Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
benbmw, Tue Nov 10 2020, 06:00PMI am planning to build a miniature DRSSTC and have been thinking of ways to overcome the high resonant frequencies inherent to small coils.
I am wondering what effects a multilayer secondary or a “grouped†multilayer secondary similar to what is explored in this paper would have on the performance of the Tesla coil.
Other than that paper one of the only other discussions specific to this idea is on stack exchange here where it is suggested that due to the non-transformer-like operation of a Tesla coil, any thing but a single layer winding would reduce performance.
Do the explanations presented there seem to be accurate with y’all’s experience and the operating theory of Tesla coils? I’ll be honest that while I understand what he is saying, the explanation does not make intuitive sense to me.
Another idea I had for reducing secondary resonant frequency is to use some sort of ferrite core. The main objection to this is the extra loss added from the core. Most discussions also pointed out that the core is likely to saturate due to the high current in the primary; but what if the core only occupied the top half of the secondary to still increase its inductance while escaping most of the flux from the primary?
Feel free to weigh in on any aspect of this discussion and correct me where my assumptions are wrong!
Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
RoV IW3IPD, Thu Nov 12 2020, 09:23PM
Hi benbmw, a classic multilayer (like a standard transformer), even ignoring the problem of inter-layer capacitance, could never work due to arcing between layers. The approach of the paper could work, but I don't see a real advantage, because the modest reduction in length doesn't compensate the very high complexity of the support and the pain of winding it completely manually. Length reduction is modest, because if you make equal slots and spaces, with 3 layers you reduce length only by 2/3. And 3 seems already borderline to avoid arcing across the wire insulation.
With ferrite you would greatly reduce Q (i.e. losses); besides, often ferrite is not a good HV insulator.
As you read, the operational mode of a Tesla coil is trasmission line-like, where uniform distributed inductance and capacitance define the line impedance and resonance is at lambda/4 (and odd multiples). Lower than that if you apply some capacitance between ends.
If you want to build a compact coil at reasonably low frequency, increase the coil diameter, use relatively thin wire, apply a top hat, place a metallic ground disk on the bottom and connect it to a larger ground (wires, metallic structures). Grounding only through the AC outlet works, but can cause damage to other equipment.
RoV IW3IPD, Thu Nov 12 2020, 09:23PM
Hi benbmw, a classic multilayer (like a standard transformer), even ignoring the problem of inter-layer capacitance, could never work due to arcing between layers. The approach of the paper could work, but I don't see a real advantage, because the modest reduction in length doesn't compensate the very high complexity of the support and the pain of winding it completely manually. Length reduction is modest, because if you make equal slots and spaces, with 3 layers you reduce length only by 2/3. And 3 seems already borderline to avoid arcing across the wire insulation.
With ferrite you would greatly reduce Q (i.e. losses); besides, often ferrite is not a good HV insulator.
As you read, the operational mode of a Tesla coil is trasmission line-like, where uniform distributed inductance and capacitance define the line impedance and resonance is at lambda/4 (and odd multiples). Lower than that if you apply some capacitance between ends.
If you want to build a compact coil at reasonably low frequency, increase the coil diameter, use relatively thin wire, apply a top hat, place a metallic ground disk on the bottom and connect it to a larger ground (wires, metallic structures). Grounding only through the AC outlet works, but can cause damage to other equipment.
Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
benbmw, Mon Nov 16 2020, 04:47PM
Roberto,
Thanks for the comments and insight! It sounds like there’s no real motivation or performance gain to be had with any of these methods.
You mentioned making the coil fatter to increase inductance. I assume at some point the aspect ratio will become a problem. I’ll have to read up more on how aspect ratio affects performance.
-Ben
benbmw, Mon Nov 16 2020, 04:47PM
Roberto,
Thanks for the comments and insight! It sounds like there’s no real motivation or performance gain to be had with any of these methods.
You mentioned making the coil fatter to increase inductance. I assume at some point the aspect ratio will become a problem. I’ll have to read up more on how aspect ratio affects performance.
-Ben
Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
Terry Fritz, Fri Dec 18 2020, 03:20AM
Why worry about the high frequencies.... The DRSSTC tech is pretty high now and MHz is an option.
In a tiny coil, that is just the game.
If your inductance is low you can use much larger primary capacitance.
In general, making the secondary basically the same in all dimensions (highest internal area) gives the highest inductance.
Helical Coil
Lh = (N x R)^2 / (9 x R + 10 x H)
Where:
Lh = Inductance in micro-Heneries
N = number of turns
R = Radius in inches
H = Height in inches
Terry Fritz, Fri Dec 18 2020, 03:20AM
Why worry about the high frequencies.... The DRSSTC tech is pretty high now and MHz is an option.
In a tiny coil, that is just the game.
If your inductance is low you can use much larger primary capacitance.
In general, making the secondary basically the same in all dimensions (highest internal area) gives the highest inductance.
Helical Coil
Lh = (N x R)^2 / (9 x R + 10 x H)
Where:
Lh = Inductance in micro-Heneries
N = number of turns
R = Radius in inches
H = Height in inches
Re: Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
Sulaiman, Thu Dec 24 2020, 03:06AM
The greatest inductance for a solenoid winding of given wire length is when H=0.9xR
the inductance will then be RxNxN/18
the self resonant frequency of the winding will be 28/Lw MHz
(Lw=length of wire in m)
adding a topload will of course reduce this frequency.
If the same length of wire is used to make a solenoid where L=0.9xD then F=75/Lw.
(from memory)
Sulaiman, Thu Dec 24 2020, 03:06AM
The greatest inductance for a solenoid winding of given wire length is when H=0.9xR
the inductance will then be RxNxN/18
the self resonant frequency of the winding will be 28/Lw MHz
(Lw=length of wire in m)
adding a topload will of course reduce this frequency.
If the same length of wire is used to make a solenoid where L=0.9xD then F=75/Lw.
(from memory)
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