Figure 7-4.--PN diode identification.
Figure 7-3.--PN junction diode characteristic curve.
barrier gets smaller, and its resistance to current flow
On the other hand, the diode conducts very little
when reverse biased. Notice at point C the reverse bias
voltage is 80 volts and the current is only 100 A. This
results in 800 kilohms of resistance, which is
considerably larger than the resistance of the junction
with forward bias. Because of these unusual features,
the PN junction diode is often used to convert
Figure 7-4 illustrates various styles of PN diodes.
H I G H R E S I S TA N C E M E A S U R E M E N T
Disconnect one of the diode leads from the circuit.
Connect the meter to the diode's pigtails. Figure 7-5
illustrates the test. You may or may not get a reading.
Reverse the leads. If you had a low reading the first
time, you should now have no reading or a high
reading. A second low reading means the diode is
shorted. Two high readings after reversing the leads
means the diode is open. A low reading and high
reading means the diode is good. One thing you should
keep in mind about the ohmmeter check--it is not
conclusive. It is still possible for a diode to check good
under this test, but break down (temporarily fail) when
F O RWA R D C O N D I T I O N -
replaced in the circuit. This can occur because the
L O W R E S I S TA N C E M E A S U R E M E N T
meter used to check the diode does not load the device
Figure 7-5.--Checking a diode with an ohmmeter.
as though it was in its operating circuit.