Page 27 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

• Low power and high power rectification
• As surge protectors in EM relay and other circuits.
• For clipping, clamping wave-forms.
For different applications, diodes of different current carrying capacity, different PIV capacity and so on are
required. Therefore, manufacturers make diodes to cater to varied applications with different specifications.
Before using a diode for a particular application, it is a must to find out whether the voltage, current, and
temperature characteristics of the given diode match the requirement or not.
Important specifications of a diodes

The material : The diode is made-of doped semi-conductor material. This could be Silicon or Germanium or
Selenium.This is important because the cut-in voltage depends upon the material the diode is made-of. For
example, in Ge diodes the cut-in voltage is around 0.3V, whereas in Si diodes the cut-in voltage is around 0.7V.

Maximum safe reverse voltage : Denoted as VR or Vr that can be applied across the diode. This is known as
peak-inverse-voltage or PIV. If a higher reverse voltage than the rated PIV is applied across the diode, it will
become defective permanently.
Maximum average forward current : If or IF that a diode can allow to flow through it without getting damaged.
Forward voltage drop : VF or Vf that appears across the diode when the maximum average current, If flows
through it continuously
Maximum reverse current : Ivr that flows through the diode when the maximum reverse voltage, PIV is applied.
Maximum forward surge current : Is that can flow through the diode for a defined short period of time.
The maximum junction temperature: The temperature upto which the diode junction can withstand without mal
functioning or getting damaged.
Testing diodes using ohmmeter: A simple ohmmeter can be used to quickly test the condition of diodes. In
this testing method, the resistance of the diode in forward and reverse bias condition is checked to confirm its
condition.

Recall that there will be a battery inside an ohmmeter or a multimeter in the resistance range. This battery voltage
comes in series with the leads of the meter terminals as in Fig 7.In Fig 7 the lead A is positive, lead b negative.
If the polarity of the meter leads are not known at first, the polarity of the meter leads can be
determined by using a voltmeter across the ohm meter terminals.
If the positive lead of the ohmmeter, lead A in the Fig 7 is connected to the anode of a diode, and the negative (lead
B) to the cathode, the diode will be forward-biased. Current will flow, and the meter will indicate low resistance.
On the other hand, if the meter leads are reversed, the diode will be reverse-biased. Very little current will flow
because a good diode will have very high resistance when reverse biased, and the meter will indicate a very high
resistance.
While doing the above test, if a diode shows a very low resistance in both the forward and reverse biased
conditions, then, the diode under test must have got damaged or more specifically shorted. On the other hand, a
diode is said to be open if the meter shows very high resistance both in the forward and reverse biased conditions.
Polarity marking on the diodes: The cathode end of a diode is usually marked by a circular band or by a dot
or by plus (+) sign. In some diodes the symbol of the diode, which itself indicates the polarities, is printed on the
body of the diode.

Special diodes: All diodes are basically PN junction diodes and are made according to the application. There are
many special purpose diodes are in use in which zener diodes widely used for voltage regulation.

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CITS : Power - Electrician & Wireman - Lesson 60-69