Normally, you will work on low-current
electronic circuits. Schematics show the voltages
at various test points. If you suspect that a certain
stage is defective, you may check the voltage by
connecting a voltmeter from the test point to
ground. If the suspected stage is good, the
voltmeter readings will match the voltages given
on the schematic.
Some technical manuals contain voltage
charts. These charts usually show the sensitivity
of the meter (e.g., 20,000 ohms/volt) you should
use to take the voltage readings for the chart. To
get comparable results, you must use a voltmeter
of the same sensitivity (or greater) as that
specified. This is to be sure the voltmeter is not
loading the circuit while taking a measurement.
Figure 2-3.-Wheatstone bridge.
If the meter resistance is not considerably higher
than circuit resistance, the reading will be
The galvanometers (an instrument that measures
markedly lower than true circuit voltage because
small amounts of current) across terminals b and
of the voltmeter's loading effect.
d shows the condition of balance. When the bridge
is in balance, no difference in potential exists
across terminals b and d; when switch S2 closes,
the galvanometers reading is 0.
The ammeter connects in series with the
current path. Circuits requiring frequent current
When the battery switch (S1) closes, electrons
readings or adjustments provide current jacks for
flow from the negative terminal of the battery to
use with a plug-in meter. Some systems have a
point a. Here, the current divides as it would in
meter installed as part of the equipment.
any parallel circuit. Part of it passes through R1
and R2; the remainder passes through R3 and RX.
OHMMETER
return to the positive terminal of the battery. The
The Navy does not have an instrument
consisting solely of an ohmmeter. The ohmmeter
and the voltmeter form a multimeter. Therefore,
of R3 and RX . The current is inversely
you must determine the choice of ohmmeter
proportional to the resistance.
by the resistance ranges available in the
Adjust R1, R2, and R3 so when S1 closes,
various multimeters. Small multi meters have
and no current flows through G. When the
a high range of R x 100; larger multimeters,
such as the AN/PSM-4, have a high range of
galvanometers shows no deflection, there is no
R x 10,000. VTVM TS-505A/D has a high range
difference in potential between points b and d.
All of I1 follows the
path, and all of I2
of R x 1,000,000.
follows the
path. This means that voltage
WHEATSTONE BRIDGE
drop E1 is the same as voltage drop E3. Similarly,
the voltage drops across R2 and RX (E2 and EX)
are also equal.
Resistance measurements taken using an
ohmmeter are not always accurate enough. The
cause of this inaccuracy is an error in meter
movement and in the reading of the meter. The
Wheatstone bridge (fig. 2-3) is widely used for
precise resistance measurements.
Resistors R1, R2, and R3 are precision,
variable resistors. The value of Rx is an unknown
value of resistance that you must determine. After
properly balancing the bridge, you can find the
unknown resistance by using a simple formula.
