Overload Protection

An overload condition is indicated by the simultaneous flashing of the display readouts. The dc voltage function can withstand up to 1,200 volts dc or 1,200 volts root-mean-square (rms) on any range.

Q.6 How is an overload condition indicated by the electronic digital multimeter?

The ac voltage function can sustain up to 1,200 volts rms on the 20-, 200-, and 1,200-volt ranges and 500 volts rms on the 200-millivolt and 2-volt ranges. The current input fuse is protected above 2 amperes rms. Protection for the resistance function is to 130 volts rms in the 200-ohm and 2-kilohm ranges, and 250 volts rms in the 20-kilohm through 20-megohm ranges.

Basic Digital Multimeter Measurement

Table 4-1 lists the proper function push buttons, range push buttons, and input terminal connections for performing specific measurements with the model 8000A.

Table 4-1. - Basic Measurement Instructions

Block Diagram Analysis

Figure 4-9 is a block diagram of an electronic digital multimeter. Note that the block diagram divides the instrument into three major sections: the SIGNAL CONDITIONING section, the ANALOG-TO-DIGITAL CONVERTER section, and the DISPLAY section.

Figure 4-9. - Model 8000A block diagram.

The signal conditioning section provides a dc analog voltage, characteristic of the applied input, to the analog-to-digital converter section. This task is accomplished by the input voltage divider, current shunts, ac converter, active filter, and associated switching.

The analog-to-digital (a/d) converter section changes the dc output voltage from the signal conditioning section to digital information. The a/d converter uses a voltage-to-frequency conversion technique. A dc voltage at the input of the a/d converter is changed to a frequency by the analog integrated circuit (ic). This frequency is characteristic of the magnitude and polarity of the dc input voltage. Counting of the output frequency from the analog ic is accomplished by the digital ic. The resulting count is transferred in binary format to the display section. (Binary number systems are covered in NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits.)

The display section takes the digital (binary) information from the a/d converter section, decodes it, and visually displays it. The decoded digital information is displayed on numerical LED readouts.

Q.7 In an electronic digital multimeter, the digital information is displayed by what type of numerical readouts?

Accessories

Several accessories are available for use with the electronic digital multimeter. One accessory is the test lead kit, shown in figure 4-10 . The kit contains two color-coded test leads with threaded adapters. These adapters attach to banana plugs, pin tips, test prod tips, alligator clips, and binding post lugs.

Figure 4-10. - Test lead kit.

Figure 4-11 shows a high-current probe. This probe extends the ac current measurement capability from 2 to 600 amperes at frequencies up to 400 hertz.

Figure 4-11. - Ac high-current probe.

Figure 4-12 shows a high-voltage probe. The probe extends the dc voltage range to 30 kilovolts.

Figure 4-12. - High-voltage probe.

Figure 4-13 shows a high-frequency probe, which allows measurements over a frequency range of 10 kilohertz to 500 megahertz.

Figure 4-13. - High-frequency probe.

AC/DC DIFFERENTIAL VOLTMETER

The DIFFERENTIAL VOLTMETER provides extremely accurate voltage measurements and is a highly reliable piece of precision test equipment. Its general function is to compare an unknown voltage with a known internal reference voltage and to indicate the difference in their values. The differential voltmeter in common use in the Navy today is the model 893A (figure 4-14).

Figure 4-14. - Ac/dc differential voltmeter.

Q.8 What is the general function of the differential voltmeter?