ELECTRONIC DIGITAL MULTIMETER

4-11 Figure 4-7.—Functional block diagram of dc current circuits. Accessories A dc high-voltage probe is available for use with the multimeter. The probe extends the range of the multimeter in a safe and convenient manner. It is primarily used to measure high-voltage, low-power, dc- current sources, such as the anode supplies in television receivers and other cathode-ray tube circuitry. CAUTION Do not use this probe on electrical equipment that can deliver high power under short-circuit conditions, such as from a large dc motor-generator set. Also available is an ac high-voltage probe. The 10,000-volt ac probe is similar to the high-voltage dc probe with the following exceptions: · The ac high-voltage probe is designed primarily to extend the range of a 5,000-ohms-per-volt VOM. · The probe is used with the VOM in the 10V AC position. · You take readings on the 0-10V AC scale and multiply by 1,000. ELECTRONIC DIGITAL MULTIMETER As you studied in chapter 3 (externally excited meters), placing a meter into a circuit causes energy to be taken from the circuit. The amount of energy taken depends on the sensitivity of the meter. In some cases, this energy loss cannot be tolerated. For example, in extremely sensitive circuits, such as oscillator grid circuits and automatic volume control circuits, degradation of normal circuit operation will occur. This often results in failure to obtain a usable indication of the fault. The use of electronic multimeters is practical in these sensitive electronic circuits. The higher the input impedance of a meter, the less the loading effect and the more accurate the measurements taken. Electronic multimeters have considerably greater input impedances than do nonelectronic multimeters. One example of a typical electronic multimeter in use within the Navy is the electronic Model 8000A Digital Multimeter. Most electronic digital multimeters overcome the disadvantage of requiring a continuous external power source by combining an external ac source with an internal rechargeable battery. Another advantage of this meter is that it can be read directly and does not use a scale. Figure 4-8 shows the model 8000A multimeter.