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

Dex Dexter dexterlabs at dcemail.com
Mon Nov 16 12:24:48 MST 2009

```Greg,
I am aware  an IGBT switch under given circumstances should
be more  efficient than a spark gap switch.
But it is not so simple.
May I ask at what frequency IGBT resistance is 0.007 ohm?
Certainly it not same at 60 Hz and 60 kHz.
And then there is some IGBT switching loss involved at
high frequencies,no?
Do you think that 0.6 ohm resistance of the spark is a constant
value during the transfer of energy from primary to secondary
(first notch primary ringdown)?
If yes why,if not what 0.6 ohm represents then?
I found an interesting design of IGBT coil here:
http://scopeboy.com/tesla/t4spec.html
I calculated primary Zchar and it turns out to be only
This coil at 4 kW puts out 81 inches sparks.
According to Freau empirical formula it should perform
with sparks over 100 inches long.In your opinion what
is more important for underperformance here :400 BPS or
very low primary impedance and increased primary losses?
Note that this coil also quenches at first notch.

Dex

--- lod at pacbell.net wrote:

From: Greg Leyh <lod at pacbell.net>
To: tesla at pupman.com
Subject: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs
Date: Sun, 15 Nov 2009 12:11:43 -0800

Hi Dex,

100BPS would probably produce longer arcs at constant wattage since Epri
would necessarily be higher.  However, the optimum BPS at constant power
would likely be somewhere 100 and 350BPS.

At normal TC primary operating currents, the IGBTs I used have a much
lower R_effective than the 120L rotary gap.  The IGBTs exhibit about
0.007Ohm where the SGTC is about 0.6Ohm.

Expressed as a ratio against Zpri however, the difference is somewhat
less.  The ratio for the SS primary would be 0.75/0.007 = 107, where the
ratio for the SGTC system is 14/0.6 = 23.  So one could say that the SS
switch is about 4.5 times better than the SG switch.

But again, the SS switch comes at a cost, both in terms of the IGBTs
themselves, the control circuitry, and the specialized coppersmithing
required for the primary circuit.   GL

> These are great (and BIG) coils Greg!
> With 100 BPS @ 25 kW operation  120L50k would create even
> longer discharges than at 350 BPS @ 25 kW I guess.
> Do you (dis)agree?
> BTW, compared only loss of IGBT in OLTC and spark gap
> loss in SGTC (at same power level and BPS) which one is higher
> at tesla coil frequencies?
>
> Dex
>
>
>> >
> --- lod at pacbell.net wrote:
>
> From: Greg Leyh <lod at pacbell.net>
> To: tesla at pupman.com
> Subject: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs
> Date: Sat, 14 Nov 2009 11:40:18 -0800
>
> Hi Steve,
>
> I'd tend to agree that low voltage silicon switched tesla coils tend to
> be less efficient than HV spark gap switched systems, for the simple
> fact that a low voltage system requires far higher currents and lower
> copper losses than a typical HV SGTC design.
>
> Most SS systems I've seen have Zchar values below an ohm, requiring
> milliohm-level copper losses to be efficient.  The coppersmithing
> required here is usually beyond the home-depot off-the-shelf approach.
>
> Still, with the relatively few coils that I've built, the SS coil
> outperforms the SGTC's by far, in terms of spark length/kW.  The SS twin
> prototype shown here is operating at ~7kW and easily bridging 16ft:
> http://www.lightninglab.org/misc/NLL_Prototype.jpg
>
> The Zchar is only 0.75ohm, yet in can just bridge 18ft at 7kW, or about
> 2.5ft/kW.  The larger 120L50k SGTC below will bridge about 25ft at 25kW,
> yielding ~1ft/kW:
> http://www.lightninglab.org/gallery/2008Teslathon/images/120L02.jpg
>
> The SS coil required a significant amount of coppersmithing to get the
> efficiency up.  But I think the perfect quenching that a SS coil offers
> may the biggest reason it outperforms the SGTC coil.  GL

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