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RE: [TCML] How linear does a ballast need to be

Hey Deano,

I have been experimenting with dual inverse E core MOT's (strips the shunts
and the filament windings in very fat cores). The advantage is it gives you
more winding window space. I have 4 identical ones 2 are stacked 2 are not-
this not for ballast, but rather for power supply efficiency tests.

It is well known that MOT's saturate at ~100 volts. NOT a good thing for
current control! It has been stated here that there should be 1.5volts per
turn and 3amps per square inch of cross section of the core. (I have been
reading here for a long time). I would want that for current control! Heavy
duty ones have 12awg primaries. Lesser ones may be 14awg. I was surprised to
find mine were aluminum under the coatings even though they are for a 1200w

My MOTs have 81 turns when it should be closer 180t/120v=1.5v/t so (2)
primaries in mutual series is perfect for 120volts if they are similar to
mine. 4 in series on inversely stacked "E" cores should be perfect for
240volt operation! Surely you will have more than 2"^2=6amps without
saturation with 1.5 volts/turn. Maybe using a current clamp or current
transformer would tell you where you are if you connect the (4) 120v series
coil MOTs across 240v fused of course!

This brings up the soft start verses inrush current which I'm not touching
and will leave open for further comments or internet searches. A heavy
enough fuse or breaker will cancel this but falsely protect the "inductor"
and down stream stuff to your spec. A slow blow may work.

You could add a shim such as PE milk carton strips in between the E cores or
one side while measuring the current.

I would love to know how such a setup works. Let us know if you try it.

Jim Mora

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of David Dean
Sent: Wednesday, July 08, 2009 7:04 PM
To: Tesla Coil Mailing List
Subject: [TCML] How linear does a ballast need to be


I just improvised a ballast from two MOT primaries in series on a single MOT

core with adjustable gap A La Richie Burnett.

Core cross sectional area 3.5 sqin

Number turns, wire gauge, unknown-- whatever a MOT primary has

I am looking for 100mA short circuit 14400V open circuit for my application,

that is about 6A at 240V input. That hits on the dark blue line (0.035" gap)
my improvised ballast from the test I just did-- see:


My Question, is this linear enough to use as is, or do I need to go back to
drawing board?

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