Amptec Research 720A User manual

13231 Rooster Springs Rd.,
Austin, TX 78737
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AMPTEC RESEARCH
MODEL 720A
OPERATION AND MAINTENANCE
MANUAL

720A Operation and Maintenance Manual – Rev S – January 25, 2019
Page 2
13231 Rooster Springs Rd.,
Austin, TX 78737
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TABLE OF CONTENTS
SECTION A: RECEIVING AND INITIAL INSPECTION
A-1 Introduction to AMPTEC 720A Digital Micro-Ohmmeter
p. 3
A-2 Unpacking and Inspection
p. 3
A-3 Setup and Preparation for Use
p. 4
SECTION B: 720A SPECIFICATIONS
B-1 Specifications for AMPTEC 720A Digital Micro-Ohmmeter
p. 5
B-2 Specifications for AC Input
p. 5
B-3 Specifications for AMPTEC 720A Battery Powered Version (720-BAT)
p. 6
SECTION C: 720A COMPATIBLE ACCESSORY ITEMS
C-1 Available Accessories and Test Leads
p. 7
C-2 Options and Interfaces
p. 7
C-3 Carrying Case
p. 8
SECTION D: OPERATION, FEATURES, AND USE
D-1 General Use
p. 9
D-2 Front Panel Features
p. 9
D-2.1 On/Off Switch
p. 10
D-2.2 Range Switches
p. 10
D-2.3 Display
p. 11
D-2.3.1 Low Battery Indicator
p. 11
D-2.3.2 Overload/Overrange Indicator
p. 11
D-2.4 Banana Jack Terminals
p. 12
D-2.5 Autorange Button and Function Information
p. 12
D-2.6 Unit of Measurement LEDs
p. 12
D-3 Rear Panel Features
p. 13
D-3.1 Fuse Holder and Fuse Information
p. 13
D-3.2 AC Inlet
p. 14
D-3.3 RS-232C Terminal Block
p. 14
D-3.4 RS-232C Connector
p. 14
D-3.5 Fan, Fan Guard, and Air Intake
p. 15
D-3.6 DC Power Jack
p. 15
D-4 Battery Holder Design and Description
p. 15
D-5 Test Lead Connections
p. 15
D-5.1 4-wire Kelvin Information
p. 16
D-6 Detailed Use Description
p. 17
D-7 Detailed Use Description for RS-232C Function
p. 19
D-7.1 RS-232C Commands and Data Format
p. 19
SECTION E: MAINTENANCE, REPAIR, AND CALIBRATION
E-1 General Maintenance, Repair, Calibration Information
p. 21
E-2 Returning the 720A to AMPTEC for Maintenance, Repair, and Calibration
p. 21
E-3 Basic Troubleshooting
p. 22
E-3.1 Battery Replacement
p. 23
E-3.2 Current Source Diagnostic Test Routine
p. 24
E-3.3 Digital Voltmeter (DVM) Test Routine
p. 24
E-4 720A Calibration Procedure Overview
p. 24
E-4.1 Equipment Required to Complete 720A Calibration Procedure
p. 25
E-4.2 Trimmer Potentiometer Adjustment Locations
p. 26
E-4.3 Calibration Procedure Adjustments
p. 27

