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 on the gate, the ohmmeter reading will be similar to
voltage limits) and as a fairly good conductor to
the diode forward reading. There is no blocking
current flow in the opposite direction. The ohmme-
reading for the reverse. Readings for the cathode to
ter may be used to measure the forward resistance
gate connection are generally only 10-50% higher
(conduction) and the reverse resistance (insulating
than those obtained in the forward mode. If a test
or blocking) of the device in order to determine its
battery (fig 72d) is not available for the conduction
overall condition. Testing should be done as shown
test, an alternate test can be done using only the
in figure 72c. Connect the ohmmeter to read resis-
ohmmeter. This simplified test is harder to inter-
tance in the forward direction. The reading should
pret and is less accurate than the procedure de-
range between 6-35 ohms. This is the range for
scribed above. To do the test, setup as in paragraph
general purpose rectifiers. Very small, signal type
rectifiers may read as high as 70-100 ohms. Very
(a) above. In place of the test battery, touch the gate
lead to the anode (+) lead. The SCR should begin to
large current capacity rectifiers may read between
conduct.
five and ten ohms. Finally, germanium diodes read
(2) Other devices. There are many styles and
lower than those made from silicon, and fast recov-
types of devices for both diodes and SCR's. It is be-
ery types read lowest of all: two to six ohms. Con-
yond the scope of this manual to describe all of them
nect the ohmmeter to read resistance in the reverse
in detail. several case styles (T62, T72, DO-200, and
direction. The reading obtained should be between
TO-200 for diodes, and R62, R72, and T9G for SCR's)
10,000 and 100,000 ohms or possibly more. The
are designed to operate clamped between heat sinks.
reading tends to be near the lower end of the range
There is a spring contact within the device that pre-
for large current capacity types. If the diode is good,
vents operation unless the unit is physically com-
the values listed above will be obtained. Readings of
pressed. These devices should be tested in place if
one (1) ohm or less mean the device is damaged or
possible. Otherwise, moderate pressure applied with
shorted. Reverse direction readings less than 10,000
the fingers is usually sufficient to "make" the inter-
ohms generally mean the device is damaged or elec-
nal connection. Apiece of insulating material should
trically leaking. In both cases, the unit should be
be placed between the fingers and SCR surface to
replaced.
(1) Semiconductor-controlled rectifier (SCR).
prevent false leakage readings.
The SCR is a diode with the ability to be forced into
d. Resistors and rheostats. A resistor is a passive
component used to hinder the flow of electric cur-
conduction by the application of a gate signal. The
rent. Many sizes, shapes, values and types of resis-
SCR cannot conduct in the reverse direction if it is a
tors are available. The most common types are wire
good unit. However, the SCR will not conduct in the
wound (resistance wire wound around an insulator)
forward direction either until a small gate voltage is
and carbon stick (pressed carbon tubes or rods). A
applied. Once in conduction, the SCR remains that
rheostat is simply a variable resistor. Like resistors,
way until its current (not the voltage) drops below
rheostats also are made in numerous sizes, shapes,
the minimum holding value for that particular de-
values and types. Again like the resistor, the
vice. The SCR should be tested like the diode recti-
rheostats are wire wound or carbon composition.
fier (fig 72d) but with the following modifications
The rheostat is normally 3/4 circular in design with
to the procedure:
a terminal at each end. A movable contact or brush
(a) Connect the ohmmeter to read forward
known as the "wiper" rides on the rheostat material
resistance. The meter needle should read infinity
surface and can be moved to select the desired re-
ohms before a gate-cathode voltage is applied. Con-
sistance value. Use an ohmmeter to accurately
nect an additional voltmeter between the gate and
cathode leads of the SCR. Apply an adjustable DC
measure the resistance of a resistor or rheostat.
voltage to these leads and measure the voltage
However to avoid false readings of devices which
needed to start conduction. The ohmmeter will give
may be connected in parallel, disconnect one side of
readings like those for the diode when conduction
the component to be tested before making resis-
has been established. Note the gate-cathode voltage
tance measurements. Replace components that do
when conduction starts. It should be between 0.6
not measure within plus or minus five percent of
and 1.3 volts for the most general purpose units.
the value given in the manual or as specified on the
schematic diagram, unless other tolerances are in-
(b) Disconnect the gate lead. The SCR will
remain in conduction until the ohmmeter leads are
dicated. Replace broken, cracked or damaged units
removed. This condition depends on two things:
and support brackets.
first, the particular SCR must have a very low hold-
e. Zener diodes. A Zener diode is a semiconductor
ing current; second, the battery in the ohmmeter
device like the rectifier diode, but the Zener device
must be fresh or fully recharged. A gate to cathode
has its composition and PN junction characteris-
resistance check may be applied also. With plus (+)
tics carefully controlled in order to produce a de-
7-4
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