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CRT into HV cap?

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LAN_403

King_of_Junk

Sun Aug 20 2006, 08:16PM

Registered Member #481 Joined: Thu Jul 06 2006, 07:10PM
Location: Frisco, Tx
Posts: 20

I've heard of making a CRT into a high voltage capacitor, but there is little to be found on the exact method. I thought you were to do something like put aluminium foil over the screen and use the regular connection for the flyback, but rather than haphazardly experiment I'd like some info. Anyone know anything- sites or anything- about using CRTs as caps?

I was planning on using one or more as the caps in a flyback driven tesla coil. Please notify me if this is unwise.

Marko

Sun Aug 20 2006, 09:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It's pretty bulky, low capacitance and leaded glass is probably even more lossy than 'normal' colorless glass.

Simple bottle caps are much less silly idea. ANd they won't implode if you drop them.

Bjørn

Mon Aug 21 2006, 12:46AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

benbradley

Mon Aug 21 2006, 02:21AM

Registered Member #312 Joined: Mon Mar 13 2006, 01:50AM
Location: Georgia, USA
Posts: 27

The CRT is a capacitor, it has a conductive coating on the inside and outside. It is used to smooth the output from the flyback. The glass is thick so the capacitance is low but it is reliable at high voltages.

What is the application? If you need high current (as in a TC resonant capacitor) I have my doubts that it would work. The coating is, I think, carbon and somewhat resistive. It works fine for the high voltage and very low current the CRT uses, but the effective series resistance could be a problem for some applications.

Terry Fritz

Mon Aug 21 2006, 10:18PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I used to fix TVs, prolly before you all were born

I don't even have a TV now

But the picture tube forms a high voltage capacitor between the inside metal coating and the outside coating to filter and stabilize the HV from the flyback and recitifier. But the current there is very "low". Tesla coils like very "high" current. TV like 0.02Amp, Tesla coil like 500Amps(!!).

However, even DigiKey sells HV caps in the $25 to $75 range that are like a thousand times better: 445-2271-ND, 445-2266-ND, 445-2257-ND. Just search on ceramic caps at like 20, 30, 50 kV

)

But for a Tesla coil, we are talking hundreds of amps!!! You need "MMC" caps for that. Just google search on "MMC capacitor" and you will find them

)

If it weren't for the lead, the old TV tube would be a fun shotgun taget

If you need "cheap" go with beer bottle caps....

Cheers,

Terry

Dr. Drone

Mon Aug 21 2006, 11:50PM

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

Avalanche

Tue Aug 22 2006, 01:49AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I second the bottle caps. I've run a successful mini TC from a flyback and the remains of a nights drinking! See my sig (shameless site plug :p )

GODSFUSION

Tue Aug 22 2006, 04:51PM

Registered Member #157 Joined: Sun Feb 12 2006, 08:00PM
Location:
Posts: 76

Ive never used the CRTs as a capacitor simply for the reasons mentioned but since i diddent have access to Al sheets or well anything to use as an electrode i used the sheets of whatever they are used as the electron mask inside thre screen. I made two rolled caps and filled the cvap wth xfrmer oil to help fill the air gaps in the hols.

I wouldent suggest trying it though it is quite lossy and diddent hold a charge well over 4kv for more than a few seconds, i tried rolling the second tighter but it only upped to 7kv before dielectric breakdown after like a second.

Was a neat learning expereience though.

- Wayne -

King_of_Junk

Tue Aug 22 2006, 05:56PM

Registered Member #481 Joined: Thu Jul 06 2006, 07:10PM
Location: Frisco, Tx
Posts: 20

Thanks for all the help. I was going to use it in a flyback coil or something like that. I'm just trying to research for now.

So, any good bottle cap schematics out there?

EDY19

Tue Aug 22 2006, 07:36PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Bottle caps are really easy- do a search on google for a bottle capacitor, and lots of stuff will show up.

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