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Re: [Amps] Shunt resistors for a bridge made of 6A10 diodes in

To: dezrat@outlook.com, amps@contesting.com
Subject: Re: [Amps] Shunt resistors for a bridge made of 6A10 diodes in
From: Gerald Williamson via Amps <amps@contesting.com>
Reply-to: TexasRF@aol.com
Date: Wed, 3 Feb 2016 12:15:45 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Avalanche diodes do not need to be identical. The diodes are in series so  
the reverse leakage current is the same in all diodes. When a diode reaches 
the  avalanche voltage the voltage drop stabilizes much like a zener diode  
does.
 
Once one of the diodes reaches it's avalanche voltage, the current is  
limited by the reverse leakage of the remaining diodes in the string. If the  
reverse voltage is high enough, a second diode will reach it's avalanche 
voltage  and stabilize. As before, the reverse current in the string is then 
limited by  the reverse leakage of the remaining diodes in the string.
 
The idea is that the sum of the avalanche voltages for the string is much  
less than the peak reverse ac voltage applied to the string and there will  
always be one or more diodes that have not reached their avalanche voltage 
and  thereby limiting the reverse leakage current for the entire string.
 
Since the reverse current is limited it does not matter what the individual 
 diode reverse avalanche voltage might be.
 
I have tested a number of 1N5408 diodes (rated 1 KV/3A) for their avalanche 
 voltage and a typical number is around 1300 volts. If there are enough 
diodes in  the string none of the diodes will ever reach the avalanche voltage 
which is ok  but does not matter. As the ac voltage is ramped up one can 
observe each diode  reach it's avalanche voltage one by one. Continuing to 
increase the ac voltage  will ultimately result in all diodes reaching their 
avalanche value and after  that the current will be limited only by the 
transformer and copious smoke  will ensue.
 
73,
Gerald K5GW
 
 
 
In a message dated 2/2/2016 10:01:28 P.M. Central Standard Time,  
dezrat@outlook.com writes:

------------ ORIGINAL MESSAGE ------------(may be  snipped)

On Fri, 29 Jan 2016 09:25:39 -0800, Jim  wrote:

>  Dont put  ANY  shunt resistors  across  a 6A10.  They don’t need them! 

REPLY:

I agree with Jim but  I would qualify that a little. The 6A10s must be
identical, i.e. from the  same manufacturer and same batch. Do not try
to mix and match. The safest  way to purchase them is "tape and reel"
so you can be sure they are all the  same. 

Unless they are truly identical, the reverse voltage will not  divide
equally across each diode and one or more may exceed their PIV  rating.
If you can not do this, then by all means use equalizing resistors.  

73, Bill  W6WRT
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