Keithley Model 168

Keithley Model 168 Autoranging Digital Multimeter Technical Description

Overview

The Keithley Model 168 is a versatile, 4-digit autoranging digital multimeter (DMM) designed for precise measurements of AC/DC voltage, AC/DC current, and resistance. Introduced in 1974, this compact, tabletop instrument is suitable for laboratory, service, and field applications, offering automatic range and polarity selection for ease of use. Its robust design and comprehensive measurement capabilities make it ideal for electronics technicians, engineers, and researchers.

Key Specifications

General

• Display: 4-digit LED with automatic decimal point, function (AC/DC), and measurement unit indication

• Autoranging: Automatic range and polarity selection

• Power Supply: 117 V or 234 V AC (selectable), 48–440 Hz; optional Model 1688 rechargeable battery set for line or battery operation

• Dimensions: 240 mm (W) x 83 mm (H) x 270 mm (D) (9.4 x 3.3 x 10.6 inches)

• Weight: Approximately 0.4 kg (0.881 lb) without battery pack

• Material: Durable plastic housing (no bakelite or catalin)

Voltage Measurement

• DC Voltage Range: ±0.0001 V to ±1000 V

• AC Voltage Range: 0.0001 V to 500 V (RMS)

• Accuracy:

  • DC: ±(0.2% of reading + 0.1% of range)

  • AC: ±(0.5% of reading + 0.2% of range) at 50 Hz–1 kHz

• Input Impedance: 10 MΩ

• Maximum Input Voltage: 1000 V DC or 500 V AC RMS

Current Measurement

• Range: 0.0001 mA to 1 A (AC and DC) in two spans

• Accuracy:

  • DC: ±(0.5% of reading + 0.2% of range)

  • AC: ±(1% of reading + 0.3% of range) at 50 Hz–1 kHz

• Maximum Input Current: 1 A (protected by internal fuse)

Resistance Measurement

• Range: 0.1 Ω to 20 MΩ in two overlapping spans

• Accuracy: ±(0.3% of reading + 0.2% of range)

• Test Current: Varies by range, typically 1 mA for lower ranges

• Maximum Open-Circuit Voltage: 5 V

Additional Features

• Measurement Functions: AC/DC voltage, AC/DC current, resistance

• Overload Protection: Input circuits protected against overloads up to specified maximums

• Indicators: LED display shows digits, decimal point, function (AC/DC), and measurement units (V, A, Ω)

• Branding Variants: Also marketed as Rohde & Schwarz Model MM635 and Racal Dana Model 4003

Power Requirements

• Line Operation: 117 V or 234 V AC, 48–440 Hz

• Battery Operation: Optional Model 1688 rechargeable battery set (specifications not detailed in sources)

• Power Consumption: Not specified, but designed for low power usage in battery mode

Environmental Characteristics

• Operating Temperature: 0°C to 50°C

• Storage Temperature: -40°C to 75°C

• Humidity: Up to 80% RH at 40°C

• Altitude: Operating up to 2,000 m

Features

• Autoranging: Automatically selects the appropriate range and polarity, simplifying operation

• High Sensitivity: Measures down to 100 μV for voltage and 100 nA for current

• Versatile Measurement: Supports AC/DC voltage, AC/DC current, and resistance with high accuracy

• Compact Design: Lightweight and portable for bench or field use

• Clear Display: 4-digit LED with automatic decimal placement and function/unit indication

• Optional Battery Operation: Enhances portability with the Model 1688 battery set

• Serviceability: Supported by Keithley’s service facilities in the USA, England, and West Germany (as of 1974)

Applications

The Keithley Model 168 is suitable for:

• Laboratory Testing: Measuring voltage, current, and resistance in circuit design and analysis

• Field Service: Troubleshooting electronic equipment with portable battery operation

• Production Testing: Verifying component specifications in manufacturing

• Education: Teaching electrical measurement principles in academic settings

Additional Notes

• Manual Availability: The instruction manual (Rev. A) is available from Tektronix, providing detailed guidance on operation, power selection, and troubleshooting.

• Historical Context: Manufactured by Keithley Instruments Inc., Cleveland, OH, and introduced in 1974. Schematics and manuals are accessible via the BAMA archive.

• Accessories: Includes standard test leads; optional Model 1688 battery set for portable operation.