1-32
the diode draws excessive current. This action is referred to as THERMAL RUNAWAY and eventually
causes diode destruction. Extreme caution should be used when working with equipment containing
diodes to ensure that these problems do not occur and cause irreparable diode damage.
The following is a list of some of the special safety precautions that should be observed 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 diode.
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 a direct replacement, or with one of the same type.
Ensure a replacement diode is put into a circuit in the correct direction.
If a diode has been subjected to excessive voltage or temperature and is suspected of being defective,
it can be checked in various ways. The most convenient and quickest way of testing a diode is with an
ohmmeter (fig. 1-28). To make the 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. Measurement 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 compared with the reverse resistance. That is, 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 measurements. However, as a rule of thumb, a small signal diode will have
a ratio of several hundred to one, while a power rectifier can operate satisfactorily with a ratio of 10 to 1.
