[Amps] Re: [Moon-Net] Motorola HV rectifiers

[Dr. David Kirkby]

Conrad G0RUZ wrote:
> 
> I have been given some HV rectifiers which are probably c 1970-80s, they
> have the Motorola M logo and are marked MDA1332H. They are quite substantial
> and I would like to know if anyone has any data for them.
> 
> 73
> 
> Conrad G0RUZ

I can't help you with data on those specific devices, but I would personally be
wary of 1970's diodes that are 'quite  substantial'. 

Some may recall the very public disagreement I had with a fellow ham  on this
matter many years ago. I obtained some diodes advertised as 24 kV, but which I
later found were pretty mediocre devices, of much lower ratings. Each HV diode
stack was huge, with 10 diodes in series, with big heatsinks, 10 resistors and
10 capacitors 

I first became suspicious when I was able to determine the value of the
resistors across the stacks. These had 390 k Ohm resistors, of a rating of about
1 W. It was clear that if 10 were in series, having 24000/10=2400 V across each
one, the 390 k 1W resistors were going to glow red with 15 W dissipated in them.
(I later found the diode spec was around 4-5 kV, not 24 kV as advertised).

If your diodes have resistors across them, it is fairly easy to put an upper
limit on ratings (recall P=V^2/R). That would be the DC holdoff voltage. With
more effort you can put an upper limit on use in bridges (taking into account
the half sine wave of current). This does not give you a rating for the diodes,
but puts an upper possible limit on it. My experience is that such a rating may
be enough to make you toss them in the bin. 

I later assembled some higher voltage, higher current devices in about a tenth
the size. The basic point I'm trying to make is that diodes have improved a lot
in 30 years, and I seriously suspect you will find it better t o buy or make new
ones rather than use very old devices. 

I've heard it said, but can't confirm this for sure, that modern diodes
avalanche non-destructively, meaning there is no need for equalizing resistors
to balance voltages. This makes sense from the physics to me (I've done quite a
bit of semiconductor physics over the year). With modern low-leagage current
devices, the reverse current through a series of didoes can not damage them at
avalanche (unless you have enough volts to make them all avalanche at once).
This fact saves resistors, capacitors, heatsinks etc.


I hope your experience is better than mine, but I finally gave one HV-rectifier
stack away to student I'm not sure she knew she was doing, which is working at
lost of kVs !! I threw the other half a dozen in the bin. 

I hope you find the data, as that is the only way to be sure, but those are my
thoughts having had a bad experience on old HV diodes stacks. 
-- 
Dr. David Kirkby PhD,
email: drkirkby@ntlworld.com 
web page: http://www.david-kirkby.co.uk
Amateur radio callsign: G8WRB