720A Operation and Maintenance Manual – Rev S – January 25, 2019
Page 3
13231 Rooster Springs Rd.,
Austin, TX 78737
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SECTION A: RECEIVING AND INITIAL INSPECTION
A-1 Introduction to AMPTEC 720A Digital Micro-Ohmmeter
The AMPTEC 720A Digital Micro-Ohmmeter is a seven (7) range product with 4 Wire
Kelvin output, capable of making resistance measurements from 200 mΩ
to 200 KΩ. The unit's high contrast LED display is readable in direct sunlight
environments and displays your measurements with 4 ½ digit resolution. For automated
resistance measurements, the AMPTEC 720A Digital Micro-Ohmmeter can be equipped
with an optional analog output and/or RS-232C interface.
The AMPTEC 720A Digital Micro-Ohmmeter can make measurements on a variety of
devices, including (but not limited to) the following:
Winding resistance and shorted turns of: small motors, alternators, transformers,
generators, coils, solenoids, relays, wire spools, chokes, and ballasts
Contact resistance of: switches, resistive trimmers, mating connectors, relay
con-tacts, and circuit breakers
Bonding or connection resistance measurement: any metal lo metal connection,
crimp lug connections for cables, weld joints, PCB to switch or connector integrity
testing, bolted joint resistance of cable connections, and earth ground testing
Component testing, matching, and sorting: resistors (all types), pots,
strain gouges, shunts, I.C. substrates, thick film circuits, small motors and small
transformers, chokes, thermistors, fuses. extruded copper wire, Nichrome wire, all
alloys of wire, healing elements, printed circuit board clad resistance, plated through
holes, and plating thicknesses
This meter has been specifically designed to keep reliability and ease of use in mind.
Due to its integrity of measurement, ease of use, and accuracy, the AMPTEC 720A
Digital Micro-Ohmmeter has become the standard for general purpose component
resistance evaluation and testing. We’re extremely proud of the engineering and quality
that goes into each and every one of our products we’ve created over the past 20 years.
We hope you enjoy working with your 720A meter and encourage you to contact us if
you have any questions or comments. Thank you for your business!
A-2 Unpacking and Inspection
Should the box you receive your product in be damaged upon its arrival, notify AMPTEC
and the shipping carrier immediately. If your 720A meter appears damaged, the carrier’s
agent should authorize repairs before the unit is returned to our facility.
If the unit fails to operate or meet performance specifications (see section B) notify the
carrier’s agent and AMPTEC immediately. Retain the shipping container for the carrier’s
inspection.

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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Austin, TX 78737
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Do not, under any circumstance, return equipment to any of AMPTEC’s facilities or
sales offices without first obtaining an RMA number (see section E) from an AMPTEC
staff member. We must obtain your full contact information in order to properly
coordinate the repair and return of the AMPTEC meter.
A-3 Setup for Preparation and Use
The following quick-step guide can be taken to power up and use the AMPTEC 720A
meter:
1) The meter may be set up to operate as soon as the power is turned on, which
can be accomplished by flipping the “ON/OFF” switch upwards on the unit’s front
panel.
2) Allow at least fifteen (15) minutes of warm up before attempting to use the
product. If the unit has been stored at a more extreme temperature, please allow
up to thirty (30) minutes of warm up instead.
a. (Optional) Perform a zero check for test lead/probe integrity:
i. Connect a set of leads/probes to the front panel via the VHI and
VLO banana jack terminals on the unit’s front panel.
ii. Short the leads/probes by touching the ends (i.e. alligator clips,
spear tips, etc.) of each cable to each other.
iii. Select the 200 Ω range on the unit’s front panel.
iv. Reading should be within 5 display counts of zero.
Your 720A meter should be ready to use immediately following its warm up period. The
product consumes little power and generates minimal heat. It may be used in any area
where the environment does not exceed the unit’s specifications (see section B). When
possible, avoid exposing the meter to extremes of temperature, which will affect
accuracy and warm up time. One should become familiar with the basic test lead usage
and the autorange feature (if applicable) described in detail in this manual in section D-
2.4 in order to properly use the unit.
For a more detailed instructional information on how to use the 720A meter, see section
D-4.

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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SECTION B: 720A SPECIFICATIONS
B-1 Specifications for AMPTEC 720A Digital Micro-Ohmmeter
Resistance Range/Resolution:
Accuracy: Below is valid for 1 year from -20° C to 70° C when stabilized at ambient
temperature after the product warms up:
•200 mΩ range: ± .02% of reading & .05% of range
•2Ω - 200 KΩrange: ± .02% of reading; ± .03% of range
Calibration: It is recommended that the product is calibrated in 1 year intervals; see
section E-4 for calibration instructions
Dimensions: 9.25” W x 18.5” D x 2.94” H
Fuse: 2A fuse; current rating is 2A and voltage rating is 250V
Instrument Display: Display is 20,000 counts, 4 ½ digit resolution, LED
Measurement Update Rate: Appox. 3 readings per second
Open Circuit Current Source Compliance Voltage: ~1.6 VDC
Overload/Overrange Indication: Display flashes zeros
Storage Temperature: -20° C to 70° C
Temperature Coefficient: 0.1 x accuracy per degree C°
Temperature Range:
•Operating temperature: 0° C to 50° C (at rated accuracy above)
•Storage temperature: -20° C to 70° C
Terminal Configuration: 4-wire Kelvin banana jack binding posts
Weight: 4 lbs.
B-2 Specifications for AC Input
The AC Input power option for the 720A allows the user to plug the meter directly from
the AC power outlet to the meter. Meters with this option are supplied with a NEMA 5-
15P US 3 pin power cord that terminates at the meter with a IEC 60320 C-14
connection. To use this meter with AC power supplies above 125V, a power cord rated
for higher voltages is required.
AC Power Cord: AMPTEC 720-AC AC power cord
•Connector: IEC60320 C-13
•Current Rating: 10A
•Plug: NEMA 5-15P
•Voltage Rating: 125V
Internal Output Power Supply: 85V to 264V
Rear AC Inlet: AMPTEC P/N 05-75897
•Connector: IEC 60320 C-14
•Current Rating: 15A
•Voltage Rating: 250V
RANGE
200mΩ
2.0Ω
20.0Ω
200.0Ω
2.0 KΩ
20.0KΩ
200.0KΩ
NOMINAL
CURRENT
100mA 100mA 10mA 1mA 100uA 10uA 1uA
RESOLUTION
10uΩ
100uΩ
1.0mΩ
10.0mΩ
0.1mΩ
1.0Ω
10.0Ω

