Re: X-Ray HT power supply

To: tesla-at-pupman-dot-com
Subject: Re: X-Ray HT power supply
From: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Wed, 25 Nov 1998 11:17:22 -0700
Approved: terryf-at-verinet-dot-com
Gavin Hubbard wrote:
>This morning a local radiology technician (who knows of my HV 
>interests) 'gifted' me a 150kV, 500mA (peak) x-ray HT power supply. 
>The power supply is operational and comes from a decommissioned 
>installation.

<SNIP>
Gavin,
 What is the make/model of your unit?  I may be able to get detailed 
info on it for you (if you don't have the original documents).

How heavy is your unit?  That will basically determine what is inside...

The newer units use a high frequency switcher device, where incoming 
3phase power is rectified and filtered into DC.  This DC is then chopped 
and applied to a large ferrite core at about 25khz.  The output winding 
is typically centerpoint grounded and may contain doublers on each leg, 
etc to reach the required +-70kv differential.  One of these 
switcher-type units weighs ~ 300 lbs.  110lbs is the weight of the 
ferrite core/doubler assembly.

The older units are basically a large high voltage PIG.  One tank I 
disassembled contained a toroidally wound xfmr core about 3ft in 
diameter, about 4x4 inches in cross section.  Several windings were 
positioned on this core, suitably insulated from each other.  The output 
(center-tapped 140kv at 60hz) was run through a dual bridge board like 
the one on my site:

http://www.geocities-dot-com/CapeCanaveral/Hangar/6160/electric.html#diode

The whole thing is placed under oil and are exceedingly heavy.  Some of 
our 60KW units are in excess of 1200 lbs.

(At work I just ordered a 50Kva 480 to 380 3phase autotransformer to 
help power some of these things...)

Voltage control of the ferrite type is controlled by the pulse width of 
the DC chopper drivers. Voltage control of the PIG type is controlled 
via IGBT/SCR phase control techniques on the incoming 2-3 phase power.  
Really really ancient ones had synchronous mechanical rectifier which 
would switch polarities in sync with the voltage!

Other smaller units don't even bother rectifying the power, as the x-ray 
tube has a rectifying action in and of itself (heated filament etc).

If you have one of the PIG kinds - I'd suggest rewinding the core to 
provide you with a lower voltage for coiling.  If you have a switcher 
unit - you might be able to scrape off the cap doublers, etc for a more 
reasonable tesla coiling voltage.

Note that the *cabling* which would run from the x-ray power supply to 
the tube contains significant capacitance, and, if left charged, will 
contain a dangerous amt of energy .  Most xray power supplies contain 
circuitry to discharge the cable energy after the exposure is completed.  
It wouldn't do to electrocute the service engineers, would it? :)

We are presently using a 50ft piece of such cable to route power from 
our pig into the backyard where the 6inch diam coil resides.

In case you are wondering - a hospital would pay about 17 to 30 thousand 
dollars for such a power supply :)

Later.
-Bill

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