Diode Question

[Doug Tymofichuk]

On Mon, 14 Jun 1999 11:16:34 EDT WeAreAs1 at aol.com wrote:

> Hello folks,
> 
> Here are two dumb questions about diodes:  I know that the 
> voltage drop across diodes connected in series is additive 
> (two diodes in series: voltage drop + voltage drop = 1.2 
> volts).  This behavior is similar to that of current 
> through series resistors, as per Mr. Ohm.  My question:  If 
> you connect two diodes in parallel (with both diodes biased 
> in the same direction), does the Ohm's Law analogy still 
> apply?  That is, will the voltage drop across them divide 
> in half (from .6 volts to .3 volts)?  (similar to the 
> behavior of current through parallel resistors)
>
NO, this is more like using batteries, in series the 
voltages are added, in parallel they are not. No two diodes 
have identical voltage drops, so putting two in parallel 
just means that all the current will go through the one 
with the lowest impedance. The other diode will not do 
anything, it is a waste of time even having it there. 
However, if you add some low resistance in series with each 
diode, the difference in voltage drop is made up there, 
causing both diodes to conduct.
 
> Second question:  is the voltage drop across a Germanium 
> diode different than the drop across a Silicon diode?
> 
Yes, usually in about the 0.3 volt range. BUT, this varies 
widely with type, some can be as high as 0.6 volts. 
Schottky diodes can have voltage drops down below 0.3 
volts. Also be aware the the voltage drop of any diode is 
somewhat dependant upon the current passing through it, it 
is not an absolute number.
----------------------
Doug Tymofichuk
dougt at cancerboard.ab.ca