[Amps] B- questions

[Bill, W6WRT]

On Sun, 8 Nov 2009 00:31:32 -0600, Rob Atkinson <ranchorobbo at gmail.com> wrote:

>
>I'm not sure of the meaning of "anti-parallel."  do you mean two
>diodes anode to cahode to anode to cathode from one end of the
>resistor to the + side of the meter, or by "back to back" do you mean
>diode cathode to anode to anode to cathode?

Anti-parallel means the diodes are connected in parallel, but with reversed
polarities. In other words, the cathode of one is connected to the anode of the
other. 
>
>Also, what about the diodes must be known in order to choose a drop of
>1/2 volt for the meter/resistor combination?  It seems like there is
>some information there that you left out.  A conduction threshold
>voltage?  it looks like if you reverse the diodes you are dealing with
>PIV which on a 6A10 is 1 kv isn't it?  So I'm obviously confused about
>some details.

When diodes are connected in anti-parallel, reverse voltage rating has no
meaning because one of them will begin to conduct long before the other reaches
its reverse rating. All conventional silicon diodes, of which the 6A10 is one,
have a forward conduction voltage that starts at about .5 volts. The forward
conduction voltage is not linear - applying additional voltage to it causes a
"clamping" effect, while the current increases dramatically. Regardless of the
amount of current passed, the voltage will not rise to more than about .7 volts,
right up to the maximum current rating of the diode. I like the 6A10 because
they are inexpensive and have a surge current rating of 400 amps, which should
be enough for any ham application.

If you can locate a graph of forward voltage vs current for a typical silicon
diode, this should become more clear. 

73, Bill W6WRT