Re: Reconditioning Neons

 * Original msg to: Ed-at-alumni.caltech.edu
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting "Edward V. Phillips" <ed-at-ALUMNI.CALTECH.EDU>

> Question:
> Is the insulation adequate to permit floating the
> center tap like that?  

Off the shelf? No way. The HV windings are too close to the iron
core with insufficent insulation in between to prevent
breakdowns. Now if you were able to get enough mils of poly in
there, you might be able to make it work.

> If so, it would permit grounding one side of the HV, which 
> seems to have safety advantages.  Anyone know about this?

Humm. My experience has been that this is one place where an
insignificant amount of safety would be traded away for a
significant amount of tank circuit Q.

There is really no difference between grounding one side of the
HV circuit... and placing the tank circuit capacitor or spark gap
directly across the HV bushings on a neon sign transformer. 


    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
         X1                              C1              O
                                         | |             O  L2    
           O-----------------------------| |-----        O
         ||O                        |    | |    |   L1   O
Line in> ||O                        |           |    O   O
   -----O||O                        *           |--->O   O
        O||O------|Gnd                G1             O   O
   -----O||O                        *                O   O
         ||O                        |                O   O
         ||O                        |                O   O
           O-----------------------------------------O   O

I have removed all bypass caps, RF chokes, AND safety gap for
simplicity. I don't think many would argue that is this a bare
bones Tesla tank circuit coupled to the resonator. When you shut
the power to this circuit off: both capacitor plates, both
electrodes on the spark gap, and one end of the primary coil are
grounded through the core of the neon. True or false?

Let's rearrange the tank circuit to another common configuration

    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
         X1                                              O
                                         G1              O  L2    
           O--------------------------->*  *<----        O
         ||O                        |           |   L1   O
Line in> ||O                        |           |    O   O
   -----O||O                     -------        |--->O   O
        O||O----|Gnd             _______ C1          O   O
   -----O||O                        |                O   O
         ||O                        |                O   O
         ||O                        |                O   O
           O-----------------------------------------O   O

Here again: You switch the power off, and all components of the 
tank circuit are grounded through the neon core. Both capacitor
plates, both spark gap electrodes, one side of the primary coil.

What is the purpose of messing with the transmformer center tap
ground (which would be a bitch anyway) and trying to say the
circuit is going to be safer?

Do you want to see a circuit that is not safe?

Look closely at the circuit below:

    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
         X1                              C1              O
                                         | |       L1    O  L2    
           O-----------------------------| |-------->O   O
         ||O                        |    | |         O   O
Line in> ||O                        |                O   O
   -----O||O                        *                O   O
        O||O----Gnd                                  O   O
   -----O||O                     G1 *                O   O
         ||O                        |                O   O
         ||O                        |    | |         O   O
           O-----------------------------| |---------O   O
                                         | |             |
                                         C2              |

This one is one of the more dangerous ones. In practice it is
easy to look at this circuit when it is wired and think that by
shunting across the gap G1 you have verified there is no charge
left in the capacitance. WRONG! NOT!  When the power is shut off
the transformer side of capacitance C1 and C2 are already common,
even grounded (X1 has a center tap ground). L1 remains hot. The
voltage left in L1 at shutdown is pretty much determined by the
location of the 60 cycle wave form when the field flux in
transformer X1 collapsed. Sometimes very hot, sometimes not. You
can play the roulette wheel with the primary coil tap a few times
and not even realize the stakes. 

I am one of those people who really don't get much of a shock
when discharging circuits through my body or making that
accidental bridge to ground with the wrong hand. I had been
messing with the circuit immediately above for some time and was
aware of the "static" charge that would sometimes be present on
the primary when I went to move the tap. Maybe my skin
conductivity is very poor, but I would touch the primary with my
finger tip a few times in rapid sucession until the coil was
safed to the point I could grab the tap comfortably. I never
really thought about what was happening because there was not a
lot of discomfort.

Until one day.

A friend came over and was helping me video tape some coil work.
He had the $800 Sony videocam in his hand and was taking a
closeup shot of the tap location when he reached out with his
index finger and touched the primary. I dove like a wide receiver
and caught the videocam inches from the floor as my friend was
shot backwards nearly six feet. I guess he must be a little more
sensitive than I am. 

Anyway I showed how the circuit was dead by shunting the across
the gap (the logical thing right?), but when I touched the
primary to show my friend the circuit was discharged, I still
felt the usual "static" buildup. Looking again I realized I was
shunting terminals that were already common. Upon bridging across
the capacitor instead of across the gap I was rewarded with a
familiar blue-white <CRACK!> that nicked the corner of the copper
ring terminal on the cable I happened to grab. My friend to this
day says that just because I had my hands on it does not mean it
is "safe" by any stretch of the imagination.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12