# [TCML] Solid state efficiency, was: mini Tesla coil specs

Frosty frosty90 at gmail.com
Thu Nov 12 16:53:22 MST 2009

```Hi all,
I think that the sparks we care about are mainly the negtive leaders. From
memory it is the negative discharges that form long leaders, and positive
coronas are dimmer and more brushy? But I dinont know how this applys to RF
discharges.

Also when we say 'efficiency', we clearly are refering to spark length vs
input power. A couple of things:how do we measure this efficiency? Do we
still use the 'Freau factor'?

k=l/sqrt(p)

where k is the 'Freau efficiency/factor', l is the spark length (in
cm.....?, I remember some posts from ages ago where we suggested
standardising this formula to cm, but the usual convention seems to be most
here are from USA, so talk in inches) and P is the wallplug real power.

If this is the case, I have a solid state coil (non double resonant) with
the following specs:

8.89 cm diameter secondary, full bridge of irfp460 mosfets, and a feedback
driving scheme

In 'CW' mode (driven by a half wave rectified mains), I get max 40cm sparks
at about 1800 to 2000 watts of power in. This gives a Freau factor of 0.89
to 0.94 cm.root watts^1 or about 0.36 inch.root watts^1., which is indeed
about 20% of a very well performing medium sparkgap coil. (For a smaller SG
coil more comparable in size to this SS coil, i think the accepted 'Freu
factor' is much lower, I cant remember the experimental evidence, but from
memory it was about 1 (in inches), making the SS coil about 36% as
'efficient'). This is not in 'interupted' mode.

One important point though is does the square root formula hold for ss coils
as well? Is there any experimental evidence to suggst this?
The other thing is we dont really have much comparison because most solid
state coils are very much smaller, than most spark gap coils, (a similarly
sized SG coil would probably have a much lower Fraeu factor, but I'm not
entirely certian).  Has anyone built, say, a 6 or 12 inch ss coil (non DR)
and run it in disruptive (interupted) mode, and made power and spark
measurements? (I have a 6" coil in progress, I should have time soon to
finally finish it and do such a thing).

I think much of the above could apply to VTTCs as well.

In 'modern' times, DRSS coils have been build closer to the sizes of
smallish to medium (and large) SG coils, so there is more of a valid
comparison there. I dont have much experience with DR coils, so Im not going
to say much, but their mode of operation is much closer to SG coils (higher
peak power, lower duty cycle), so I'd agree that they probably have the same
'spark' efficiency. Is there any data out there on 'Fraeu factors' for DR
coils?

Perhaps we should attempt to redefine the Fraeu factor as a measure of
efficiency, and introduce a new factor to take into account secondary size
(or perhaps inductance, this seems to be the most important parameter.) A
good test might be to build one secondary, and drive it from a solid state
source, and from a SG source, and compare.

This all relates to spark length, but in terms of actual power arriving at
the secondary, clearly solid state coils (DR and non DR), and VT coils beat
the SG coil any day. This is more difficult to measure, but running a 2000w
SS coil, and the heatsinks and dirver barely gets warm, nearly all of the
2000w must be going into the secondary. Its quite apparent too, the sparks
are much thicker and flame like than a SG coil.

On Fri, Nov 13, 2009 at 7:17 AM, Dex Dexter <dexterlabs at dcemail.com> wrote:

> Spark propagation for a positive or a negative spark?
> I think I read there were some differences ,and tesla coil
> sparks are generated by bipolar waves.
>
>
```