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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B-3 Specifications for AMPTEC 720A Battery Powered Version (720-BAT)*
Unless indicated separately below, all other specifications for the battery powered
version of the 720A match those listed in section B-1.
The 720-BATT power option for the 720A allows the user to power the meter with
rechargeable Nickel–metal hydride (NiMH) batteries. This power option comes with a
smart battery charger that has a red LED indicator when the meter is charging, and a
green LED indicator when the meter is fully charged. It is safe to leave the charger
plugged in even on a full charge, as the charger will adjust the output current on a full
charge and not damage the batteries.
Batteries: 8 D Cell NiMH; nominal capacity is 5000mAh; nominal voltage is 1.2V
Battery Charger: AMPTEC 720-DC smart charger
•Battery Charger Charging Current: 800mA typical with 1A peak
•Battery Charger Input Voltage: 90 VAC to 264 VAC
•Battery Charger LED Indicator: black LED indicates power on but no
battery connected, red LED indicates charging, green LED indicates
charged/trickle charging
•Battery Charger Output Voltage: 6.4V
•Battery Charger Trickle Current: <80mA
Charge Time: Recommended 8+ hours of continuous charge to fully charge batteries
Input: AC-DC adaptor, input voltage of 100V-240V AC
Low Battery: Left side of display will show “-“
Weight: 5.6 lbs.

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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SECTION C: 720A COMPATIBLE ACCESSORY ITEMS
C-1 Available Accessories and Test Leads*
AMPTEC is also well-versed in making custom accessories and lead/probe sets.
Contact us for any custom requirement you may have to see if AMPTEC can provide
you with a solution.
*Please note that this is not a complete list, as we offer a wide range of accessories.
Contact AMPTEC to receive the most current list of 720A compatible accessory items.
For pricing information on our packages, accessories, and lead/probe sets, please
contact AMPTEC.
C-2 Options and Interfaces*
*Please note that this is not a complete list, as we offer a wide range of options,
modifications, and interfaces. Contact AMPTEC to receive the most current list of 720A
compatible options.
720A Product Packages:
720A COMMERCIAL:
720A meter, lead pouch (OP-110), clip leads (OP-300), and
mini-probes (OP-403)
720A NSN:
720A meter, RS-232C interface (OP-232), autorange feature,
lead pouch (OP-110), clip leads (OP-300), and mini-probes
(OP-403)
720A Meter Accessories:
OP-100:
Hard shell case for meter
OP-110:
Test lead storage pouch
720-AC:
AC power cord for the 720A meter (commercial version)
720-DC:
Battery charger for the 720A meter (720-BAT version)
NIMH:
Set of 8 NiMH D cell batteries
720A Compatible Test Lead/Probe Sets:
OP-290:
Alligator clip test lead set
OP-300:
Gold plated 4-wire Kelvin clip leads
OP-302:
5-way banana jack backed Kelvin clips
OP-401:
Gold plated single tip Kelvin probes
OP-402:
Surface mount Kelvin micro-probes
OP-403:
Handheld Kelvin mini-probes
720A Compatible Options and Interfaces:
OP-232:
Optically isolated RS-232C interface
OP-RFI:
Conductive conformal coating to reduce R.F. interference
720-BAT:
Battery powered version of 720A meter
720-NS:
No spark version of 720A meter

