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.

5
LOM 510A Series
Circuit Descriptions
Chapter 4
Circuit Descriptions
The test leads can easily pick up line related noise
orders of magnitude greater than this.
TheAC voltage at the transformer secondary is con-
verted into a square wave by U7 and its associated
circuitry. R34 feeds CR8 and CR9 with theAC volt-
age. The diodes clip the AC voltage, reducing it to
about 1.2 volts peak to peak. This reduced voltage is
then amplified by U7 to produce a 60Hz square wave.
R33 and C10 provide some hysterisis to insure clean
switching. The 60Hz signal is then reduced to 6Hz
by the decade counter US. U9 then takes this 6Hz
signaland produces 4 equallyspacedtimeslots. Each
time slot lasts for a period of 1/6 seconds, giving a
total measurement cycle time of 4/6 seconds. Nor-
mally U9 would reset after 10 pulses but is instead
reset after every 4 pulses via the RS latch formed by
two gates in U9. This latch is set every time the first
time slot goes high, forcing the reset line of U9 low.
When the forth time slot has been completed the latch
is reset, causing the reset line of U9 to go high. This
resets U9, starting a new measurement cycle.
System timing in the Switched DC Mode is as fol-
lows; During time slot one (U9, pin 3 is high) current
to the unknown is switched off allowing the measure-
ment circuit to auto zero itself. During time slot 2
(U9, pin 2 is high) test current is applied to the un-
known and the resultant voltage appearing across the
unknown is amplified. During time slot 3 (U9, pin 2
is high) and time slot 4 the measurement circuitry
converts the analog voltage proportional to the
unknown’s resistance to a digital number to drive the
display.
TiminginthePulsedModeisidentical to the Switched
DC Mode except that the cycle is not automatically
repeated. The RS latch formed by U28 is inhibited,
4.1 Power Supply
The power supply produces 3 regulated DC voltages,
namely+5V DC, +8VDCand-SVDC.The 120 VAC,
60 Hz enters the unit via the rear panel three pronged
connector and the high side immediately pas-sea
through F1, a 1/2 amp slow blow fuse. The AC high
then runs up one edge of the pc board to the front
panel power switch and back down to one side of the
power transformer primary. The other side of the pri-
mary connects directly to the AC low terminal of the
ACconnector,providinga return path for the 120VAC
input.
Thetransformersecondarycontainstwocenter-tapped
windings. The first set of windings (transformer pins
10-12) feed CR11 and CR12 which in turn charge
C11 and C12 to about 9 VDC. This raw voltage is
then regulated to +5 VDC by U16.
The second set of windings (transformer pins 7-9)
feed RA1, a full wave bridge, which generates posi-
tive and negative unregulated DC voltages of about
13VDC. The three terminal regulators,U17and U18
reduce and regulate these voltages to the required
+8VDC and -SVDC.
4.2 Timing Circuitry
NOTE: The digital signals in the timing circuitry
switch from -8V to +av.
All measurements are synchronized with the AC line
to reduce the effects of line related noise. This is
especiallyimportantin the Model 510A since the least
significantdigit in the displayisonly 1 microvolt DC.

6Circuit Descriptions
LOM 510A Series
holding the timing circuitry in reset. In the Continu-
ous DC Mode switch S2 inhibits the auto zero, forc-
ing a constant current to flow continuously through
theunknown. The AC toDCconversiontiming how-
ever is the same as in the other modes so that the
60Hz noise rejection remains.
4.3 Component Drive Circuitry
The drive circuitry drives the unknown with one of 5
constant current levels, depending upon the range
selected. In measuring resistors a constant current is
very convenient since the voltage appearing across
the unknown will be directly proportional to its resis-
tance.
The network formed by CR1, R1 and Cl produces a
stable voltage source of about 1.23VDC. This volt-
age is used as a reference by both the drive circuitry
and measurement circuitry. In the Switched DC
Mode, during time slot 1 U1 pins 9 and 8 are shorted
driving the input of U2 to 0 VDC. During time slot 2
U1, pins 10 and 11 are shorted driving the input of
U2 to about 1VDC. When the input of U2 is driven
positive the output also goes positive. This turns Q1
on. Q1 in turn drives a network of precision current
sensing resistors (SIP-1,R10-R19 and R21). The cur-
rent sensing resistors then drive the unknown.
