When the variable resistor is adjusted properly,
A, which is under test. The power should be off.
When checking conductors A, B, and C, the
with the leads shorted, the pointer of the meter
will come to rest exactly on zero. This reading
current from the ohmmeter flows through plug
2 (female), conductor A, B, or C, to plug 1
indicates zero resistance between the test leads.
When the test leads of an ohmmeter are separated,
(female). From plug 1, current passes through the
the pointer of the meter will return to the left
jumper to the chassis, which is grounded to the
side of the scale. This happens because of the
aircraft structure. The aircraft structure serves as
the return path to the chassis of unit 2 and
interruption of current and the spring tension
completes the circuit through the series-connected
action on the movable coil assembly.
ohmmeter.
After you adjust the ohmmeter for zero
reading, it is ready to be connected in a circuit
The ohmmeter shows a low resistance because
no break exists in conductors A, B, or C.
to measure resistance. The power switch of the
However, checking conductor D reveals an
circuit to be measured should always be in the
open. The ohmmeter shows maximum resistance
OFF position. Circuit voltage applied across the
because current cannot flow in an open circuit.
meter could damage meter movement.
With an open circuit, the ohmmeter needle is all
Connect the test leads of the ohmmeter across
(in series with) the circuit to be measured. The
the way to the left, since it is a series-type ohm-
meter (reads right to left). When you have finished
current produced by the meter must now flow
using the ohmmeter, turn the ohmmeter switch
through the circuit being tested. The reading
OFF to conserve the batteries,
obtained from the ohmmeter between zero and
infinity shows the number of ohms resistance in
High-voltage Insulation Tester
that circuit. An infinity reading shows there is an
open circuit.
The high-voltage insulation tester detects or
In open circuits, current flow stops. The
measures leakage of high-voltage insulation.
cause may be a broken wire, defective switch,
Figure 9-13 shows one of the several types of
etc. To detect open circuits, you must perform
testers now in use.
a continuity test, which will tell if the circuit is
The high-tension voltage tester is composed
complete or continuous.
of four sections. They are as follows:
Look at figure 9-12. It shows a continuity test
of a cable connecting two electronic units. You
A vibrator invertor, which converts a
can see that both plugs are disconnected, and the
24-volt dc source to 110-120 volts ac.
ohmmeter is connected in series with conductor
Figure 9-12.-Continuity test.
9-16