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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C-3 Carrying Case
The AMPTEC 720A Digital Micro-Ohmmeter and its accompanying accessories can be
carried and stored safely within an OP-100 meter transit case.
An OP-100 meter transit case is capable of fitting one 720A meter, two sets of
probes/leads, and one AC power cord or battery charger. The below photo
demonstrates how a 720A meter and its accessories are
stored within a transit case.
Item A: AMPTEC 720A Digital Micro-Ohmmeter. Transit
case has custom die-cut foam insert that allows the unit to
fit snugly, allowing for minimal movement or damage to
occur while transporting.
Item B: AC power supply or battery charger (depending
upon product version). Transit case has custom die-cut
foam insert that allows power supply to fit snugly.
Item C: Two lead/probe sets (may vary from photo).
Transit case has custom die-cut foam insert that
allows two lead and/or probe sets to fit snugly.
AMPTEC is capable of creating various custom transit case configurations, as well as
outfitting the unit with a more heavy duty or waterproof transit case, across all of our
product lines. Please contact AMPTEC if interested in receiving a quote for a custom
carrying case or other accessory.

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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SECTION D: OPERATION, FEATURES, AND USE
D-1 General Use Information
The AMPTEC 720A is a micro-ohmmeter designed to accurately measure low level DC
resistance from 200 mΩto 200 KΩ. The 720A meter may be used immediately after the
unit has been turned on and been given a fifteen (15) minute warm up period
(temperature range limits permitting).
To use the 720A meter to make a resistance measurement, first connect the desired
measurement leads to the appropriately colored banana jack terminals on the unit’s
front panel. If your unit does not have the autorange feature, select your desired range
by pressing in the push button switch that matches the value needed. If your unit has
the autorange feature, you may select your desired range manually or press in the push
button switch below “AUTO” and the unit will automatically select the appropriate range
when it takes a measurement.
Next, place the measurement leads/probes across the unknown resistance value that
needs to be measured. At this point, the 720A meter should start updating the LED
display once an electrical connection has been made. If your 720A is using the
autorange feature, one of the three LEDs to the right of the display should indicate the
unit of measurement being shown (either mΩ, Ω, or KΩ). Once the measurement leads
are removed from the component that was being tested, the meter will wipe the
resistance reading from the display.
If you have any other general use questions, please contact AMPTEC.
D-2 Front Panel Features
Sub-sections D-2.1 through D-2.6 provide information on the features, design, and uses
of the 720A meter’s front panel. The below photo (Figure 1) shows the details of a
commercial 720A front panel. The key to the right of the front panel highlights the main
features that will be described further in this section.
KEY
1
On/Off Switch
2
Range Switches
3
Display
4
Banana Jack
Terminals
Figure 1
Commercial Version of 720A Front Panel

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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The below photo (Figure 2) shows the details of the front panel of the NSN version of a
720A meter. There are a few key features which differ from the previous photo, and they
will be described further in this section.
D-2.1 On/Off Switch
This switch (refer to item 1 within Figure 1 on the previous page) controls the power of
the unit. When flipped upwards, the unit is on and operational. When flipped
downwards, the unit is off and the only operation the unit can perform is the charging of
its batteries, if equipped with the 720-BAT option.
Once the switch has been flipped into the “on” position, the unit’s display should
immediately light up, indicating that the unit is on and operational. If the unit’s display
does not engage when the switch is flipped into the “on” position, check to ensure that
the unit’s AC power cord is properly inserted into both the unit’s rear panel and a power
outlet. If equipped with the 720-BAT option, this issue may indicate dead batteries,
which can be resolved by charging the unit’s batteries with the power supply provided
by AMPTEC.
If you have continued difficulty troubleshooting a unit that won’t operate when turned on,
contact AMPTEC for assistance.
D-2.2 Range Switches
These switches (refer to item 2 within Figure 2) control which range the unit is making
measurements in. The available ranges of measurement are as follows: 200 mΩ, 2 Ω,
20 Ω, 200 Ω, 2 KΩ, 20 KΩ, and 200 KΩ. When one range button is pressed, all other
buttons should be fully extended. If this is not the case, then your range switches may
be broken, and you should contact AMPTEC for assistance.
If you attempt to take a measurement that is too high for the range you have selected,
the unit will enter overrange mode. This will be indicated by the display flashing four
zeros on the screen. The unit will automatically exit overrange mode if you press the
appropriate range switch for the measurement being taken. For more information on
how the overrange mode works, please visit section D-2.3.2 on the following page.
KEY
5
Autorange Button
6
Unit of Measurement
LEDs
Figure 2
NSN Version of 720A Front Panel

