1-98. Diodes are rugged and efficient. They are also expected to be relatively trouble
free. Protective encapsulation processes and special coating techniques have even further
increased their life expectancies. In theory, a diode should last indefinitely. However, if
diodes are subjected to current overloads, their junctions will be damaged or even
destroyed. The application of excessively high operating voltages can also damage or
destroy junctions through arc-over or excessive reverse currents. One of the greatest
dangers to the diode is heat. Heat causes more electron-hole pairs to be generated, which in
turn increases current flow. This increase in current generates more heat and the cycle
repeats itself until the diode draws excessive current. This action is referred to as
THERMAL RUNAWAY and eventually causes diode destruction. Use extreme caution
when working with equipment containing diodes to ensure that these problems do not
occur and cause irreparable diode damage.
1-99. There are a number of ways to prevent diode damage. Observe the following
special safety precautions when working with diodes.
Never remove or insert a diode into a circuit with voltage applied.
Never pry diodes to loosen them from their circuits.
Always be careful when soldering to ensure that excessive heat is not applied to the
When testing a diode, ensure that the test voltage does not exceed the diode's
maximum allowable voltage.
Never put your fingers across a signal diode because the static charge from your
body could short it out.
Always replace a diode with one of the same type or with a direct replacement.
Ensure a replacement diode is put into a circuit in the correct direction.
1-100. You can check a diode in a number of ways if it has been subjected to excessive
voltage or temperature and is suspected of being defective. The most convenient and
quickest way of testing a diode is with an ohmmeter (see Figure 1-28). To make this check,
simply disconnect one of the diode leads from the circuit wiring and make resistance
measurements across the leads of the diode. The resistance measurements obtained depend
upon the test-lead polarity of the ohmmeter; therefore, two measurements must be taken.
The first measurement is taken with the test leads connected to either end of the diode and
the second measurement is taken with the test leads reversed on the diode. The larger
resistance value is assumed to be the reverse (back) resistance of the diode and the smaller
resistance (front) value is assumed to be the forward resistance. Measurements can be
made for comparison purposes using another identical-type diode, known to be good, as a
standard. Two high-value resistance measurements indicate that the diode is open or has a
high forward resistance; two low-value resistance measurements indicate that the diode is
shorted or has a low reverse resistance. A normal set of measurements will show a high
resistance in the reverse direction and a low resistance in the forward direction. The diode's
efficiency is determined by how low the forward resistance is as compared with the reverse
resistance. Therefore, it is desirable to have as great a ratio (often known as the front to
back ratio or the back to front ratio) as possible between the reverse and forward resistance
23 June 2005