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First tesla coil thread - Dual Mot

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MrFlatox

Thu Oct 17 2013, 07:37PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Overcharged ? I don't get it. How my capacitor could be charged with more than 1/2 * C * UÂ², where U is a sine. PFC capacitor are wired across the mains, and hopefully, they do not explode. You may be right that my capacitor could have survived, but I do not understand why. If it failed with 4200vac, then the insulation was not strong enough i guess, spark gap or not.

redruM69

Fri Oct 18 2013, 05:10AM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

MrFlatox wrote ...

Overcharged ? I don't get it. How my capacitor could be charged with more than 1/2 * C * UÂ², where U is a sine. PFC capacitor are wired across the mains, and hopefully, they do not explode. You may be right that my capacitor could have survived, but I do not understand why. If it failed with 4200vac, then the insulation was not strong enough i guess, spark gap or not.

Sorry, "overcharged" wasn't the best term to use. Overloaded? You probably experienced resonant rise, and the cap most likely experienced voltages well exceeding 4200v. Read up:

It only needs to be in the ballpark of resonance, or divisible, to experience voltage peaks of up to 50x the supply voltage! A spark gap will prevent it from peaking that high.

Your PFC caps are fine, different situation entirely.

MrFlatox

Fri Oct 18 2013, 10:09AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Ah ! Thank you, it explains a lot of thing. So to test my future capacitor, I should add a spark gap to prevent the voltage to rise anywhere up to my power supply voltage due to resonance, I get it.

MrFlatox

Sat Oct 19 2013, 08:53AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

I just made a 1.5 liter PE bottle capacitor and only got 3nF. Considering the surface area of a bottle, i cannot see how you could end up with a 50nF bottle.

Andy

Sat Oct 19 2013, 08:58AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

It could be the water, with the electrode make a bias voltage, I fried my multimeter capacitor setting, trying to measure a water cap.

Ash Small

Sat Oct 19 2013, 09:33AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

I just made a 1.5 liter PE bottle capacitor and only got 3nF. Considering the surface area of a bottle, i cannot see how you could end up with a 50nF bottle.

Andy wrote ...

It could be the water, with the electrode make a bias voltage, I fried my multimeter capacitor setting, trying to measure a water cap.

I can see I'm going to have to try this again. I did use a DMM to measure the capacitance last time. This time I can parallel it with a known inductance and 'scope it.

Hydron

Sat Oct 19 2013, 09:57AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I tried once (maybe 10 years ago) with 10L polyprop plastic buckets, and got ~2nf per bucket. I also tested them across a 12kV neon transformer with a reasonably small spark gap to limit the voltage - made lots of ozone but survived.

Given the price and effort though it was an easy choice to go with a MMC for my first coil (DRSSTC built this year) - I'm using 48x 2uF 1kV snubber caps which were ~2USD each as used surplus, though that doubled by the time I got them to New Zealand.

Edit: I should say though that your coil looks like a great start to my (inexperienced) eye - your setup is pretty similar to what I was planning ~10 years ago when I first tried building a coil on a low budget.

Ash Small

Sat Oct 19 2013, 11:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

There is more to capacitance than just surface area, but I can see that it's 'possible' that copper + sodium chloride 'could' confuse a cheap DMM.

Sulaiman

Sat Oct 19 2013, 01:45PM

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

I have never mastered DIY TC Primary capacitor construction, I have tried.
I think that in the '80's PUPMAN was busy with home rolled caps.
I've tried a few and flat plate.
Initially rapid results (smoke, fumes, arcs, corona ..)
occasionally a semblance of reliability, but only if too large to store !

Homemade TC primary capacitors seem to be a rite of passage ... to mmc

Ash Small

Sat Oct 19 2013, 02:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

I have never mastered DIY TC Primary capacitor construction, I have tried.
I think that in the '80's PUPMAN was busy with home rolled caps.
I've tried a few and flat plate.
Initially rapid results (smoke, fumes, arcs, corona ..)
occasionally a semblance of reliability, but only if too large to store !

Homemade TC primary capacitors seem to be a rite of passage ... to mmc

Pupman was my main source of reference when I made mine. It was before the days of 4HV.

Mine were for a very low power application, it only needed to arc 3/16" under argon.

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