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If your unit has autorange, you can still use the range switches to manually select your
range of measurement. Simply press the appropriate range switch and the unit will exit
autorange mode. For more information on how the autorange feature works along with
the range switches, please visit section D-2.5 on the following page.
D-2.3 Display
The unit features a 4 ½ digit red LED display (refer to item 3 within Figure 1 on page 9).
The unit’s display should engage and light up automatically when the unit is turned on.
All resistance measurements made by the unit are presented on this display up to
20,000 counts. The 720A’s display is durable and high quality, meaning it should have a
life lasting many years. Should the display become damaged or stop operating properly,
it can be replaced at an AMPTEC facility.
D-2.3.1 Low Battery Indicator
This feature is only present in the 720-BAT version of the 720A meter. The above
Figure 3 demonstrates the low battery indicator on the unit’s front panel. Whenever the
unit’s batteries are nearly depleted (having only 4V or less of remaining power) the unit
will indicate this with a dash appearing on the leftmost LED of the display. Once the
unit’s low battery indicator is triggered, the unit has approximately 30 minutes of usage
left before the batteries will be entirely depleted.
D-2.3.2 Overload/Overrange Indicator
If you attempt to take a measurement that is too high for the range you have selected,
the unit will enter overrange mode. An example of this would be if you were to attempt
measuring 500 mΩ on the 200 mΩ range. The unit’s way of letting the end user know
that it is not in the correct range to take a measurement will be to enter overrange
mode. Once in overrange mode, this will be indicated by the display flashing four zeros
on the screen. The unit will automatically exit overrange mode if you press the
appropriate range switch for the measurement being taken.
If your unit has the autorange feature it will automatically select the appropriate range of
measurement and will not enter overrange mode unless you are attempting to take a
resistance reading over 200 KΩ.
Figure 3
Low Battery Indicator on 720A Front Panel

720A Operation and Maintenance Manual – Rev S – January 25, 2019
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D-2.4 Banana Jack Terminals
These gold-plated banana jack terminals (refer to item 4 within Figure 1 on page 9)
enable the end user to make 4-wire measurements by acting as connectors for
cable/lead/probe sets. They are compatible with any 4-wire cable, lead, or probe set
that has banana jacks. Simply connect your cable set by inserting your black cable(s)
into the VLO terminals, and your red cable(s) into the VHI terminals. You may also
connect bare wires to these terminals by unscrewing the plastic caps, which then
exposes gold posts that the wires can be attached to. For more information on test lead
connections to these terminals and how to use test leads to make measurements, refer
to section D-5.
D-2.5 Autorange Button and Function Information
This button (refer to item 5 within Figure 2 on page 10) is only present in the NSN
version of the 720A meter. Pressing this button enables the unit to automatically select
its range of measurement, thus activating the unit’s autorange mode. To use this
button/feature, first ensure that all other range buttons are fully extended (deselected)
and press the AUTO button. If you would like to disable this feature and manually select
your range of measurement, you may simply press the appropriate range switch and the
autorange feature will turn off. The autorange function will automatically select the
correct resistance range, given that the reading is between 0 Ωand 200 KΩ. If the
reading is above 200 KΩ, the meter will enter overrange mode until a lower reading is
applied or the autorange is turned off/deselected. For more information on overrange
mode, see section D-2.3.2 on the previous page.
D-2.6 Unit of Measurement LEDs
This feature (refer to item 6 within Figure 2 on page 10)is only present in the NSN
version of the 720A meter. These three LEDs indicate which unit of measurement is
being displayed (either mΩ, Ω, or KΩ) when using the unit. Only one LED should be lit
per measurement. No action is required by the end user to enable this feature – these
LEDs should engage automatically when the unit is turned on.

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D-3 Rear Panel Features
The following sub-sections, D-3.1 through D-3.6, provide information on the features,
design, and uses of the 720A meter’s rear panel. The below diagram (Figure 4) shows
the details of the NSN version of a 720A rear panel. The key to the right of the rear
panel highlights the main features that will be described further in this section:
The below diagram (Figure 5) shows the details of the Commercial version of a 720A
rear panel. The key to the right of the diagram highlights which feature will be described
further in this section:
Please note that the appearance and layout of your unit’s rear panel may vary from
these diagrams slightly depending upon the options that your unit is outfitted with and its
year of manufacture.
D-3.1 Fuse Holder and Fuse Information
This cap (refer to item 1 within Figure 4) holds the unit’s 2A fuse in place. The unit will
accept any 3AG fuse that has a current rating of 2A and a voltage rating of 250V. The
cap can be turned clockwise to provide access to the fuse. Once the cap has been
unscrewed from the unit, you may delicately pull on the fuse to remove it. To insert a
KEY
1
Fuse Holder
2
RS-232C Terminal
Block
3
Fan Guard
4
AC Inlet
5
RS-232C Connector
KEY
6
DC Power Jack
Figure 4
NSN Version of 720A Rear Panel
Figure 5
720-BAT Version of 720A Rear Panel

