As I understand it, the diode doesn't go low resistance when it
avalanches, it's more like the characteristic we associate with a zener
type diode so it maintains its own particular breakdown voltage while
conducting - obviously within limits.
Steve G8GSQ
On 14/12/2022 00:52, Ron W4BIN wrote:
MU 4CX250B wrote:
An interesting point, Steve. If I understand you, you’re saying that
if a modern diode in a series string approaches reverse breakdown, it
will start to conduct, thus shifting its overload voltage to other
diodes in the string. Before the advent of controlled avalanche
diodes, however, an overloaded diode would just short circuit, thus
permanently shifting its voltage to the remaining diodes in the
string. This process, once started, would likely lead to the
destruction of the entire string. To me, the takeaway message is that
No that has never been the case, "controlled avalanche diodes" of a
particular type all avalanche at the same Voltage.*
Otherwise the lowest reverse Voltage device could avalanche and present a
very low resistance, (causing an increase in reverse Voltage across the
rest)
then the next lowest Voltage device could avalanche until they would all
avalanche at which time, the current would destroy most until one "blows
open"
ending the catastrophe.
Zener diodes below about 6.2 Volts are true Zener diodes, above about
6.8
Volts are "controlled avalanche diodes" and as long as they are conducting
reverse current less than Imax they are not harmed.
* in our high Voltage operations a tighter grouping of avalanche Voltages
can be maintained by buying in lots like of 50 or 100 and keeping the
batches separate and constructing strings all from the same batch.
there’s little to be gained these days by adding .01uF caps to each
diode. This is a helpful explanation, Steve. Thanks!
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