Leader Electronics Corp. LCR-740

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Overview

Important Technical Specifications
Resistance Measurement
Capacitance Measurement
Inductance Measurement
Dissipation Factor (D) and Quality Factor (Q) Measurements
Measuring Sources
Power Supply
Physical Characteristics
Environmental Conditions
Accessories Supplied
Usage Features
Measurement Procedures
Maintenance Features

The Leader LCR-740 is a self-contained LCR bridge designed for precise measurements of resistance (R), capacitance (C), and inductance (L) across a broad spectrum. It features a 3-digit readout for measured values and includes an internal battery for DC resistance measurements, as well as a 1kHz oscillator and null detecting amplifier system for AC measurements of capacitance and inductance. The instrument also provides input connections for external DC or AC (50Hz to 40kHz) sources and an AC adapter (LPS-169).

Important Technical Specifications:

Resistance Measurement:

Range: 0.001Ω to 11MΩ in eight ranges, with a +10% extension at each range.
Minimum Resolution: 1mΩ (0.001Ω).
Accuracy (at 20° ±5°C):
- 1Ω to 100kΩ: ±(0.5% + 0.1% f.s.)
- 1MΩ: ±(1% + 0.1% f.s.)
- 0.1Ω: ±(2% + 0.1% f.s.)
Residual Terminal Resistance: Approximately 3mΩ.

Capacitance Measurement:

Range: 1pF to 11,000µF in eight ranges, with a +10% extension at each range.
Minimum Resolution: 1pF.
Accuracy (at 20° ±5°C):
- 1000pF to 100µF: ±(0.5% + 0.1% f.s.)
- 100pF: ±(1% + 0.1% f.s.)
- 1000µF: ±(3% + 0.1% f.s.)
Residual Terminal Capacitance: Approximately 3pF.

Inductance Measurement:

Range: 0.1µH to 1100H in eight ranges, with a +10% extension at each range.
Minimum Resolution: 0.1µH.
Accuracy (at 20° ±5°C):
- 100µH to 10H: ±(0.5% + 0.1% f.s.)
- 100H: ±(1% + 0.1% f.s.)
- 10µH: ±(3% + 0.1% f.s.)
Residual Terminal Inductance: Approximately 0.3µH.

Dissipation Factor (D) and Quality Factor (Q) Measurements:

Range: 0.01 to 30, at 1kHz, in two ranges.
Accuracy: ±10% + 3 scale divisions.

Measuring Sources:

DC: Internal or external for resistance measurements.
AC: Internal 1kHz, or external 50Hz to 40kHz, for capacitance and inductance measurements.

Power Supply:

9V DC (006P, NEDA 1604, or equivalent).

Physical Characteristics:

Size and Weight: 240(W) x 85(H) x 170(D) mm; approximately 2kg.

Environmental Conditions:

Operating Temperature Range: 0 to 40°C.
Operating Humidity Range: 0 to 85%.

Accessories Supplied:

Earphone (1)
9V battery (1)
Mini-plug-clip cable (1)
AC adapter, LPS-169 (Option, Separate Order)

Usage Features:

The LCR-740's front panel includes several controls for intuitive operation:

POWER switch and SENSITIVITY control: This combined control turns the unit on/off and adjusts the detector sensitivity for AC measurements.
RCL indicator: A 3-digit display shows measured values, which are determined by the knob rotation and the settings of the RANGE MULTIPLIER switches.
Lock knob: Used to lock the RCL indicator at a specific setting for tolerance testing, normally kept in the "free" position.
NORMAL +1.00 switch: Typically set to NORMAL for direct readout. The "+1.00" setting extends the upper measuring limit by 10% when readings exceed 9.99 in a given range.
RANGE MULTIPLIER switches: These buttons select the appropriate measurement range for the component under test.
SELECTOR switch: Set to R, C, or L depending on the component type being measured.
NULL indicator: A 10-0-10 scale for DC resistance and a 0-10 scale (right of 0) for capacitance and inductance measurements, indicating bridge balance.
Mechanical zero adjust: A screw for fine-tuning the NULL indicator's zero point.
D, Q dial: Features two scales: an outer scale for dissipation factor (D) and an inner scale for equivalent series resistance (RE), calibrated at 1kHz.
X1 - X10 switch: Selects the multiplier for the D and RE readings on the D, Q dial.
SOURCE switch: Selects the internal bridge operating source (DC for resistance, AC at 1kHz for R, C, and L).
RED HI and BLUE EXT. +DC terminals: Connections for the unknown component. These terminals are "floating" from ground.
BLACK terminal: For grounding the unit to the case.

