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.
DC VOLTMETER
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
