Gene,
That was true for the old diodes before the 80's or so. I can't remember
when the modern ones came on the market.
The diodes today have an avalanche region that is similar to a zener diode
across the reverse junction that will start to conduct when reverse voltage
reaches a certain level. It will not handle a lot of current, off hand I
seem to remember 20 microamps. If one diode conducts sooner than others in
the string it will not allow the reverse voltage of that diode to reach a
level that will damage it. The reverse voltage will just be a little higher
on each of the other diodes in the string and since they are all in series
the reverse current will be negligible in all diodes.
With modern diodes it is recommended NOT to put resistors across them as
that can upset the reverse current balance and force excessive reverse
current on an individual diode.
A good way to test diodes is with a hi-pot tester. Crank the voltage up
until you read 1 or 2 microamps and note the voltage. That is the start of
the reverse current avalanche conduction point.
It is not a bad idea to check all diodes that way before putting them into a
supply. I have found a few that were flakey with varying reverse current at
a lower voltage than they were rated for. Usually a 1kv piv diode is good
for anywhere from 1100 to 1300 volts and some consistently higher.
Diodes that have unequal reverse breakdown points are usually not a problem
if they are not too far apart as they will never conduct in normal
operation. For there to be reverse current of any significance in one diode
there must be reverse current in all in the string as they are in series.
Normal leakage reverse current below the breakdown voltage will be in the
low pico amps in a good diode.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Gene May
> Sent: Wednesday, April 25, 2012 8:24 PM
> To: Amplifier Mailing List
> Subject: [Amps] PIV requirements for identical, individual diodes
>
>
>
>
> Reference the thread about this: The PIV across each diode in a bridge
> will be the peak voltage of the transformer voltage. . . . . , I offer
> some words of caution:
>
> How the reverse voltage disributes across a chain of diodes is
> problematic in a way that is similar to the way power supply output
> voltage distributes across a chain of filter capacitors. One of the
> important functions that bleeder resistors serve across such a string of
> capacitors is to equalize the voltage drops. Just as electrolytic
> filter capacitors vary A LOT in how much they "leak" current, meaning
> how much they vary as resistors across the output voltage, rectifier
> diodes vary considerably in how much they leak when they are reverse
> biased. Once when I had access to the instruments that I could do this
> with, I got a batch of about 200 1 KV PIV diodes, reverse biased them to
> about 800V and measured the currents through them. The currents varied
> from several microramperes to a mil or so, easily a ratio of 20:1 or
> more. This is the reason that you see resistors of about 470K-ohms to
> about a megohm across a chain of diodes -- to "swamp" the leakage and
> equalize the voltage dr
> ops when they are reverse biased. I put about a megohm across each
> diode in a chain or leg of diodes, and use enough diodes to give a total
> PIV of 5X the output voltage. Nothing blows. I concede that I am
> conservative here, but diodes and resistors are cheap.
>
> Gene May
> WB8WKU
>
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