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new fuse, first ensure that the polarity of the fuse is correct, and then gently push it into
the receiver on the inside of the cap.
D-3.2 AC Inlet
This AC inlet (refer to item 2 within Figure 4 on page 13) acts as a receiver for the AC
power cord (P/N 720-AC) that comes with the unit. Once an AC power cord has been
plugged into this AC inlet, you may connect its other end to an outlet. This AC inlet is an
IEC 60320 C-14 connector. Its current rating is 15A and its voltage rating is 250V. If
operating this unit from outside of the U.S., you may use any AC power cord that has a
60320 C-13 connector (this ensures that is compatible with the unit’s inlet) and a max
voltage rating of 250V or less.
D-3.3 RS-232C Terminal Block
This terminal block (refer to Figure 6 above) allows resistances readings to be taken
from the rear of the meter. Each gold terminal is electrically connected to the front panel
post of the same label.
To take a reading from the rear panel, connect the measurement leads to each pin of
the rear terminal block using the rear terminal screws shown in Figure 6. Clip or ring
tongue termination ends fasten securely to the screws. Connect the opposite ends of
the measurement leads to the unit under test (UUT) in the same configuration as a front
panel reading. Note that the AMPTEC leads/probes supplied with your unit are not
compatible with the rear panel terminal.
Lastly, please note that readings taken from the front panel and the rear panel may
differ from each other slightly slightly (within 5 counts).
D-3.4 RS-232C Connector
This connector (refer to item 4 within Figure 4 on page 13) is a DE-9 female IDC
connector that supports RS-232C connections. You may use any cable that has at least
one DE-9 male end to connect this port to your computer. As it is rare for modern
computers to come standard with an RS-232C port, you may use an RS-232C to USB
Figure 6
Rear Terminal Block

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adapter to connect your 720A meter to your computer. Connection to this port from your
computer allows you to send commands directly to your 720A meter. For more
information on how to use the RS-232C function, see section D-7.
D-3.5 Fan, Fan Guard, and Air Intake
The fan guard (refer to item 5 within Figure 4 on page 13) protects the unit’s internal fan
and prevents obstructions from the vent. It is important that you assist the fan by not
unnecessarily obstructing or covering the vent while your unit is on. Additionally, the unit
has seven (7) holes on the bottom plate that draw in outside air. These intake holes
work with the fan to cool the unit, and it is important that they are also not obstructed or
covered while operating your 720A. A unit that cannot properly intake air through the
bottom holes or expel heat through the vent may overheat, which can lead to less
accurate measurements.
D-3.6 DC Power Jack
This feature (refer to item 6 within Figure 4on page 13) is only present in the 720-BAT
battery powered version of the 720A meter. This DC power jack acts as a receiver for
the battery charger (P/N 720-DC) that comes with the unit. It is through this jack that the
unit receives power that charges its batteries. This DC power jack has a current rating
of 5A and a voltage rating of 12V. We do not recommend attempting to charge the unit’s
batteries with any charger not supplied by AMPTEC. If operating this unit from outside
of the U.S., please contact AMPTEC for assistance in purchasing a different variation of
charger or an outlet adaptor.
D-4 Battery Holder Design and Description
The battery powered version of the 720A Digital Micro-Ohmmeter has two battery
holders inside which are affixed to the bottom plate of the unit. Each battery holder is
made of a durable black plastic and houses 4 D cell NiMH batteries for a total of 8
batteries per unit (for more information on battery specifications see section B-3).
One safety feature the unit has is a temperature fuse that is installed on each battery
holder. This fuse ensures the batteries do not overheat and cause damage to the unit.
Another safety feature is the addition of a flame-retardant laminate cover plate to protect
the batteries from coming in contact with any leads on the PC board, which would in
turn short out the unit and prevent it from functioning properly.
Lastly, both battery holders are then sealed with a layer of durable fiberglass tape to
prevent any battery shifting, as well as to provide a final level of protection.
D-5 Test Lead Connections
The AMPTEC 720A meter test lead connection uses 4 banana jack terminals on its front
panel. These inputs are gold-plated and accept a variety of connections. Most
commonly, cables with a banana jack on one end may easily plug into the front of these
terminals. If you would like to use a different cable set that does not have banana jacks,
this is achievable by unscrewing the plastic that covers these jacks. This exposes a
horizontal hole through the gold-plated post which you can insert a bare wire into. Once

