Re: X-Ray Transformer on eBay

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Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Godfrey,

You wrote:

> Hello Ivan
Also x-ray transformers are rated in peak
> voltage, so this one is really 21,000v rms. However this particular
> transformer does looks interesting in that the secondary coil is
> encapsulated and not placed over the primary. If one could disassemble
> the core and get it back together again, the primary could be replace
> with a lower gauge wire to run hotter. One should count the number of
> turns on the discarded primary. Only the big x-ray transformers counts
> towards impressive arcs.
> Godfrey Loudner

One would think that, from the kv(p) designation, where (p) = peak.
From personal experience in measuring the transformation ratio it
would seem that the AC rms value reaches or even exceeds  the kvp
rating, though, with 240 volt input.  For example, a 140 kvp x-ray tranny 
that I once had showed about a 750:1 secondary to primary voltage ratio.
Therefore 240 volt(750) = 180 kV, which is actually considerably
higher than its kVp rating. Its nameplate did say 220 volts input
though, not 240. But at 220 volt input, that would still yield a 165 kV
AC rms output. This transformer would make sparks nearly 14"
long, too ;^) but it finally burned out on me ;^(( I'm thinking that
the kVp rating is designed for the load of the x-ray tube and the
this would allow for a voltage drop which may very well bring the recti-
fied output into the AC peak x (.707 range). For that reason, it's
NOT a good idea to run an x-ray tranny at its full input voltage
with the HV secondary output open circuit as this allows the po-
tential of the secondary to go higher than what the transformer is designed 
for. That's obviously what happened to my ill-fated 140 kVp,
500 mA tranny that weighed nearly 1000 lbs when I was streching
out the output leads 14" apart to get ever longer sparks. It finally 
"sparked" at the wrong place ;^O

BTW, you're correct in that big x-ray transformers are what make
the impressive arcs. I don't think that I have ever heard of a voltage
as low as 30 kV for diagnostic x-rays, either. Most dental x-ray
units (which require the lowest voltage for diagnostic puposes-
only penetration of the flesh of the cheak is required) are 60 to
70 kV.

David Rieben