Re: neons & rsg

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Sunday, August 03, 1997 8:09 AM
To: 	Tesla List
Subject: 	Re: neons & rsg

Tesla List wrote:
> From:   Mad Coiler[SMTP:tesla_coiler-at-hotmail-dot-com]
> Sent:   Friday, August 01, 1997 11:15 AM
> To:     tesla-at-pupman-dot-com
> Subject:        neons & rsg
> All,
>      I know many have said not to use rotary gaps with neon xformrs.
> What is the theory behind this? Could you use a rsg with a neon if you
> use protective chokes on the neon outputs?
> ______________________________________________________
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Mad Coiler,

There's a number of theories about why, and plenty of empirical evidence
by those who've got many years of practical coiling experience. The most
likely explanation has to do with a combination of two factors -
variability in gap breakdown voltage and low frequency resonant rise. 

Most tesla coil design tools will calculate an "optimum" value of
primary capacitance for a givne size transformer voltage and current.
For a 15,000 volt 60 MA neon 60 Hz transformer, this works out to be
about 0.01 uF, and scales linearly with current - at 120 MA it would be
about 0.02 uF. It turns out that this value also corresponds to the
value where the transformer's effective inductance (the portion which
causes it to current-limit) resonates with the tank capacitance at 60 Hz
in a series LC circuit. Under these conditions, the voltage across the
tank cap OR the neon can rise to a level of Q times the normal voltage,
where Q typically lies in the 5-10 range. The secondary current also
tries to rise by a factor of Q, so your poor neon is really taking a
beating! Unless your main spark gap breaks down at a voltage which is
less than the maximum voltage standoff capability of the transformer or
the primary cap, you're going to smoke something - and usually its the

Now unlike a static gap, a rotary gap under certain conditions can
exhibit quite a lot of variability in breakdown voltage. For example,
suppose you just "miss" firing on one electrode presentation, allowing
the tank voltage to rise substantially before the next electrode
presentation. If this voltage exceeds the breakdown voltage of the
transformer or cap _just once_, the resulting arc then destroys that
component. Series chokes alone will NOT help you. However, an
appropriately adjusted set of safety gaps across the neon combiined with
HV RF chokes may help extend the neon's life somewhat. In those
instances where the voltage rises too high, the safety gap would fire,
saving your expensive components. Unfortunately, the wider the safety
gap, the greater ITS variability in breakdown voltage, so even this may
not always save your poor neon. Most coilers at this stage migrate to a
pole pig or potential transformer, since the breakdown voltages of these
tend to be in the 100+ KV range.

Safe coilin' to you!

-- Bert --