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the wire is through the hole in the post, you can then re-tighten the plastic by screwing it
clockwise, which secures your electrical connection.
The OP-300 48” gold-plated Kelvin clip lead set and the OP-403 48” Kelvin mini spear
probe set that come standard with the 720A meter both plug directly into the banana
jack terminals on the front panel. The spacing of the jacks only allows horizontal
connection of the dual banana jack test leads to prevent misconnection.
D-5.1 4-wire Kelvin Information
In many resistance measurement applications, the connection contact resistance can
exceed the value of the test resistance by several orders of magnitude. However, the 4
terminal configuration of the AMPTEC 720A meter eliminates measurement errors
typically caused by in series test lead or test lead contact resistance. Figure 7below
and the rest of this section provide a detailed description of how this is achieved.
Figure 7illustrates the 4-wire Kelvin principle which eliminates lead wire length and test
lead connection contact resistances as potential sources of measurement errors. The
internal constant current source inherently overcomes all in series resistance (within
voltage compliance limits) and delivers a precise constant current. Separate DC to DC
isolation circuitry provides independent circuit voltage supplies for both polarities of the
constant current source circuit. The internal high impedance digital voltmeter then
senses the voltage drop across the test resistance.
Fundamentally, the 4-wire Kelvin resistance method avoids measurement errors
induced from the resistance of the test lead/probes and the contact resistance of the
lead/probes to the device under test. It doesn’t matter if the current loop is longer or if
there is some contact resistance with the connection under test, because the constant
current source’s compliance voltage works to maintain constant current flowing in the
loop.
Figure 7
4-wire Kelvin Diagram

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D-6 Detailed Use Description
This section provides a detailed description of how to use the 720A Digital Micro-
Ohmmeter to take resistance measurements.
Firstly, ensure that the unit is appropriately plugged into a power outlet. If using the 720-
BAT battery powered version of the meter, it is ideal that the unit’s batteries will be
charged before attempting to use the product. If your unit’s batteries are dead you can
operate the meter while charging the batteries at the same time but should allow an
additional ten (10) minutes in addition to the standard fifteen (15) minute warm up time
before attempting to take measurements.
The meter may be set up to operate as soon as the power is turned on, which can be
accomplished by flipping the “ON/OFF” switch upward on the unit’s front panel. The
unit’s display should engage immediately. One should allow the unit at least fifteen (15)
minutes of time to warm up before attempting to make measurements. If operating a
720A meter in a more extreme climate, a warm up period of thirty (30) minutes is ideal.
This warm up period allows the unit to adjust to the room’s ambient temperature and
enables it to accurately take/display measurements. During this warm up period, one
may also choose to perform a zero check for test lead/probe integrity if desired. Unless
performing test lead/probe integrity checks, the unit should otherwise be left alone while
turned on during this warm up period.
(Optional) Procedure to perform a zero check on a test lead/probe set:
i. Connect a set of leads/probes to the front panel via the VHI and
VLO banana jack terminals on the unit’s front panel.
ii. Short the leads/probes by touching the ends (i.e. alligator clips,
spear tips, etc.) of each cable to each other.
iii. Select the 200 Ω range on the unit’s front panel.
iv. Reading should be within 5 display counts of zero.
Figure 8
720A Usage Example Diagram