The value of the current sensing resistance is deter-
mined by the range selected and is either
1,10,100,1000 or 10000 ohms. Trims are provided
for each of the five ranges. U3 feeds the voltage ap-
pearing across the current sensing resistor back to the
negative input of U2 where it is compared to the ref-
erence voltage such that the current flowing through
the current sensing resistor and the unknown is held
constant.
R9 provides short circuit current limiting during
switching.
R8/C32 and R7/C3 add stability to the constant cur-
rent loop. The function of R2-R4 is to null out any
small offset voltages which might produce a differ-
encein reading between the Continuous andSwitched
DC Modes.
When the “DRY CKT” button is (S8) depressed the
voltage at the output of U4 is sensed. When this volt-
ageexceedsapproximately 3 volts U29 becomes posi-
tive. As this voltage becomes more positive CR15
forward biases, closing a loop which clamps the out-
put voltage to less than 50 millivolts (20 millivolts on
the Model 510A/20MV).
4.4 Measurement Circuitry
U4 is a differential amplifier wired to have a voltage
gain of 100. The inputs of this amplifier are the sense
terminals connecting to the unknown. Since on any
givenrangethe voltage appearing across the unknown
will vary from 0 to 20 mV then the output of U4 can
go from 0 to 2V. For the Switched DC Mode, during
time slot one the current through the unknown is 0.
Howevertherecanbeasmallvoltageappearingacross
the unknown due to thermal EMF’S. There is also an
offset associated with U4. These offsets are multi-
plied by 100 and used to charge the sample and hold
circuit formed by U1, pins 3-5 and C4. During time
slot 2 U1, pins 4 and 5 are opened and U1, pins 1 and
2 are closed, causing C5 to be charged. The differ-
ence between the voltages appearing across C4 and
C5 is proportional to the resistance of the unknown.
This differential voltage is applied to the 7135 DVM
integrated circuit. The analog to digital conversion
takes place when pin 25 of U6 goes high at the begin-
ning of time slot 3. The conversion time takes all of
time slot 3 and most of time slot 4.
Al50 kHz clock isrequiredby the DVM chip tomake
the conversion. This oscillator is formed by U5 and
its associated passive components. The output of the
DVM circuit is multiplexed BCD and is used to drive
the display circuitry and I/O circuitry. The DVM cir-
cuit also requires a number of passive components
(C7-C9 and R30) for proper operation. R31 and R32
function as a voltage divider, reducing the reference
voltage to 1.0VDC.
CR13 and CR14 assure that the common mode range
ofthe DVM circuit isnotexceededwhen the test clips
are open circuited. R42 and R43 provide current lim-

7
LOM 510A Series
Circuit Descriptions
iting to the DVM circuit during open circuit. S2-C
and R24 discharge C4 in the Continuous DC Mode.
R22 in conjunction with R40 is used to zero the meter
when in the Continuous DC Mode. Although this
offset control is still active in the Switched DC Mode
it has no effect on the reading because offsets from
any source are cancelled by the auto zero circuitry.
4.5 Display Circuitry
The 20,000 count (4 1/2 digit) display is multiplexed
to reduce power consumption and the number of dis-
play drivers. The DVM circuit supplies both digit
select and BCD outputs. All five digits (UlO-Ul4)
have their segments wired in common. The common
anodes for each digit each have their own drivers.
U15 converts the 4 BCD lines to the proper 7 seg-
ment code and pulls those particular segments low.
Which digit is selected is determined by which of the
digit select lines (DI-D5) is high. When a digit select
line goes high it turns on the associated transistor
which in turn pulls the common anode of that digit
on.
RN-1 provides the proper current level to the seg-
ments.
