of medium- and high-impedance electronic circuits

With this information, you can figure the

found in aviation electrical/instrument systems.

value of unknown resistor RX. Divide the voltage

A vacuum tube voltmeter, because of its high

drops across R1 and R3 by their respective voltage

input impedance, is the ideal instrument for

drops across R2 and RX as follows:

measuring low voltage in oscillators, automatic

gain control, automatic frequency control, and

other electronic circuits sensitive to loading, When

measuring voltages of more than 500 volts, a

multimeter having a sensitivity of 20,000 ohms per

volt has an input impedance comparable to most

vacuum tube voltmeters. The input impedance of

most vacuum tube voltmeters is between 3 and

10 megohms. A 20,000-ohm-per-volt meter, when

reading a voltage of 500 volts, has an input

Then, multiply both sides of the expression by RX

impedance of 500 x 20,000 or 10 megohms.

to separate it:

Therefore, on the 500-volt scale, a multimeter of

this sensitivity has an input impedance at least

equal to the vacuum tube voltmeter. For voltage

readings over 500 volts, a 20,000-ohms-per-volt

multimeter offers an input impedance higher than

For example, in figure 2-3, you know that R1 is

the vacuum tube voltmeter.

60 ohms, R2 is 100 ohms, and R3 is 200 ohms.

You can use any multirange voltmeter, though

To find the value of RX, use the formula as

its sensitivity may not exceed 1,000 ohms per volt,

follows:

to get fairly reliable readings in a dc circuit. If

you do not know the impedance of the circuit

under test, a comparison of two voltage readings

will show if the meter is having a loading effect

on the circuit. Take the voltage readings on the

lowest usable range and the on the next higher

range. If the two readings are approximately the

same, the meter is not causing appreciable voltage

variations, and you can accept the higher reading

as the true voltage. If the two readings differ

considerably, the true voltage may be found by

the following formula:

The Wheatstone bridge is widely used to get

precise resistance measurements. You can measure

capacitance, inductance, and resistance for precise

accuracy by using ac bridges. These bridges consist

of capacitors, inductors, and resistors in a wide

where E is the true voltage

variety of combinations.

E1 is the lower of the two voltage readings.

E2 is the higher of the two voltage readings.

R is the ratio of the higher voltage range to

When selecting a dc voltmeter, you should

the lower voltage range.

consider the sensitivity of the meter movement

and its effect on the circuit under test. Navy

As an example of how the formula works,

Electricity and Electronics Training Series

make the following assumptions:

(NEETS), Module 16, discusses meter sensitivity.

1. A reading of 22 volts was obtained between

Use a multimeter having low sensitivity for

two terminals with the meter on the 0-30 volt scale.

quick, rough readings where approximations are

2. A reading of 82 volts was obtained from

adequate. When desiring a high degree of

the same terminals with the meter on the 0-300

accuracy, use a meter having high sensitivity. Such

volt scale.

a meter has wide application in the maintenance

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