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Tube-type bridge rectifier
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From: vonpostel [SMTP:vonpostel-at-prodigy-dot-net]
Sent: Monday, February 09, 1998 8:29 AM
To: Tesla List
Subject: Re: Tube-type bridge rectifier
Tesla List wrote:
> From: Bill the arcstarter [SMTP:arcstarter-at-hotmail-dot-com]
>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...
Bill:
Here is a reference that will help you figure out how to build your
rectifier so you can move it to your pig:
Terman, F. E. "Radio Engineers' Handbook", McGraw-Hill.
My copy is ed. 1 4th printing 1943.
There is a table on page 598 showing the voltage and current
relationships for rectifier circuits. What follows is based on that
table. It assumes the d.c. componen of the output voltage is 1.
Given:
Circuit: Single phase , full-wave bridge.
D.C. output voltage: 15,000 volts
D.C. output current: 0.2 amp.
RMS value of transformer secondary voltage:
1.11 x 15,000 volts d.c. = 16,650 volts rms
Maximum inverse voltage. (aka. peak inverse voltage or PIV)
1.57 x 15,000 volts d.c. = 23,550 volts peak.
If we work the other way from a transformer having an output
voltage of 14,000 volts rms. and the same circuit:
D.C. component of output voltage:
14,000 volts / 1.11 = 12,126.6 (d.c. component)
Maximum inverse voltage (PIV)
1.57 x 12,126.6 volts = 19,801.8 volts peak
That means that any rectifier tube you use will have to be rated with a
PIV high enough to do the job.
As mentioned by some one else, the 866-A and the 872-A mercury vapor
rectifiers both have a PIV of 10,000 volts.
10,000 volts / 1.57 = 6369.4 volts r.m.s. Running them in a single
phase full wave bridge from a transformer putting out more than 6000
volts rms is asking for trouble. What happens is that they arc inside,
the plate gets from dark red to white, the tube is lost, and the H.V.
transformer sits there looking at a short circuit. That is when you
have to be quick on the off switch or have good over load protection.
An old GE Transmitting Tube manual give these gaseous or mercury vapor
rectifier tubes:
GL-266-B PIV 22,000 volts, Average Amp. 10
GL-857-B PIV 22,000 volts, Average Amp. 10
High vauum rectifiers are generaly available with a higher PIV
KC-1 PIV 100,000 volts, Max. Amp. 1
KC-3 PIV 150,000 volts, Max. Amp. 1
GL-218 PIV 50,000 volts, Max. Amp. 0.750
GL-411 PIV 100,000 volts, Max. Amp. 0.3
GL-8020 PIV 40,000 volts, Max. Amp. 0.075 Average Amp. 0.1
The foregoing is only a part of the story. That is why I suggested the
reference. You didn't say if you wanted "pure" d.c. which would mean
the deseign of a filter system. The filter configuration must consider
both the rectifier tubes and the load. Every thing interacts with
everything.
As a practical matter, the filament transformers must be insulated for
the PIV. The use of mercury vapor rectifiers can be tricky where there
are large amounts of r.f. around. Operation can suffer because of the
ionization of vapor in the tube by the r.f. Shielding i not difficult,
but you should plan for it.
Hope this helps.
Ray