The AMPTEC 720A Digital Micro-Ohmmeter is a seven (7) range product designed for accurate low-level DC resistance measurements, ranging from 200 mΩ to 200 kΩ. It features a 4-wire Kelvin output, which is crucial for eliminating measurement errors caused by test lead and contact resistance. The unit's high-contrast LED display provides 4 ½ digit resolution, making measurements readable even in direct sunlight. For automated resistance measurements, the 720A can be equipped with optional analog output and/or an RS-232C interface.
Function Description:
The 720A is a versatile micro-ohmmeter capable of measuring resistance in a variety of applications. These include:
- Winding resistance and shorted turns: Used for small motors, alternators, transformers, generators, coils, solenoids, relays, wire spools, chokes, and ballasts.
- Contact resistance: Ideal for switches, resistive trimmers, mating connectors, relay contacts, and circuit breakers.
- Bonding or connection resistance: Measures resistance in metal-to-metal connections, 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: Suitable for resistors (all types), pots, strain gauges, shunts, I.C. substrates, thick film circuits, small motors and small transformers, chokes, thermistors, fuses, extruded copper wire, Nichrome wire, all alloys of wire, heating elements, printed circuit board clad resistance, plated through holes, and plating thicknesses.
The 4-wire Kelvin principle employed by the 720A eliminates lead wire length and test lead connection contact resistances as potential sources of measurement errors. An 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. A high-impedance digital voltmeter then senses the voltage drop across the test resistance. This design ensures that the constant current source's compliance voltage works to maintain constant current flowing in the loop, regardless of the length of the current loop or contact resistance.
Important Technical Specifications:
- Resistance Ranges/Resolution:
- 200mΩ: 100mA nominal current, 10uΩ resolution
- 2.0Ω: 100mA nominal current, 100uΩ resolution
- 20.0Ω: 10mA nominal current, 1.0mΩ resolution
- 200.0Ω: 1mA nominal current, 10.0mΩ resolution
- 2.0KΩ: 100uA nominal current, 0.1mΩ resolution
- 20.0KΩ: 10uA nominal current, 1.0Ω resolution
- 200.0KΩ: 1uA nominal current, 10.0Ω resolution
- Accuracy: Valid for 1 year from -20°C to 70°C when stabilized at ambient temperature after warm-up.
- 200 mΩ range: ±0.02% of reading & ±0.05% of range
- 2Ω - 200 KΩ range: ±0.02% of reading; ±0.03% of range
- Calibration Interval: Recommended 1 year.
- Dimensions: 9.25" W x 18.5" D x 2.94" H
- Fuse: 2A, 250V (3AG type)
- Instrument Display: 20,000 counts, 4 ½ digit resolution, LED
- Measurement Update Rate: Approx. 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°
- Operating Temperature: 0°C to 50°C (at rated accuracy above)
- Terminal Configuration: 4-wire Kelvin banana jack binding posts
- Weight: 4 lbs. (720A), 5.6 lbs. (720-BAT)
- AC Power Cord (AMPTEC 720-AC): IEC60320 C-13 connector, 10A current rating, NEMA 5-15P plug, 125V voltage rating.
- Internal Output Power Supply: 85V to 264V
- Rear AC Inlet (AMPTEC P/N 05-75897): IEC 60320 C-14 connector, 15A current rating, 250V voltage rating.
Battery Powered Version (720-BAT) Specifications:
- Batteries: 8 D Cell NiMH, 5000mAh nominal capacity, 1.2V nominal voltage.
- Battery Charger (AMPTEC 720-DC smart charger):
- Charging Current: 800mA typical with 1A peak
- Input Voltage: 90 VAC to 264 VAC
- LED Indicator: Black (power on, no battery), Red (charging), Green (charged/trickle charging)
- Output Voltage: 6.4V
- Trickle Current: <80mA
- Charge Time: Recommended 8+ hours for full charge.
- Input: AC-DC adaptor, 100V-240V AC input voltage.
- Low Battery Indicator: Left side of display shows “-“.
RS-232C Interface Specifications (Optional OP-232):
- Baud Rate: 9600
- Data Bits: 8
- Stop Bits: 1
- Parity: None
- Connector: DE-9 female IDC connector.
- Commands: Case-sensitive.
C: Continuous Read Mode (meter outputs readings for every DVM computation).S: Single Read Mode (meter outputs one reading per R command; default mode).R: Read Command (in Single Read Mode, outputs one reading).r0: Deselects all ranges.r1 to r7: Selects specific resistance ranges (200 mΩ to 200 KΩ).
- Data Format:
1.2345E+4 (measurement in ohms, E indicates exponent multiplier). Overrange indicated by 9.9999E<pn>.
