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Steve Ward

Sat Aug 05 2006, 10:09PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

If you really want to measure the resonant frequency of the secondary, you can hook the base of it up to a signal generator (ground the other output of the gen). Hang a scope probe in the air near the coil, and sweep the frequency until you find the lowest freqency that really peaks on the scope. It should be close to whatever you calculated. If you get a number thats way off, be suspicious. If you dont have a signal generator, you could use a simple 555 circuit too, it doesnt have to be fancy.

To measure the primary inductance, the best way would be to dry fire the primary (that is, remove the secondary). You shouldnt need but a few volts on the tank cap before firing. You can watch the voltage across the tank cap, or across the primary on the scope. Try to capture the ringdown, and work out the oscillatory frequency from that. Once you know this frequency, and you are fairly certain of the tank cap value, then you will know the *total* inductance of the tank circuit (which is really what matters for tuning). There might be ways of calculating the inductance, but my method would be the most direct way of determining the total loop inductance of the setup.

I think most guys tune their OLTC's by changing the number of capacitors in their bank, rather than trying to tap the primary.

...

Sat Aug 05 2006, 10:26PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Inudctance of primary: Calculate inductance of a single turn of whatever diameter the primay is (uH~[diameter in inches]/18 if I remember correcly, is a very rough estimate) then count divide by the number of turns.

Frequiency of secondary: Hook it to a function generator and see when it resonates. I just take the the 600ohm output of a function generator anf hook one side up to the gnd of the secondary, the other up to a ground plane under it, then set a scope probe a few inces away from the top of the secondary, set the time base of the scope to about what I expect it to resoante at, then sweep the frequiency until I get the biggest signal picked up... works great

It also works great to show how object getting near the coil accect the resonay frequiency

Steve Ward

Sun Aug 06 2006, 02:04AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

then count divide by the number of turns.

Not so fast... Coupled inductors do follow the rule 1/Ltot = 1/L1 + 1/L2... that *uncoupled* inductors do. The inductance is reduced somewhat from an ideal single turn (maybe by a few %), but its definately not 1/5th, in this case. Just trying to keep the facts straight.

Jrz126

Tue Aug 08 2006, 06:44PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Well I was able to measure the primary frequency, 53 khz, which gives me an L of 0.66uH, my C is 13.44 uF.

So now I think I'm in a bit of trouble. My secondary calculates out to 399mH and 17.73 pF. With a 6x24" topload (30.02pF), I'll have a resonant frequency of 36.45 kHz.

In order to match this with my current primary, I'll need to increase my cap bank to 28.4 uF. Which means I will have to buy some new caps. Or I could try to make a 2 turn primary...Is it possible to parallel several 2-turn primarys?

Do my numbers make sense? Are they too extreme?
I'm using TeslaMap to estimate my secondary frequency.

Steve Ward

Tue Aug 08 2006, 07:50PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Your numbers sound OK to me for the primary (though i calculate .67uH for the primary, good enough).

Given your L and C values for the secondary, i calculate nearly 50khz for resonance, but this doesnt take into account all the distributed effects in the secondary, which may change that rough number.

In any case, you might want to measure the secondary frequency since there is some question here...

I suggest getting more capacitors so you get more bang energy. If you charge your current tank cap to 1kV, thats a bang energy of 6.72J. Thats something like 806W input power at 120bps. Anyway, i think you might only need a few more caps to bring the system in tune, so maybe you will end up at around 8-9J bangs, which should make for 5' sparks if you really wanted

.

Id really suggest eventually making a new MMC for 2kV operation if you really want to get some long sparks. In that case, it may be worth doing 2 primary turns so that your tank cap wouldnt be a 30J monster (i dont know that your coil is tall enough for 30J bangs, which would be about 8-9' sparks if its efficient).

Dr. Drone

Tue Aug 08 2006, 07:55PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Steve Conner

Wed Aug 09 2006, 11:16AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I second what Ward said, pleeeeese use 2kV, it'll make the thing so much better. My OLTC2 ran at a charging voltage of 1kV (DC resonant charged from a 600V bus) and it was never really more than "OK" performance wise.

Jrz126

Wed Aug 09 2006, 05:47PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Chris, I dont quite follow your diagram there...I'm not sure what that weird box thing is by the 10K pot.

There's only one problem with using 2kV to power it, and that is that I dont have a 2kV source. How much current should I expect to draw from the line at that voltage? Would I be able to use a MOT to power it?

Also, where would I be able to get some nice caps from? I'd like to be able to use the ones I have if possible since they were free.

Can I run several 2-turn primarys in parallel? Maybe I'll attempt it with some 10 gauge wire to see how well it will work.

...

Wed Aug 09 2006, 07:27PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I think that Chris is trying to rectify the output to give an average voltage (which would be highest when it is in tune) but as long as you have a scope you don't need any of that.

The current will be proportional to the break rate... If you are using 10J bangs at 2kv 100bps is going to draw 500ma, just about right for a mot... If you want to crank things up you would need to parallel a few more on

As to the caps, a standard CDE 942 .15ufs/2kv cap is about 1/3J, so for for well under $100 you could make a mmc... You should be able to get away with just paralleling them up, but you would need to make sure you don't have very many spikes...

I would assume that multiple 2 turn primaries should work fine...

Dr. Drone

Wed Aug 09 2006, 07:47PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

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