[TCRA] diodes. wordpad doc

K9YLI at aol.com K9YLI at aol.com
Mon Mar 13 09:59:13 CST 2006


     DIODES      
. Banded end is cathode.

Diodes are rated by maximum current flow (I) and peak inverse voltage.
PIV is the voltage applied with polarity to make the diode NOT conduct. 
The diode is forward biased when a positive voltage is on the anode.
   HOW THEY WORK
Diodes conduct only one way. (they do have a small reverse current flow)
The  K shape of the symbol is the cathode( Kathode)  then you 
lay a "plate" accross the open end. The plate of a tube being the plus side  
the tube symbol configuration or ANODE.
Electron flow is  AGAINST the arrow. Back when 'current 'flow as taught, 
current flow is plus to minus which would then have been with the arrow.
The way to remember the symbol is using some tube type nomenclature. You  
start with the cathode (kathode)
then you lay a plate accross the open end.
 ---K |-----  then  push it together
cathode     anode.
If the voltage accross the diode is more plus on the anode side it conducts 
fully.   If the anode is minus in reference to the 
cathode, then it does not conduct   Called  'reverse bias'.     If the 
reverse bias exceeds the  PIV  rating, the diode will  'avalanche' and conduct  
usually destroying it.  Most common diodes are at least 50 piv. 

ZENER diodes require a fixed forward bias voltage before they conduct, thus 
they are used as voltage sensing devices, as they 'turn on' at a pre-determined 
voltage.

SCRs are Silicon Controlled Rectifiers. They only conduct when the 'gate lead 
' is triggered. Then they don't stop until either
one lead is opened or they "blown out".  The way to 'blow them out' is to 
apply a zero or negative pulse on the anode, thus momentarily stopping 
conduction.                  
                                                                             
gate 
                                cathode               anode
Triacs look similar ro  SCRs.  They are effectively   A.C.  SCRs and are used 
to control motors, etc in AC circuits. They look like
back to back reverse diodes. With gate leads. 
 



More information about the members mailing list