Tube-type bridge rectifier

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: Tube-type bridge rectifier
From: Tesla List <tesla-at-stic-dot-net>
Date: Sun, 8 Feb 1998 20:46:03 -0600
Approved: tesla-at-stic-dot-net


From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Saturday, February 07, 1998 10:48 AM
To:  Tesla List
Subject:  Re: Tube-type bridge rectifier


> >Coilers,
> > I've been contemplating constructing a HV bridge rectifier out of 
> >tubes, to power (among other things) my coil.
> >
> > I noticed Gary Weaver has used the 1B3GT tube, which looks like 12KV at

> >2.0 ma average current.
> >
> > Can anyone suggest a rectifier tube capable of perhaps 100-200 ma of 
> >plate current at about 15KV or so?  I'd eventually like to move this 
> >onto my pig...
> Am I missing something here ? - why not use strings of 20 or so
> 1N4007's - 20KV at 1 amp for a couple of bucks - too fragile maybe ?
> how about 1N5408's?
>      ____                                                           ____
>    _/ L_/  Mike Harrison / White Wing Logic / wwl-at-netcomuk.co.uk  _/ L_/
> 

Or, a bunch of $15 microwave oven replacement rectifiers (that's for the
expensive NTE variety). Or, some rectifiers from K2AW (1 A, 15 kV)...

Having destroyed a number of semiconductor rectifiers inadvertently in cap
charging circuits, I can recommend that you carefully analyze your circuit
to determine what happens if there is a reverse voltage applied to the
output of your supply. My first unexplained death (until later) of the
rectifiers was a standard 4 rectifier bridge on a Neon transformer charging
a 14 uF cap to 10 kV or so. When I discharged the cap, the rectifiers died.
Turns out the discharge was highly underdamped, and when the voltage across
the cap reversed, it forward biased the rectifiers, dumping many, many amps
through the bridge, cooking some, but not all of the elements in the series
string encapsulated in the microwave oven rectifiers. Then, I thought I had
a cap failure because I couldn't charge higher than 7 kV, then the max
voltage kept decreasing. Actually, it was the reverse biased diodes in the
strings failing one by one (they fail shorted).

Hence forth, I have always charged through a resistor of several hundred
ohms, which at least limits the short circuit current to a few tens amps,
which is within the surge rating of the devices.

I might also suggest that you can get 6kV 250 mA HV diodes from surplus
places (like All Electronics) for about $0.40 each. Series them up, and pot
them in silicone RTV and you've got a dandy bridge. These units MUST be
potted though, because they are quite small (say .15 inches long), and at
rated voltage, they will arc around the package. I've also run them
immersed in an oil tank.  

Of course, one advantage of tubes is that they are, in general, more
forgiving of momentary screw ups. With semi's, they're cheap, but they die
in milliseconds if you make a mistake.