🚀 go-pugleaf

music@erich.triumf.ca (FRED W. BACH) writes:

>    My, my!  All you young folks out there...  Ever taken a Van De Graaff
>  accelerator apart (I mean a commercial or laboratory model) ?  Ever taken
>  a course in Television Repair ? at your local Technical College?  or at
>  your high school?
> 
>     The 24-v. (nay, the 5-v.) electronics generation has little experience
>  with high-voltage components.  The oil-filled, metal-can, line-charge-power-
>  supply capacitors in the VDG accelerators (they supply the power to the
>  circuit which charges the VDG belt(s)), once they're pulled out of service
>  and placed on the shelf, may develop a LETHAL voltage across the open
>  terminals.  This is common knowledge in the industry.  That is why all such
>  high-voltage capacitors must be stored with a short across the terminals.
>  We always called it "dielectric relaxation", this effect which 're-charged'
>  the capacitor all by itself.  The same effect is known to happen in the
>  glass in TV picture tubes, which is why every course you'll ever take in
>  TV repair tells you to find a ground on the TV chassis and hook a lead to
>  it and then very carefully (so as not to damage the glass or the cap) stick
>  the probe under the 30-kv rubber boot on the CRT's HV button and ground
>  the button before you disconnect the HV wire.  Many a tech has been knocked
>  on his butt by forgetting this step, at least in the old days.
> 
>     This effect depends on the type of dielectric material and the internal
>  leakage in the capacitors.  Could someone explain the physics of this 
>  dielectric relaxation, on a molecular or atomic scale??
> 
>     Incidentally, there are other self-charging capacitors out there.  They
>  are made of a Carbon-14 (or some other radio-active element which spits
>  out a charged particle) and are real handy as current sources for sensitive
>  integrators.  Since the half-life of the sample is known,( and the mass and 
>  the isotopic purity), it's electrical emission can be calculated.  These
>  make very nice current-calibration sources for remote, rugged applications.
>  They are always to be kept shorted when they are not feeding current into a
>  circuit.  Otherwise, the insulation in the connector will break down after
>  enough charge has accumulated  (Voltage = Capacitance / charge ).
> 

Fred,

        Once you drain a picture tube, it is drained- the 
dielectric is the glass envelope itself, and is not effected by 
chemical action.  You mentioned electrolytics from Van de Graaf 
generators- I think you mean Tesla coils, as the Van de Graaf is 
nothing more than a static electric generator- other than a rubber 
belt and felt "electron pick-offs" there is nothing else inside!

        Another big capacitor is found in X-ray machines.  100kv 
electrolytics- needless to say, you die if you touch!

        Ionizing radiation, radioactive decay is responsible for 
DISCHARGING capacitors.  One can prove this by holding a cap on a 
high Z meter, after charging, and noting the discharge curve.  
Example: it may take many minutes for the voltage to drop by 50% 
or so.  Now, place the same capacitor charged to the same 
potential close to a gamma or beta source and watch the reaction.  
The cap will discharge MANY TIMES FASTER.  This is due to the fact 
that the radiations are "playing pool" and knocking valence shell 
electrons off the negative charged plate atoms, and pushing them 
over to the positive plate, where they are natually attracted.  
This action causes a nulling of the capacitors potential.

        I am not an older individual, but I too worked with vacuum 
tube TV's as a kid, and remember knocking myself hard when I stuck 
a screwdriver under the aquadag of a 19" color set!  I was about 
12 at the time, and learned the HARD way!  I was also educated 
about flybacks and plate voltage using the same "school of hard 
knocks" technology................sq