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Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
I am beginning to understand better how the SSTCs work and why they need suc high voltage on the primaries to throw sparks.
At such high frequencies (200kHz or so) there is not enough time for the current to build up beyond a peak of 100mA or so with say 12V input at the primary.
I used the standard AC formulas to do a rough calculation of current and around 0.1A is what came out out using AC impedance on the primary coil inductance and half bridge capacitors.
And this fits with my observation that my Tesla coil can only excite a fluorescent bulb but not throw any sparks with 12VDC input.
The GDT puts out a high frequency sine wave (on my oscilloscope) so surely the current flowing through the power FETs will effectively be high frequency AC.
Which in turn means the AC impedance formulas ARE good enough to calculate the peak current if you factor in the duty cycle that changes the effective voltage, e.g. 50% means half the input voltage.
So my fear about peak current getting so high that it starts melting wires etc is a great deal less likely than I had assumed.
Next on the agenda is to finish off my 120V DC power source.
I have one of those fuse box safety switch things (8A) on the primary coils of my series connected modified MOTs.
But I think I need to put in some fuses on the DC side just in case.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Comments from someone who has never made a Tesla coil of any kind.
Most "wound" devices can be rewound for operation with different voltage and current. Frequency, coil sizes and weights, magnetic field strength, electrical & mechanical power, and efficiency are invariant.
>>At such high frequencies (200kHz or so) there is not enough time for the current to build up beyond a peak of 100mA or so with say 12V input at the primary.
One remedy might be to use a primary coil with fewer turns, in proportion to the lower working voltage. Thus lower inductance by factor of about N^2. Resonant capacitance would have to increase by that same factor. Of course the number of turns can't be less than one.
For TC's of a given size and frequency, a useful metric for (say) the ability to throw sparks might be the primary voltage per turn.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
klugesmith wrote ...
Comments from someone who has never made a Tesla coil of any kind.
Most "wound" devices can be rewound for operation with different voltage and current. Frequency, coil sizes and weights, magnetic field strength, electrical & mechanical power, and efficiency are invariant.
>>At such high frequencies (200kHz or so) there is not enough time for the current to build up beyond a peak of 100mA or so with say 12V input at the primary.
One remedy might be to use a primary coil with fewer turns, in proportion to the lower working voltage. Thus lower inductance by factor of about N^2. Resonant capacitance would have to increase by that same factor. Of course the number of turns can't be less than one.
For TC's of a given size and frequency, a useful metric for (say) the ability to throw sparks might be the primary voltage per turn.
I was merely testing the primary with 12V so I was not really expecting anything spectacular.
I am feeling my way slowly forward with this thing and not making any assumptions.
I was not sure at all how much current was likely to be drawn by primary but my multimeter indicated something of the order of a few 10s of mA at 12V input voltage.
It was never the less enough to light up a fluorescent bulb held near the secondary so I know the circuit is working as expected.
Next I will test it with my 120V DC power supply.
If that goes OK then I may consider increasing the voltage to perhaps 220V DC - my caps are rated for 250V
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Primary coil current