Usage Features:
- Power On/Off: Controlled by a front panel switch. Allow 15-30 minutes warm-up for optimal accuracy.
- Range Selection: Manual range selection via push-button switches (200 mΩ, 2 Ω, 20 Ω, 200 Ω, 2 KΩ, 20 KΩ, 200 KΩ).
- Autorange (Optional): The "AUTO" button (on NSN version) automatically selects the appropriate range between 0 Ω and 200 KΩ. Manual range selection disables autorange.
- Overrange Indication: Display flashes four zeros when the measurement exceeds the selected range.
- Banana Jack Terminals: Gold-plated, 4-wire Kelvin terminals (VHI, VLO, IHI, ILO) on the front panel for connecting test leads/probes. Compatible with banana jacks or bare wires (by unscrewing plastic caps).
- Rear Terminal Block (Optional): Allows resistance readings from the rear panel via screw terminals. Electrically connected to front panel posts.
- RS-232C Connector (Optional): DE-9 female IDC connector for serial communication with a computer, enabling remote control and data acquisition.
- Zero Check: Optional procedure to check test lead/probe integrity by shorting leads and selecting the 200 Ω range; reading should be within 5 display counts of zero.
- Bond Testing: Requires clean contact areas on the part under test and metal chassis. Pointed probes (e.g., OP-403) are effective on aluminum. Hold probes steady for 3-5 seconds.
- Component Testing: Similar to bond testing, but both probes/leads are placed on the part under test. Kelvin clip leads (e.g., OP-300) may be more appropriate.
Maintenance Features:
- Fuse Replacement: The unit uses a 2A, 250V 3AG fuse, accessible by unscrewing the fuse holder cap on the rear panel.
- Troubleshooting:
- Unit not turning on: Check battery charge (if applicable), power outlet, and external fuse.
- Inaccurate readings: Verify operating temperature within specifications, allow sufficient warm-up time (15-30 minutes), and consider recalibration.
- Internal checks (voids warranty): Ensure batteries are not dislodged (720-BAT), LED display board is securely plugged in, and ICs (IC1, IC2, IC9) are properly seated in their sockets. An ESD bracelet is recommended if opening the unit.
- Battery Holder Design (720-BAT): Two battery holders, each housing 4 D cell NiMH batteries. Features a temperature fuse to prevent overheating and a flame-retardant laminate cover plate to protect against shorts. Sealed with fiberglass tape to prevent shifting.
- Current Source Diagnostic Test: Select 2 Ω range, connect output leads across a 1 Ω resistance standard, measure voltage drop with DC Voltmeter (should be 0.100V).
- Digital Voltmeter (DVM) Test: Select 200 mΩ range, short output leads (should read 0.000 ± 5 counts), then connect across a 0.100 Ω resistance standard (should read 100.00 mΩ ± 10 counts).
- Calibration Procedure:
- Overview: Recommended annually, but can be done more often. Requires 30 minutes warm-up, fully charged batteries (if applicable), and optimal ambient conditions (21°-25°C, 37%-42% humidity, minimal RF interference). Opening the unit for calibration voids the warranty.
- Equipment Required: Precision standard resistors (0.001 Ω to 100 KΩ with 0.005% or better accuracy), trimmer potentiometer adjustment tool, AMPTEC OP-300 or OP-304 lead set.
- Trimmer Potentiometer Adjustment Locations: RV1 (full-scale on all ranges), RV2 (zero-scale on all ranges), RV5 (zero-scale on 200 mΩ range), RV6 (full-scale on 200 mΩ range). RV3 and RV201 are not used for standard recalibration.
- Adjustment Steps:
- Zero Adjustment: 200 Ω range, 1.0 Ω standard, adjust RV2 for 1.00 Ω reading.
- Full-Scale Adjustment: 100 Ω standard, adjust RV1 for 100.00 Ω reading.
- 200 mΩ Adjustment: This step calibrates the 200 mΩ range, which is affected by RV5 and RV6.
- 200 mΩ Zero Adjustment: 200 mΩ range, 0.001 Ω standard, adjust RV5 for 01.00 mΩ reading.
- 200 mΩ Full-Scale Adjustment: 200 mΩ range, 100 mΩ standard, adjust RV6 for 100.00 mΩ reading.
- After calibration, verify all ranges against standards before closing the unit.
- Factory Service: AMPTEC RESEARCH provides maintenance, repair, and calibration services. An RMA number is required before sending units to the facility. Diagnosis is free, but opening the product for diagnosis incurs a calibration fee if not under warranty, as all instruments are calibrated before leaving the facility.
- Fan, Fan Guard, and Air Intake: The rear fan guard protects the internal fan. Ensure the vent and seven bottom intake holes are not obstructed to prevent overheating and maintain measurement accuracy.
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