2-21
different magnitudes and frequencies. Some diodes may be damaged by the excessive current produced
by some range settings of a standard multimeter. Therefore, you should use a digital multimeter when
performing this measurement.
Q-14.
As a rule of thumb, what is an acceptable ratio of back-to-forward resistance for a diode?
SILICON-CONTROLLED RECTIFIERS (SCR)
Many naval electronic equipments use silicon-controlled rectifiers (SCRs) for the control of power.
Like other solid-state components, SCRs are subject to failure. You can test most SCRs with a standard
ohmmeter, but you must understand just how the SCR functions.
As shown in figure 2-12, the SCR is a three-element, solid-state device in which the forward
resistance can be controlled. The three active elements shown in the figure are the anode, cathode, and
gate. Although they may differ in outward appearance, all SCRs operate in the same way. The SCR acts
like a very high-resistance rectifier in both forward and reverse directions without requiring a gate signal.
However, when the correct gate signal is applied, the SCR conducts only in the forward direction, the
same as any conventional rectifier. To test an SCR, you connect an ohmmeter between the anode and
cathode, as shown in figure 2-12. Start the test at R 10,000 and reduce the value gradually. The SCR
under test should show a very high resistance, regardless of the ohmmeter polarity. The anode, which is
connected to the positive lead of the ohmmeter, must now be shorted to the gate. This will cause the SCR
to conduct; as a result, a low-resistance reading will be indicated on the ohmmeter. Removing the anode-
to-gate short will not stop the SCR from conducting; but removing either of the ohmmeter leads will
cause the SCR to stop conducting — the resistance reading will then return to its previous high value.
Some SCRs will not operate when you connect an ohmmeter. This is because the ohmmeter does not
supply enough current. However, most of the SCRs in Navy equipment can be tested by the ohmmeter
method. If an SCR is sensitive, the R 1 scale may supply too much current to the device and damage it.
Therefore, try testing it on the higher resistance scales.
Figure 2-12.—Testing an SCR with an ohmmeter.
