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Choosing secondary for TC

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mihlikos

Sat Dec 24 2011, 04:54AM

Registered Member #3947 Joined: Mon Jun 13 2011, 11:06PM
Location:
Posts: 16

I was thinking about the secondary of a tesla coils.Why most choose to use wire like 24 or 26 awg ?
Why not using a 18 or even a 10 awg for secondary ?
Is it because that the most people like the higher voltage and sacrifice the higher frequency?
Anyone build any tesla coil with these ranges of gauge?

Sulaiman

Sat Dec 24 2011, 09:39AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For fixed secondary and top load sizes,

for an sgtc the primary capacitance is fixed by the NST etc.
so primary turns reduce proportionally to secondary turns
which results in a very low impedance primary circuit with very high currents causing large spark gap losses which reduces performance.

for a sstc the switch transistors don't work as well at the resultant high frequencies

mihlikos

Fri Dec 30 2011, 06:45PM

Registered Member #3947 Joined: Mon Jun 13 2011, 11:06PM
Location:
Posts: 16

Thank you Sulaiman,
Another question i have is about how to choose a power supply.
Lets say that i already have primary and secondary coils and are both made for a frequency of 500kz .The capacitor on the primary is 0.02uf
So the capacitor must discharge 500.000 times per second on to primary, right?
If this is correct how much energy do i need ?
how do i calculate the supply needed ?
If each discharge is 1joules or 1 watt that is mean 1/2million watt per second. Hm... i got confused.

Fulmen

Fri Dec 30 2011, 11:46PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

mihlikos wrote ...

Lets say that i already have primary and secondary coils and are both made for a frequency of 500kz .The capacitor on the primary is 0.02uf
So the capacitor must discharge 500.000 times per second on to primary, right?

No, the cap will be charged by the PSU at mains frequency. When charged to the spark gap breakdown voltage it will discharge through the primary, forming an LC-circuit resonating at 500KHz.

mihlikos

Sat Dec 31 2011, 05:24AM

Registered Member #3947 Joined: Mon Jun 13 2011, 11:06PM
Location:
Posts: 16

Thanks Fulmen, now it makes sense.
What is the formula to estimate the power supply needed having in mind that i have only 2 numbers,the 500kz and the 0.02uf ?
On many sites there are calculations to determine the coils based on psu but not the opposite.
I need to understand not just have the answer that is why i am asking.
Thanks

Fulmen

Sat Dec 31 2011, 11:54AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

I', sure there are others that can answer this better, but I'll have a crack:

First off, the voltage will be determined by the voltage rating on the cap.
The current needed can then be from the energy stored in the capacitor, which is 1/2CV^2.

So for a 10KV cap @ .02uF the energy in the cap would be 0.5*,02u*10K^2= 1J. This energy will have to be delivered in 1/4 cycle (time from 0 to full voltage in a sine curve). 1J is 1Ws, so you need 1J/0.005s=200W. And 200W/10KV = 20mA.

Does this make sense?

Sulaiman

Sat Dec 31 2011, 11:54AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

How many ways are there to power a Spark Gap Tesla Coil?

If you are unable to calculate the power required then I guess that you'll also be unable to design the required high voltage switching power supply,
so it will be a case of looking to see what others managed to buy or construct,
see which power sources you can obtain, and go from there.
Reverse to the 'correct' practice of designing the power supply last.

The answer is; 0.5 x bps x Cpri x (Vgap)squared
Power = Rate of energy transfer = (bangs per second) x (energy per bang)
Ideally.

mihlikos

Sat Dec 31 2011, 03:21PM

Registered Member #3947 Joined: Mon Jun 13 2011, 11:06PM
Location:
Posts: 16

Thanks Sulaiman, again your right.My question wasnt the right one.My English or language skill is not good.
The proper question it should be
What is the relation between the capacitor discharges (bps) and the primary coil so to give the optimum results.If for example is 100bps why not 150 or 1500.How we calculate the right bps?

Please dont take as a rule the 50hz from the grid and answer based on that.But assume that we can have any hz on our psu and we only need to find the right bps for the specific primary coil.

Sulaiman

Sat Dec 31 2011, 04:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Getting a little out of my depth as this usually applies at higher power than I use but here goes;
Spark 'spectacularity' is proportional to SQRT(Power) approximately
so more bps is good,
and more energy per bang is good

For a particular supply/gap/capacitor/primary/ secondary........etc
there will be an optimum operating point
with experience you could design the operating point and be quite close in practice
for the rest of us it's more a case of continual 'tuning' and modifications.

The simple fact is that the only available simple high voltage sources seem to be

OBIT, NST, MOT, Pole-Pig or other mains frequency transformer
hence 50/60 Hz or 100/120 bps.
Or some type of electronic hv dc supply/charger, e.g. flyback- or ignition-transformers,
maybe with dc resonant charging, where bps can be varied.
OR you can go the SSTC/DRSSTC.and others route of direct electronic drive at the TC resonant frequency etc.

IF you can start with high voltage low current (e.g. NST) rather than high power lower voltage (e.g. MOT) it makes things easier and 'safer'
but you have to accept that it will not look like the spectacular coils on youtube
still exciting though! (and more affordable)

mihlikos

Sun Jan 01 2012, 01:21PM

Registered Member #3947 Joined: Mon Jun 13 2011, 11:06PM
Location:
Posts: 16

Really appreciate your time Sulaiman.Almost understood the most , because by
using the Tesla Coil CAD program to calculate the secondary found that there are 2 resonant frequencies
one is the Approximate resonant frequency and the other is the Secondary quarter wavelength resonant frequency.
Can you explain me what are these 2 different frequencies ?

Secondary Coil Design

Diameter of secondary coil : 180.00mm
Winding height of secondary coil : 950.00mm
Wire diameter for secondary coil : 2.00mm
Spacing between windings : 0.09mm
Secondary turns : 454.00
Secondary wire length : 257.04m
Secondary inductance : 6.39mH
Approximate resonant frequency : 508.84kHz
Secondary quarter wavelength resonant frequency : 291.78kHz
Secondary self capacitance : 15.31pF
Toroid capacitance required to form quarter wavelength coil : 31.25pF

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