[ARC5] Solid State 6AL5

[Dennis Monticelli]

The fast diode problem is unequal release of stored charge.  Quite a bit of
instantaneous current flies around when fast diodes are suddenly reversed
in a PWM circuit.

Yes, a dead short in one diode of a string increases the reverse BV of the
survivors, thus stressing them further.  I generally design for 50% of BV
rating when using common Si power diodes in 60Hz applications.  1N4007's
are so cheap these days while 1KV rated resistors are not.

I wonder how modern day HV 60Hz rectifier modules are built? Do they do
anything to match BVs or leakages before assembly?  Do they use resistors?
I haven't broken one open.  My guess is that they a simple test to throw
out the out-liers to insure they are using only the middle of the
distribution and then dispense with resistor balancing.

Dennis AE6C

On Thu, Feb 8, 2018 at 12:23 PM, Peter Gottlieb <kb2vtl at gmail.com> wrote:

> Yet, I still find manufacturer app notes, such as STMicro AN443, that talk
> about this, although in the context of fast diodes.
>
> application-notes.digchip.com/005/5-10702.pdf
>
> One diode shorts in that HV bridge and the whole rest of them will not be
> far behind.
>
> I’m not saying you’re wrong, just that I’d like to see a manufacturer or
> academic reference justifying eliminating the resistors before I’d accept
> it in anything beyond hobby use.
>
>
> Peter
>
> On Feb 8, 2018, at 3:04 PM, Dennis Monticelli <dennis.monticelli at gmail.com>
> wrote:
>
> FYI.  The practice of using equalization resistors dates way back to when
> early Si manufacturing had uniformity issues.  If you are using modern
> 1N4007's (particularly from the same batch) don't bother with the
> resistors.  Just design with the usual BV margins and you will be fine.
>
> Dennis AE6C
>
> On Thu, Feb 8, 2018 at 12:00 PM, Peter Gottlieb <kb2vtl at gmail.com> wrote:
>
>> In my test set the 5R4 is set up as a full wave rectifier from a 1700
>> volt CT winding. At one point I thought the tube was weak and I’d replace
>> with diodes. I have a bunch of 1N4007 but looking at the voltages I
>> realized I needed three diodes for each plate!  And then equalization
>> resistors, but with their max rated voltages I needed a lot of them. This
>> started to be more trouble than it was worth and I was able to locate a
>> replacement tube in my stash and just went with it.
>>
>>
>> Peter
>>
>> > On Feb 8, 2018, at 2:47 PM, Richard Knoppow <1oldlens1 at ix.netcom.com>
>> wrote:
>> >
>> >    I don't remember if I ever measured the resistance of the 5R4
>> replacement I have and don't know where it is now. However, the B+ with it
>> installed was substantially higher than with the tube rectifier and it was
>> small so I don't think it had anything but diodes in it.
>> >
>> >> On 2/8/2018 10:17 AM, John Watkins via ARC5 wrote:
>> >> My solid state 6AL5s have just diodes and possibly a resistor in
>> series with them (2 diodes).  They measure 16 Ohms forward resistance using
>> a Simpson VOM on the Rx1 scale.off the upper end on both Rx1 and Rx100 for
>> reverse.  Nothing in mine for heater load.  That is a no connection.  I
>> haven’t removed the amber epoxy yet to see if the diodes have a 1N number.
>> Will report on that when I get back to town.
>> >> John WD5ENU
>> >
>> >
>> > --
>> > Richard Knoppow
>> > 1oldlens1 at ix.netcom.com
>> > WB6KBL
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