It is difficult to determine the polarity of a diode when its marking is already erased. The color ring at one end usually indicates the cathode lead.
But if this color ring is erased, what are you going to do? This problem can be solved by using a multimeter. However, when one must test a large number of
unmarked diodes, the multitester becomes uncomfortable to use. The best way is to build a tester circuit that is specially designed for testing diodes alone like the
one featured here. This circuit not only determines the polarity of the diode but also tests whether the diode is open or short circuited.
The diode to be tested is connected to the terminals as shown. When the LED lights, the diode is connected correctly and the symbol shows the polarity. To check whether
the diode is shorted, reverse the diode and if the LED remains lighted, then the diode has an internal short circuit. On the other hand when the LED does not light up in both
directions, the diode is open.
The circuit can test both germanium and silicon diodes. The tester must be housed in a plastic box and the test electrodes must be marked with cathode(K) and anode
(A) respectively. A box with fixed contact points installed on it is highly practical in testing a large number of diodes. A suggested design is shown in Fig. 61.3
Parts Placement Layout
External Wiring Layout
Electronic Circuits volume 1.0 - Circuit Nr. 61
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.