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Re: X-rays from light bulbs/Tesla Coils
Jim, All
Yes the graphite coating on the outside and the iron oxide coating on the
interior of the funnel which is connected to the anode the shadow mask does
form a capacitor at a value of about 25 mfd
which is used as a filter for the high voltage, I imagine they would work
for a TC if a few were hooked together leyden jar fashon.
The CRTs which had excessive X-rays was glass from Europe which was too
thin and also the wrong glass formula, These were never approved for sale in
the US.
I won't get to technical because some of this is a bit out of my area of
expertise but here goes
A CRT (in this case a color picture tube) mainly generates X-rays in the
following manner, The 3 electron beams from the electron gun are accelerated
by the intense electric feild, gaining kinetic energy. When the beam strikes
the material between the apertures of the shadow mask and/or the rare earth
screen itself energy is liberated in the form of light, heat and X-rays.
(note: the mask is coated with black iron oxide and also barium oxide from
the getter, it can reach a temperature of 300 to 400degrees C) As the anode
voltage increases the intensity of the X-rays increases, the wavelength also
decreases and the X-rays become more penetrating. X-radiation is
proportional to approximately the twentieth power of the anode voltage and
rises linear with the current. X-ray production is also proportional to the
atomic number of the target. (materials making up the internal coating,
mask, getter, anode connector etc) The limit for X-rays from CRTs in 1999
is .5mR/hr at 40kv 3ma 5cm from the front. Of course in the home the anode
voltage on a large TV won't be much more than 28kv. so the test condition is
at a higher level than the consumer chassis can go. X-rays are also
generated close to the anode button and is slightly higher than the level at
the face under NORMAL conditions
Up until 1957 the limit was 6.25mR/hr and this was at a16kv anode voltage!
The tube was also made of thinner leaded and or strontium glass. This is why
imho an old tube would be a much better canidate, although none new or old
are really
good due to the construction and glass makeup.
I visited the X-Ray lab today and asked a few questions of the engineer in
charge. He had a 25" tube set up in his test rig and kindly offered to do a
couple of experiments for me. The tube was first setup under the normal
aforementioned testing conditions. The movement on the geigers lowest range
was barely above zero. At 25kv the meter did not move at all. I then had
him collapse the scanning in so the lit screen was only about 4"x4" the
cathode current was set at .6ma and the anode voltage at 48kv 3ma (the limit
of the test set) the mR/hr went from 0 to .65mR/hr. If a higher current high
voltage power supply were available and the beam were collapsed down to an
inch or so I would have to think the X-ray level would rise dramatically
from what I saw.
What would happen if a Tesla coil were used to supply the anode I don't know
but it would be interesting. I think I would use a Tesla coil that put out a
lower voltage and a higher current as current seems to be more important
than excessive voltage. As a side note the X-ray engineer told me he once
had a "dummy load" made up for some special testing and calibration
purposes which consisted of a 3 beam CRT electron gun sealed in a 3' long
evacuated glass tube with an anode at the other end which put out some
serious X-rays, He jokingly called it his "Death Ray".
To anyone wanting to play with CRTs in the ways I have mentioned be sure to
use junk and not Dads' new RCA 37" because turning the scan down to very
small areas (like disconnecting the yoke windings) and over driving the gun
(turning up the brightness to full) will quickly ruin the tube, the electron
beam is easily capable of heating the shadow mask and or screen above the
melting point of steel!!! Unlike a real X-ray tube there is no water cooling
in a picture tube :<)
Mike KB9NZQ
> Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
>
> Very interesting.. There was a discussion a while back on this list where
> someone was going to try and use picture tubes as the primary cap in a TC.
>
>
> This is really interesting.. So what is the mechanism that makes CRT's
> generate xrays? With the normal anode voltages I wouldn't think you'd get
> much if any (of course, now that I think about it, if that same voltage
> made xrays in the HV rectifier, it would make it in the CRT). Is it one
of
> these ion bombardment things, like the cold cathode xray tubes that have
> been mentioned? The tubes that fail the test: what sort of
characteristics
> do they have? Is the test condition at a much higher 2nd anode voltage
than
> rated? Most of the TV's I have taken apart recently have a spark gap to
> limit the 2nd anode voltage.
>
> I don't know that I'd use a CRT as a cap: big, bulky, and not very much C
> per volume or pound. But, they are cheap (free)..
>
> Of course, if you zap the CRT with the secondary voltage at several
hundred
> kV from the TC, you have a good chance of making xrays.
>
> ----------