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Re: [TCML] DC SGTC charging inductor



Hi Again

I've never used fluorescent tube ballasts but cannot see why in principle an array (sized to match the size of the tesla) of these could not be suitable . I have no idea of their characteristics and how much DC current they will take before they start to saturate, I've some ballast for a 33 watt tube but not the 4 tube ballasts you have

I suspect they will not support much DC current and quite a number of ballast inductors will be needed for a tesla of any size.

I created a test jig in which I could inject DC thru the core of a MOT while simultaneously measuring the AC impedance of the winding. I have the circuit and can send that to you separately, it used quite a number of external bits to create a DC supply with a very high AC source impedance and a method to isolate that DC from an AC injection source measuring the AC component across a small resistance in series with the winding, I needed several variacs and inductors and meters to do it. This is how I was able to determine the saturation characteristics with DC flowing as I adjusted the MOT air gap and found about 0.3mm was a good compromise still maintaining high inductance but greatly increasing the DC current the winding could support before it saturated

Given the free availability of MOT cores and knowledge about how they perform it may be the easiest way to go.

If time permits I will re-create the test set up and look how a florescent lamp ballast inductor saturates with DC in it. Given most of these are for 33 watt tubes and are small I would guess it will only be in the region of 100 mA or less before they saturate. I think they will support your 30 mA system

Saturation occurs when all of the magnetic domains in the iron core become aligned so if the DC current has already aligned all (or most) of the available domains in the iron the AC impedance drops rapidly as there are none left for the AC component to use causing very high resistive currents to flow in the winding as the inductance value drops due that saturation.

Regards
Ted L in NZ

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-----Original Message----- From: Mark X2
Sent: Tuesday, February 19, 2013 8:26 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] DC SGTC charging inductor

Thanks Ted,
I might be mistaken, but isn't saturation caused by a too low turns/volt
ratio?
Current will be max 30 mA, depending on the flyback's voltage drop, so no
worries there.
Do you think a series of 4 fluorescent lamp ballast might be any good or a
waste of time?





2013/2/18 Tesla <tesla@xxxxxxxxxxxxxxx>

Hi

I suggest using another MOT secondary , Break open the E/I by using a
hacksaw, I put about 0.3mm piece of plastic in there and then glue it
together with a building adhesive like "no more nails"

modified in this way they do not saturate nearly as much and over 1 amp of
DC is possible. Arraying these in series and parallel combinations to get
the required inductance and DC current ability can then be done. I lost my
measurements of MOT's modified in this way and can only recall the
inductance was about 10 Hy, this is easy to check by exciting the MOT with
a modest AC voltage (30 to 120)  and checking the current draw. The
inductance will be somewhat less when it is used as a charging inductance
due to the DC flowing in the core BUT the gap ensures the inductance does
not reduce too much

Hope this assist
Ted L in NZ

-----Original Message----- From: Mark X2
Sent: Sunday, February 17, 2013 9:28 AM
To: Tesla Coil Mailing List
Subject: [TCML] DC SGTC charging inductor


Hi all,

I'm experimenting a bit with a small spark gap Tesla coil to get acquainted
with dc resonant charging.

The HV source is a HVDC tester which has a huge 250W flyback and the
primary cap is a string of 10 942Cs.
For the de-Q-ing diode I intend to use 3 microwave oven diodes in series.

But I'm going to need some pointers on how to build a proper charging
inductor,
like size, app. number of windings and HV-insulation. Any advice is
welcome, thanks.

Mark
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