvoltage and then measuresthevoltage with a cali-
3.1 General Operating Instructions.
1. Plug the power cord into the rear panel AC
connector and the test leads into the connec-
tor provided on the front panel.
2. Push the power switch in to turn the Model
510A on and wait 30 minutes to allow the
instrument to stabilize. If the test leads are
left open circuited or the resistance of the
unknownexceeds maximum value forthese-
lectedrange, the display willflash. Thisdoes
no harm to the instrument.
3. Select the proper measurement drive mode.
The Continuous and Switched DC drive
modes are selected by the front panel mode
select switch. The Pulsed Mode is selected
through the rear panel I/O connector. Refer
to the next section to determine which mode
is beat for a particular measurement task.
If the Continuous DC Mode is selected then the
front panel zero control should be adjusted
periodicallyto assure thatatruezero is main-
tained. Zeroing the meter is beat done on the
highest (200 ohms full scale) range since a
trueshort circuit can be difficult to attainwith
Kelvintypeclipson the more sensitive ranges.
(Almost any chunk of metal has at least a
micro-ohm of resistance.). Connect the test
clips to a heavy, short piece of wire and care-
fully zero the display.
Push the “DRY CKT” button in if the open cir-
cuit voltage is to be clamped to 50 millivolts
maximum. (20 millivolts maximum on the
Model 510A/20MV)

4Operation
LOM 510A Series
brated current source flowing through the unknown.
The displayed reading is the difference between the
two measurements.
TheSwitched DC Mode is the appropriatedrivemode
for most measurements. No zeroing is required, in
fact the front panel offset control has no effect. All
sources of zero offset are cancelled by the auto zero
circuitry. In addition the instrument is less sensitive
to AC pickup in this mode, although the instrument is
quite insensitive to AC pickup in all modes. Power
dissipation of the unknown is 1/4 that of an equiva-
lent measurement in the Continuous DC Mode.
3.4. Pulsed Mode
The Pulsed Mode drives the unknown with a single
167 millisecond pulse of current each time a short is
removed between pins 18 and 19 of the rear panel I/
O connector. The short must be removed for at least
1 microsecond but less than 667 milliseconds. If the
open circuit lasts for longer than 667 milliseconds
then the instrument will take an additional measure-
ment. The front panel mode switch must be in the
Switched DC Mode for proper operation.
Anormallyclosed, momentary pushbutton switch can
be wired to the rear panel connector to manually trig-
ger the Pulsed Mode. If remote electronic control of
the Pulsed Mode is desired an analog switch such as
a Motorola MC14066 or Siliconix DG200 can be
used. A small relay also works well.
The Pulsed Mode should be used when the compo-
nent to be measured is extremely temperature sensi-
tive, such as in the case of small bead thermistors.
The Pulsed Mode is the only way to externally trig-
ger the Model 510A.
If the Model 510A is pulsed into an open circuit, the
first measurement after the open circuit measurement
will read about 4 counts higher than the actual value.
All subsequent readings will be within specifications.
3.5 Dry Circuit Measurements
Switchcontacts can oxidize, causingahighresistance
contact when closed. This oxide layer will usually
break down when the voltage drop across the con-
tacts approaches a volt or so. The “DRY CKT” but-
ton, when depressed will limit the voltage to 50 milli-
voltsmaximum(20millivoltsmaximumontheModel
510A/20MV). If the switch is being used in a low
level signal application then the dry circuit measure-
ment will give a better indication of how the switch
willperform. SeveralMILspecificationsdealing with
contact resistance measurements in switches and re-
lays require dry circuit measurements.
All Cambridge Technology, Inc. test clips, probes and
fixtures include 100 kW resistors between drive(+)
and sense(+) and between drive(-) and sense(-). The
resistors assure that the output voltage is monitored
(and limited to 50 millivolts) even if the sense leads
fail to make contact with the sample. If custom made
test fixtures are used with the Model 510A and the
dry circuit feature is desired, make sure the resistors
are installed.