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source for good HV resistors?

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YoshiEggz

Wed Feb 14 2007, 06:58PM

Registered Member #521 Joined: Wed Feb 14 2007, 06:03PM
Location: Ontario - Canada
Posts: 9

I bought a bunch of doorknob caps rated at 40Kv each and built a makeshift marx generator with it. It works good but the resistors tend to break down near the end stages. I am starting with 5Kv from a NST and using six stages. my resistors are 1meg 2 watt carbon style.

Does anyone know a good source for resistors that will work in my application? I looked on Ebay and the glass "tiger-stripe" ones are very expensive and I think overkill for my application?

J. Aaron Holmes

Wed Feb 14 2007, 08:21PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I haven't built a marx before so I can't speak from experience, but there was talk in the not-too-distant past about using homebrew water resistors as well as black rubber for the resistors in a marx. The black rubber idea was pretty intriguing. A search of the archives would probably turn it up.

Another option would probably be to just stack the small carbon resistors, but use corona rings at the ends and in between sections to keep things from corona-ing to death. Maybe you're already doing that, though.

Regards,
Aaron, N7OE

Marko

Wed Feb 14 2007, 08:29PM

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

The black rubber idea was pretty intriguing. A search of the archives would probably turn it up.

I didn't try building a marx yet, but vulcanised rubber indeed seems to work quite well as a resistor and is easy to use.. just cut two strips and arrange the caps on it!

It may, although, have too high resistance per cm for our liking, so you may need to mount the 'electrodes' just few mm from each other. I didn't try it on a marx, so I don't know.

Nik

Wed Feb 14 2007, 08:47PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I belive hot glue works to prevent flashover as long as you aren't going for good looks.

...

Wed Feb 14 2007, 11:59PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Hot glue should never be used for anything with an H or a V in the name. Trust me.

I have found silicone caulk to work pretty good. With my homewound flyback transformer I used OH transparencies and clear silicone to insulate it, nd haven't had a flashover even putting out 20kv at .5mhz

ShawnLG

Thu Feb 15 2007, 05:43AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

"Hot glue should never be used for anything with an H or a V in the name. Trust me."

What happens when you mix the two? I thought It would be a good potting agent. I have never used the stuff for HV.

...

Thu Feb 15 2007, 06:41AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well, it starts to conduct at a few kv, then it starts to boil... I suppose there is probably glue that doesn't conduct, but I haven't found it

I am not sure if it is just the capacitance of it and me using AC, but hot melt glue is not a good potting agent. I have completely killed operation of 3 things I built by using it

Electroholic

Thu Feb 15 2007, 06:47AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

the black rubber stuff has actually too low a resistance for marx applications.

as for my marx, im using 3 X 500k 2W metal film in series.
mines 20kv per stage btw.

Marko

Thu Feb 15 2007, 02:18PM

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

the black rubber stuff has actually too low a resistance for marx applications.

It depends heavily on rubber type and 'hardness' of vulcanisation. Some small rubber bands I have here measure few hundred kiloohms per centimeter, while some enforced black rubber 'tape' I couldn't measure at all, <2000 megaohms.

But it did conduct, although poorly, when I hooked an obit to it.

I guess it doesn't conduct until voltage is high enough to ''arc over'' between conductive particles in the strip, similary as ferrite.

Some 'surface tracking' could be seen at points where I stabbed the electrodes, so I don't know if it would be best for a marx.

I think it's better idea to find some lower resistance rubber and use longer strips if needed.

Self Defenestrate

Thu Feb 15 2007, 03:01PM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

I was in the resistor challenge a while ago, and made a few resistors with water putty and graphite. They had horrible tolerances, and were fragile, but worked alright. These days I find it much easier to use 1/4w resistors in series(usually 100k, i have a reel of them) and put them in a length flexible pvc tube. Fill that up with oil, and seal the ends with some epoxy, and your set.

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