[Amps] HV Supplies and Diodes

[Gary Schafer]

The reverse current will be set by the sum of the leakage resistance of ALL
of the diodes in the string that are not in avalanche. The current will be
the same in all the diodes, they are in series.

 

73

Gary  K4FMX

 

  _____  

From: TexasRF at aol.com [mailto:TexasRF at aol.com] 
Sent: Thursday, January 03, 2008 8:01 PM
To: garyschafer at comcast.net; larry at w7iuv.com
Cc: amps at contesting.com
Subject: Re: [Amps] HV Supplies and Diodes

 

re: reverse current in an avalanche diode string: in fact, the reverse
current of the string will be set by the diode with the lowest leakage
current. This is as good as it gets!

 

73/k5gw

 

In a message dated 1/3/2008 5:45:22 P.M. Central Standard Time,
garyschafer at comcast.net writes:

The 1980 ARRL handbook section on power supplies does recommend using
resistors across each diode in a string.

The 2000 ARRL handbook recommends NOT using resistors across each diode in a
string. 
Times have changed!

Diodes these days have an avalanche area in them that makes them operate
like a zener when maximum reverse voltage is seen. So if one diode in a
string has a lower reverse breakdown voltage its zener will conduct a small
amount of current while protecting its junction. Since current in a series
circuit is the same everywhere, the amount of current conducted will be
limited by the total leakage current of the whole diode string. (very small)

By placing resistors in parallel with these diodes you may force excess
reverse current through a diode that has reached avalanche because the
resistors provide a low impedance source of current. 
Without the resistors there is only the very high leakage resistance of the
whole string to supply current.

73
Gary  K4FMX