Fluke 887AB Differential Voltmeter  
Written by AnubisTTP on 2006-09-17  


The Fluke 887AB AC/DC differential voltmeter functions like a comparator. Instead of displaying the voltage of the object being tested, the user selects a voltage on the large numeric readout and the meter tells the user how close it is to the actual voltage of the tested device. Though unwieldy, the 887AB can measure voltage increments as small as 0.01mV. This is far more precise than most of its contemporaries, perhaps making it worth dealing with the comparator-style operation.


The selection of the actual comparison voltage on a 887AB is provided by a large mechanical display. Each digit, as well as the polarity, is controlled by a mechanical rotary switch located below the corresponding digit. The mechanical switches configure a Kelvin-Varly divider, which generates the comparison voltage. A worm gear attached to each switch controls the large mechanical display, which is updated every time a switch is rotated. The knob on the far right is an exception: its shaft is connected to a variable resistor for fine control of the comparison voltage. The display also has a mechanical floating decimal point, which is actuated by a cam in the range selection switch, which moves a metal plate to shift the location of the decimal point. The range switch also mechanically changes the scale displayed above the 'null' switch. In contrast to the huge mechanical main display, feedback for the zero-in process is provided by an inconveniently small analog display.

Surprisingly, the 887AB is designed for field use, and is equipped with permanently installed NiCd rechargeable batteries.

Fluke 887AB Differential Voltmeter
Fluke 887AB differential voltmeter, dated to 1972.

Fluke 887AB Differential Voltmeter Teardown
Inside the 887AB is very clean, a mass of gold and teflon. Most of the transistors and other semiconductors are socketed for easy removal.

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