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While not mandatory, it is also recommended that one perform a quick visual inspection
of the test leads/probes being used to locate any obvious damage. Damaged cables,
banana jacks, or probe/lead ends can negatively impact the validity and accuracy of
measurements.
Once the unit’s warm up period has elapsed, you may then connect your supplied
AMPTEC test leads/probes to the appropriate corresponding jacks on the unit’s front
panel. On any AMPTEC test lead/probe set, the red cable should connect to the unit’s
HI terminals while the black cable should connect to the unit’s LO terminals.
Next, select the desired range of measurement for the test you are completing. If the
resistance value of the part you are testing is unknown, select the 200 KΩ range. If your
unit has autorange and you would like to enable this feature, select the “AUTO” button
on the unit’s front panel.
To complete bond testing measurements (see Figure 8), find a clean area on both the
part you need to test (meaning an area with no paint or coating) and the metal chassis
of your unit under test (UUT). Pointed probes (such as the AMPTEC OP-403) work best
on aluminum parts, as this material often has a thin coating that is a non-conductor.
Press and hold each probe with a steady hand – one on the part under test and one on
the metal chassis – and do not move the probes for 3 to 5 seconds. If using the
autorange function, the meter needs a little settling time to automatically determine
which range is best before displaying a resistance value. If your 720A meter does not
have an autorange function and the resistance value is unknown, start on the 200 KΩ
range. Once a resistance reading is displayed on the screen, you can then select/move
to a lower range to decrease the resolution value until the desired range is met.
Resistance testing of singular parts/units can be completed much the same way, but
instead of using one probe/lead on the chassis and one on the part under test, both
probes/leads would be placed on the part under test. Depending on the circumstance,
the supplied AMPTEC OP-300 Kelvin clip leads may be more appropriate for use than
the supplied AMPTEC OP-403 mini spear probe set.
The 720A Digital Micro-Ohmmeter consumes little power and generates minimal heat. It
may be used in any area where the environment does not exceed the unit’s
specifications (see section B). When possible, avoid exposing the meter to extremes of
temperature, which will affect accuracy and warm up time. If using your unit in areas
with an extreme temperature, please see section B to determine the temperature
coefficient that will need to be applied to your measurements.
If you have any further questions on how to use your 720A meter, please contact
AMPTEC.

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D-7 Detailed Use Description for RS-232C Function
The RS-232C Interface for the AMPTEC 720A Digital Micro-Ohmmeter allows the user
to communicate with the meter through a serial connection from a computer. The RS-
232 communication interface can be used to switch measurement ranges, query
resistance readings from the meter, and read the firmware version of the meter. Any
software application that can send and receive serial communication messages can be
used to communicate with your 720A meter (e.g. HyperTerminal, PuTTY, Termite).
Once the communication protocol settings (listed in section D-7.1) are met, the meter
should be connected to a computer using a DE-9 connector. Most modern computers
do not have a built-in DE-9 serial port, but a USB-to-DE-9 converter will work if properly
installed. If a command is sent from the user to the meter, the meter will respond that a
command has been received. To test that your unit and the computer are connected
correctly, send a “V” command to read the firmware version back from the meter.
Additional commands are listed in the following section D-7.1.
Note: Readings taken through the RS-232C communication interface may be from
either the front or rear panel.
D-7.1 RS-232C Commands and Data Format
This section details the commands necessary to communicate with your 720A meter via
its RS-232C connection. The RS-232C communication protocol for the 720A is listed
below:
Baud Rate: 9600
Data Bits: 8
Stop Bits: 1
Parity: None
Please note that the RS-232C Interface will not function correctly unless all the physical
range selection buttons are deselected (all the way out).
RS-232C Commands
Please note that the following commands are case-sensitive.
CContinuous Read Mode: Meter will output readings to computer for every reading
computed by the digital voltmeter (DVM). To turn this mode off, enter a ‘S’
command to switch to Single Read Mode.
SSingle Read Mode: Meter will output one reading every time it receives a ‘R’
command from the computer. User must enter this command to turn off Continuous
Read Mode. Meter is in Single Read Mode by default.
RRead Command: When in Single Read Mode, meter will output one reading when
this command is received. When in Continuous Read Mode, this command will not
appear to have any effect.

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r0 Deselects all ranges.
r1 Selects the 200 mΩ range.
r2 Selects the 2 Ωrange.
r3 Selects the 20 Ωrange.
r4 Selects the 200 Ωrange.
r5 Selects the 2 KΩ range.
r6 Selects the 20 KΩ range.
r7 Selects the 200 KΩrange.
Data Format
The RS-232C Interface outputs reading data in the following format:
1.2345E+4 The measurement is always in ohms and the number following “E” is the
exponent multiplier. For example, 1.2345E+4 is equal to 12.345 KΩ.
Range Exponent
200 mΩE-1
2 ΩE+0
20 ΩE+1
200 ΩE+2
2 KΩE+3
20 KΩE+4
200 KΩE+5
Please note that an overrange reading will be indicated by 9.9999E<pn>, where p is the
polarity of the exponent and n is the multiplier of the range. The range exponents are
listed above.