Measuring ac Voltages

To measure ac voltages, you must set the function switch to the AC position. The same procedure used to measure dc voltages applies, except that in reading the voltage, you use the AC volts scale (the polarity of the test leads is not important). When measuring very low or high frequencies of ac voltages, you should be aware that the multimeter has a tendency to be inaccurate. View A of figure 4-6 is a functional block diagram of the ac and output voltage circuits in the multimeter. View B shows the jacks and switch positions used to measure ac voltages.

Figure 4-6. - Functional block diagram of ac and output voltage circuits.

Measuring Output Voltages

You will often measure the ac component of an output voltage where both ac and dc voltage levels exist. This occurs primarily in amplifier circuits.

The multimeter has a 0.1-microfarad, 400-volt blocking capacitor in series with the OUTPUT jack. The capacitor blocks the dc component of the current in the circuit under test, but allows the ac component to pass on to the indicating circuits.

When using OUTPUT, do not attempt to use the meter in a circuit in which the dc voltage component exceeds the 400-volt rating of the blocking capacitor.

To use the multimeter to measure output voltage, you must follow these steps:

Measuring Current

The multimeter can function as an ammeter to measure current flow.

When using the multimeter as a current-indicating instrument, NEVER connect the test leads directly across a voltage. ALWAYS connect the instrument in series with the load.

To use the multimeter as an ammeter, you must take the following steps:

• 1. Set the function switch at +DC (assuming the current to be positive).
• 2. Plug the black test lead in the COMMON jack and the red test lead into the + jack.
• 3. Set the range switch at one of the five ampere-range positions.
• 4. Ensure the equipment is OFF and then physically open the circuit in which the current is being measured.
• 5. Connect the VOM in series with the circuit, ensuring that proper polarity is observed when making this connection.
• 6. Turn the equipment ON and then read the current on the DC scale. (This is the same scale used to measure dc voltages.)

The setting of the range switch determines the maximum value represented by the DC scale. Always use the range scale that corresponds to the range switch setting.

Never attempt to measure currents greater than the setting of the range switch. Increase the range with a shunt, if necessary, but do not exceed the marked current.

When measuring unknown currents, follow the same procedures as when measuring voltages. Always start with the highest range available and work down. Use the range that gives approximately half-scale deflection. If this procedure isn't followed, the meter could be burned out.

Figure 4-7 is a functional block diagram of the dc current circuits in a multimeter.

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.

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.

Figure 4-8. - Digital voltmeter 8000A operating features.

Operating Features

The locations of all controls, connectors, and indicators are shown in figure 4-8. The INPUT terminals, located on the left-hand side of the meter face, provide input connections for voltage or resistance (V-?) and milliampere current (MA) measurements with respect to the common terminal. The readout section, located across the upper half of the meter face, contains light-emitting diode (LED) indicators. They display the measured input and polarity signs for dc measurements. The POWER switch, located on the lower right-hand side of the meter face, is a push-button switch used to energize the instrument. The RANGE switches, located on the lower, middle, right-hand portion of the meter face, select the voltage (200 millivolts, 2, 20, 200, or 1,200 volts), current (200 microamperes, 2, 200, or 2,000 milliamperes), and resistance (200 ohms, 2, 20, 200, or 2,000 kilohms) ranges. The FUNCTION switches, located on the lower, middle, left-hand portion of the meter face, select the voltage, current, or resistance modes. The MA input terminal is also a fuse holder for the current protection fuse.

Internal Battery Models

Power is supplied by internal rechargeable batteries that allow the instrument to operate for at least 8 hours. Recharging the batteries is accomplished by switching the POWER switch to OFF and connecting the instrument to an ac power line. You can use the instrument when recharging the batteries on ac power, but the recharging time will be extended.

Q.5 Power for the electronic digital multimeter is normally supplied by what internal power source?