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

[Greg Leyh]

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