Rear and front side connections include:

Cover fastener: Secures the battery compartment.
Carrying handle: For portability.
EXT. SIG. IN jack: For external AC sources (50Hz to 40kHz). Inserting a mini-plug automatically disables the internal 1kHz oscillator.
PHONE jack: For earphone use, providing an audible signal in conjunction with the meter for null detection.
EXT. PWR IN jack: For connecting the LPS-169 AC Adapter, which automatically turns off the internal battery.

Measurement Procedures:

Resistance (DC) Measurements: Connect the component, set SELECTOR to "R" and SOURCE to "DC/R", turn POWER on, and rotate the RCL knob to achieve a null (center 0) on the NULL meter. Read the value from the RCL indicator and apply the range multiplier. For the 1MΩ range, external DC sources may be preferred for better null definition. Residual terminal resistance must be accounted for in the 0.1Ω range.
Capacitance Measurements: Set SELECTOR to "C", SOURCE to "AC/R.C.L.", D,Q dial to 0, D,Q switch to X1, and POWER on. Adjust SENSITIVITY for a null meter reading around "5". Connect the component and adjust RANGE MULTIPLIER and RCL knob for minimum pointer swing. Fine-tune the D,Q dial for null. Read capacitance from the RCL indicator and apply the multiplier. Dissipation factor (D) and equivalent series resistance (Rs) can also be determined. Residual terminal capacitance should be considered for measurements up to 1000pF.
Inductance Measurements: Similar to capacitance, set SELECTOR to "L", SOURCE to "AC/R.C.L.", D,Q dial to approximately 0.3, D,Q switch to X1, and POWER on. Adjust SENSITIVITY to move the meter needle to the right (between 6 and 7). Connect the component and rotate the D,Q dial to find a clear "dip" point. Then, adjust the RCL digital dial for the largest dip. If a dip is not found, adjust the D,Q switch to X10 or change the range. Read inductance from the RCL indicator and apply the multiplier. Quality factor (Q) and equivalent series resistance (Rs) can also be calculated. For iron core inductors, external oscillators are recommended to measure inductance at desired frequencies and account for B/H curve non-linearity.
External Measuring Sources: The LCR-740 supports external DC sources for high-value resistance measurements and external AC sources (50Hz-40kHz) for C and L measurements. When using external DC, protective series resistors are crucial to prevent damage. For external AC, connect the generator to the EXT. SIG. IN jack.
Large Capacitances: For high-capacitance components (e.g., electrolytics), measurements should be made at a low frequency (e.g., 120Hz) using an external AC source to minimize inductive effects.
Transformer Measurements: The turns ratio can be determined by measuring the inductance of windings under no-load conditions. Leakage inductance can be measured by shorting secondary windings and measuring from the primary side.
Tolerance/Error Measurements: For checking components within given limits, a "standard" component is measured and its null point locked. Subsequent components are then tested, and the error is noted from the NULL meter.

Maintenance Features:

Battery Replacement: The internal 9V battery (006P, NEDA 1604, or equivalent) should be replaced when the 1kHz oscillator becomes inoperative. This is indicated if the meter pointer does not swing when the SOURCE switch is set to AC/R.C.L. The battery compartment is accessed by pulling out the captive fastener on the back cover.
General Precautions:
- Always turn the POWER switch OFF when the bridge is not in use to prolong battery life.
- Regularly check the NULL meter's center 0 setting for DC resistance measurements and adjust if necessary.
- Keep connections to the measuring terminals (RED and BLUE) as short as possible, especially for small value measurements.
- Do not force rotation of the RCL indicator knob if it is locked.
- Only use the specified AC adapter (LPS-169).
- When using external DC sources, always set high voltage input to OFF before making lead connections, avoid contact with high voltages, use a protective resistor, and turn off DC input before changing RANGE MULTIPLIER settings to prevent damage.