Re: Blown Neon transformers

In a message dated 96-04-01 10:46:53 EST, you write:

>   I completed my large transformer today, and two of the four neon 
>transformers went bad after just a few minutes of operation.  I was 
>using four 15kV neon transformers in parallel, one 60mA and three 30ma. 
> I checked the phasing before connecting them to the rest of the 
>circuit, and it was correct.  I was tuning the primary and had just 
>begun to get some very intense blue arcs when everything was reduced to 
>a hum from the blown 60mA neon.  I am wondering if I have caused this, 
>or if it is to be expected with old transformers.  I am also wondering 
>if there is anything that I can do in addition to the normal RF 
>filtering that will help prevent this type of failure in the future.  A 
>brief description of my circuit follows.  Please excuse me if I have 
>mentioned these details to you before.
>   From the neons, my primary is protected by a typical protection 
>circuit.  The bypass caps equal 500pF in each leg, and the chokes equal 
>about 50uH.  My primary capacitor is a .025uF-at-20kV Condensor Products 
>capacitor.  The spark gap consists of two cylinder gaps in series with a 
>rotary gap.  The wheel of the rotary gap is 10" in diameter, has eight 
>tungsten electrodes, and it is driven by a universal motor capable of up 
>to 15krpm.  The primary coil is wound with 50' of 5/8" (OD) 
>refrigeration tubing spaced 3/8" apart;  the inductance measured 49.5uH 
>before I tightened the spacing up slightly (from 7/16" to 3/8").
>  The secondary is wound on 10.5" OD polyethylene sewer pipe (the walls 
>are 5/8" thick).  It has approximately 1150 turns of #21 magnet wire, 
>which are a few too many, but I figured it would be easier to remove 
>turns than to put them on.  The inductance measured 87mH and 40.19 
>ohms.  The toroid consists of a 16" diameter aluminum plate with 4" 
>aluminum tubing (24" overall).  The dedicated RF ground that I am using 
>consists of a single copper clad ground rod.
>   When I was tuning this coil, the only sparks that I got were some 
>very intense thick blue arcs that went from one point on the toroid to 
>either the beginning of the primary (about 3') or the beginning of the 
>RF gorund.  The arcs were very fast, went straight down, and I could not 
>tell precisely where they were striking.  The point that the arcs 
>originated from was the blunt end of a brass screw that was pointing 
>down (it is now very smooth and is pointing up).
>   I would appreciate any comments or suggestions that you can find the 
>time to make.  I would like to think that the transformers were weak, 
>but I just don't really believe they were.  I don't want to replace them 
>and have the same thing happen again.
>          Jim Watson
>          Pensacola, Florida


The first thing that comes to my mind is the spark gaps.  You might have too
much gap allowing the primary voltage to go too high and wipe out the
transformers.  When I was using neon transformers, I had 12 kv at 120 ma and
used a single cylindrical gap with the gaps set at .030 each for 6 gaps.
 This is with a 6" dia. secondary.  I also lost a few transformers.  Since
you are using internally current limited transformers, you don't need a
rotary gap.

When I tune at low power, I usually place a wire connected to the RF ground
near the toroid and also place a wire on the toroid facing the ground wire.
 This way, you know where the sparks are going to go.

I would also suggest beefing up your RF ground system by adding another
couple of ground rods.

Ed Sonderman