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Dual MOT coil capacitors.

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Patric

Sun Feb 09 2014, 04:22PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

I have 10 MOT capacitors of 1 micro-farad. When I connect them in series, i have a 100nF, 20kV capacitor. Can I do that and use the capacitor in a dual coil Tesla?

Kizmo

Sun Feb 09 2014, 04:43PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Nope. Microwave capacitors have dielectric that is not suitable for high frequency operation. They might work poorly for a short while but overheat rather quickly.

Sulaiman

Sun Feb 09 2014, 05:24PM

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

I've not tried and I'm sure there are many reasons why these capacitors are less than ideal ... but ...
they are probably better than what was available to N.Tesla
and if you do kill them, you're no worse off than not trying !

I've heard of magnetrons and transformers failing in microwave ovens
but not the capacitors (they must fail because spares are available)
considering the temperatures in a typical microwave oven enclosure,
the capacitors must be quite rugged.

There are three main problems with T.C. primary capacitors
1) insulation
... these will be better than what the average constructor will fabricate (me included)
just be sure to insulate the cases from each other

2) heating due to I2R of plates and terminations
... should be quite decent for these

3) heating due to dielectric losses
... this is where I'd expect problems due to the dielectrics
paper in oil isn't the best, but it's better than most
e.g. much better than common soda glass

If you go ahead (I think you should, let us know the results)
. insulate the cases from each other and everything else
. be prepared for hot pressurised oil to explode out (not a huge explosion)
. run for a short burst, say 1 second, discharge caps and check temperature
. repeat run for 5 seconds and check
. again for 1 minute.

warm = excellent,
very warm = ok,
too hot to hold = risky,
the maximum operating temperature will be printed on the cans.

lower frequency will produce lower heating, so consider as large a secondary as practical, at least 4" diameter and 12" or more diam. topload.

The spark gap will be the weak point I guess
LOTS of airflow required between the electrodes.

If other members have tried and failed;
A) ignore everything above
B) try anyway as they may have tried at a very high frequency
(small secondary)

Worst case you will have to use a different primary capacitor,
killing TC primary capacitors is NOT new !!

.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

Kizmo, is your objection theoretical or from experience ?

Patric

Sun Feb 09 2014, 07:29PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

Thanks for the reply! I will try it, the frequency will be about 300 kHz, and I will use a rotating spark gap. Thanks again Sulaiman!

Kizmo

Sun Feb 09 2014, 10:03PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I have personal experience. These caps have mylar or paper and oil dielectric which will overheat very badly at 100s of kHz. Also their ESR is typically something like 0.5-3 ohms, resulting quite poor primary circuit Q.

I would not recommend wasting your time with these

Patric

Mon Feb 10 2014, 03:25PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

Any hints? Some experience?

Patric

Wed Feb 12 2014, 12:12PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

Started...

Sulaiman

Wed Feb 12 2014, 12:34PM

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

just so that I don't live in fear of bad advice causing you danger,
could you put an enclosure (e.g. wooden box) over the cap bank
with a fan and exhaust port.

Dr. Dark Current

Wed Feb 12 2014, 02:10PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The construction of these caps is not really suitable for pulse duty. The first thing to fail will be the the thin foil connections between the plates (just 1 point) and the terminals. The construction is also "high inductance".

omegalabs

Wed Feb 12 2014, 02:27PM

Registered Member #1521 Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128

Patric wrote ...

Started...

They're not even the same type, if these caps have different capacity, the voltage won't be equal on them, and the internal 10Mohm resistor will not help in this case.

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