The correct decimal point location for a particular
range is selected by a single pole in each of the range
buttons (S3-C through S7-C). R39 provides the cur-
rent limiting for the decimal point.
To facilitate zeroing the meter when in the Continu-
ous DC Mode segment “g” (the center horizontal seg-
ment) is driven when the polarity to the DVM circuit
reverses.
4.6 I/O Circuitry
The multiplexed BCD information produced by the
DVM circuitry is latched and presented to the I/O
connector in a parallel format. U21 through U25 are
4 bit latches. The multiplexed BCD lines are wired
in common with each 4 bit latch. This BCD informa-
tionistransferredtotheappropriatelatchwhenagiven
digit line goes high and the strobe line from the DVM
chip is pulsed. The latch will present the BCD infor-
mation to the I/O connector until the next measure-
ment cycle.
If pine 18 and 19 are shorted together the meter will
finish the current measurement cycle and then go into
a hold mode until the short is temporarily removed (1
microsecond or greater). Each time the short is
opened the meter will take a single measurement and
display the results. When the two lines are shorted,
U28, pin 13 is pulled low. This prevents the associ-
ated latch from changing state, which in turn holds
the reset line of U9 high, forcing U9 into a reset con-
dition.

8Calibration
LOM 510A Series
Danger !!!
The calibration procedure requires that the top cover
of the instrument be removed. This exposes the op-
erator to hazardous voltages within the instrument.
Inparticularseveralpointsaroundpoweron/offswitch
and fuse assembly are at AC high (120 VAC).
In order to calibrate the Model 510A Micro-ohmme-
ter 5 resistor values are required. The value of each
calibration resistor should be known to better than
.005% (50 PPM). The actual value of a particular
calibration resistor is important only to the extent that
it produces a display reading of 10,000 counts (half
of full scale) or greater (but less than full scale).
Nominal values are as follows;
15 mΩ(10 to 19.999 mΩ)
150 mΩ(100 to 199.99 mΩ)
1.5 Ω(1.0 to 1.9999 Ω)
15 Ω(10 to 19.999 Ω)
150 Ω(100 to 199.99 Ω)
Ideally, all 5 resistors shouldbe fourterminaltoelimi-
nate any error due to component lead resistance.
However this is only an absolute requirement for the
three lowest ranges and is quite unimportant on the
highest range (199.99 ohms full scale).
1. The top cover must first be removed to gain
access to the trimming potentiometers. Re-
move the line cord and turn the instrument
upside down. Remove the two screws clos-
est to the rear panel, then remove two screws
holding the tilt stand to the instrument that
are closest to the front panel. Then, while
holdingthetwoclamshellhalvestogetherturn
the instrument right side up and remove the
top clamshell.
2. Reattach the line cord and turn the instrument
on. Allow the instrument to stabilize for 5
minutes before proceeding.
3. Place the instrument in the Continuous DC
Mode (mode switch is in the out position).
Push the 200 ohm range button in. Attach
the test clips to the 15 milliohm standard and
carefully adjust the front panel zero control
such that the display reads +00.01 to +00.02.
4. Push the 20 milliohm range button in and ad-
just R19 for a reading of 15.000 milliohms.
5. Place the instrument in the switched DC mode
and adjust R2 for a reading of 15.000 millio-
hms.
6. Repeat steps 3 through 5 until no further ad-
justmentis required. Normally, once through
steps 3 - 5 is all that’s required but sometimes
small thermal EMF’A cause drifts of several
counts between readings.
The switched DC mode must be used for steps 8
through 11
7. Push the 200 milli-ohm range button in and
attachthe test clips tothe150milli-ohm stan-
dard. Adjust R17 such that the display reads
the correct value.
8. Push the 2 ohm range button in and attach the
test clips to the 1.5 ohm standard. Adjust
R15 such that the display reads the correct
value.
9. Push the 20 ohm range button in and attach the
test clips to the 15 ohm standard. Adjust R13
such that the display reads the correct value.
10. Push the 200 ohm range button in and attach
the test clips to the 150 standard. Adjust R11
such that the display reads the correct value.
Chapter 5
Calibration