[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Spark Gap



Original poster: "christopher carsten by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ctom-at-toast-dot-net>

CJ,  Mov's are metal oxide varistors, from what I understand they are used
as overvoltage protection divices. they are placed accross the mains line
(usually) and present a high impedance at rated voltage but as the voltage
rises above the mov's rating the impedance begins to drop and the part
clamps the voltage to a safe level dissapating transient energy into the
varistor. After a really high spike I have found them vaporized in
industrial power supplys.
Chris

----------
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: Spark Gap
>Date: Fri, Jul 20, 2001, 7:02 AM
>

> Original poster: "CJ Moore by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <wizard1234-at-home-dot-com>
>
> I hate to ask this, but since most people seem to be mentioning them and I
> gather they are important, what are MOV's?
>
> ----- Original Message -----
> From: "Terry Fritz" <twftesla-at-qwest-dot-net>
> To: "CJ Moore" <wizard1234-at-home-dot-com>
> Sent: Wednesday, July 18, 2001 3:46 PM
> Subject: Re: Spark Gap
>
>
>> Hi CJ,
>>
>> At 03:26 PM 7/18/2001 -0700, you wrote:
>> >If I haven't made the primary coil yet, can I just wrap some thin wire
>> >around a pipe and use it to limit the current?
>>
>> Sure,  Since the RMS current is low, even thin wire will work fine.
>>
>> >Also I put the fuses on each
>> >input of the NST's? So 4 total? You say that the fuse will limit the
> current
>> >if you internal shunts forget to, so if the internal shunts go haywire,
> the
>> >fuses will limit the current supplied to the NST's thereby saving the
>> >capacitor hopefully?
>>
>> Yes.  When NSTs start to go nuts, they draw a ton of current.  Simple
> fuses
>> will stop the destructive process.
>>
>>
>> >Does this mean that the NST's are shot now?
>>
>> They should be fine.  When/if they will no longer draw an arc to the case
>> or anything, then they are toasted.
>>
>> >About the
>> >NST hookup, they are each 9kv 60 ma, and I want to get 9kv 120 ma. They
> are
>> >center grounded. Do I not ground the cases to the ac ground?
>>
>> Yes, ground the metal casses and hook them up in parallel like the diagram
>> I sent.
>>
>> Cheers,
>>
>> Terry
>>
>>
>> >
>> >----- Original Message -----
>> >From: "Terry Fritz" <twftesla-at-qwest-dot-net>
>> >To: "CJ Moore" <wizard1234-at-home-dot-com>
>> >Sent: Wednesday, July 18, 2001 11:37 AM
>> >Subject: Re: Spark Gap
>> >
>> >
>> >> Hi CJ,
>> >>
>> >> At 09:19 PM 7/17/2001 -0700, you wrote:
>> >> >
>> >> >
>> >> > ----- Original Message -----
>> >> > From: <mailto:wizard1234-at-home-dot-com>CJ Moore
>> >> > To: <mailto:twftesla-at-qwest-dot-net>Terry Fritz
>> >> > Sent: Tuesday, July 17, 2001 5:31 PM
>> >> > Subject: Re: Spark Gap
>> >> >
>> >> > I don't want to sound like an idiot, but I don't really have that
> much
>> >> > knowledge (actually very little). So I was wondering if you could be
> a
>> >little
>> >> > more specific? I really don't want to damage my cap, and the setup I
>> >> > described is very similar to how my tesla coil operated. If this is
> bad
>> >I
>> >> > would like to know how to correct it. Several questions:
>> >> > 1) How do I limit the current?
>> >>
>> >>
>> >> Normally, a Tesla coil has a primary inductor that the cap discharge
>> >through.
>> >> This inductance acts much like a resistor in that it will limit the
>> >current.
>> >> With the gap right across the capacitor, there is not too much to limit
>> >the
>> >> current except the wire resistance which is really low.  If you have a
>> >primary
>> >> coil, I would wire in series with the cap.
>> >>
>> >>
>> >> >
>> >> > 2) Where do I put the device in the circuit?
>> >>
>> >>
>> >> In series with the cap would be fine.
>> >>
>> >> >
>> >> > 3) Why do I want to limit the current if I want the big sparks?
>> >>
>> >>
>> >> The current can get too high.  10,000, 20,000, 50,000 amps....  No one
>> >knows
>> >> how much current a good short across a high energy pulse cap is.  But
> if
>> >you
>> >> get too much, the internal connections in the cap will blow out.
>> >>
>> >> >
>> >> > 4) Is an inductor similar to a choke?
>> >>
>> >>
>> >> Yes.   But, a primary coil is built to take high voltage where a common
>> >chock
>> >> may short.
>> >>
>> >>
>> >> >
>> >> > 5) Would something like your NST protection circuit work in a
> situation
>> >like
>> >> > this?
>> >>
>> >>
>> >> It would not limit the current from the primary cap but it would help
>> >protect
>> >> the transformer.
>> >>
>> >> >
>> >> >
>> >> > If you can answer these questions more directly with a simple answer
>> >please
>> >> > do so, as I don't want to waste your time. I attached a picture that
> I
>> >drew
>> >> > of how I have it hooked up, because I don't trust myself to draw the
>> >> > schematic correctly.
>> >>
>> >>
>> >> I would also put a fuse on the input of each NST.  About 3 amps.  There
> is
>> >a
>> >> chance that your cap and NST combination could saturate the NST current
>> >> limiting cores and blow the NST.  The fuse will limit the current if
> the
>> >> internal shunts in the NST forget to ;-)
>> >>
>> >> I would also think you would want to hook up the NSTs like the enclosed
>> >> diagram.  If your NSTs are center grounded, then you only get 9kV at 30
>> >MA.
>> >> This way you get 9kV at 60 mA.  If your NSTs are not center grounded,
> then
>> >you
>> >> would get 18kV at 30mA.
>> >>
>> >>
>> >> >
>> >> >
>> >> > Thanks again for all your help
>> >> >
>> >> >
>> >> > BTW: I got some caps rated at .02 uf 16 kvdc and I used them in
> series
>> >(I
>> >> > think) with the two transformers and I got 4-5 inch arcs from a
> Jacobs
>> >ladder
>> >> > type device, but after a few minutes of operation the a little hole
> in
>> >the
>> >> > cap blew out. It looks sort of like somebody shot a small bb through
> it.
>> >So
>> >> > they do blow up
>> >>
>> >>
>> >> Hard to say why.  Could have been the wrong cap material.  If they got
>> >hot,
>> >> they probably were not polypropylene which is about the only suitable
>> >material.
>> >>
>> >> Hope this helps...
>> >>
>> >> Cheers,
>> >>
>> >>         Terry
>> >>
>> >>
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > _________________________________________________________________
>> >> > Hi CJ,
>> >> >
>> >> > A cap connected just to the NST may ring up the voltage really high.
> If
>> >you
>> >> > hit the 60Hz resonant point you can get like 80000 volts!  that will
>> >blow
>> >> > something up so be careful.  When the cap arced, was there anything
> to
>> >limit
>> >> > the current?  If you have 20000 volts but no inductor and no
> resistance,
>> >the
>> >> > current will be super high.  20000 volts into 1 ohm is 20000 amps.
> That
>> >will
>> >> > be loud :-)  Super high current like that can damage the cap too.
> Best
>> >to
>> >> > add
>> >> > an inductor to act as a current limit.
>> >> >
>> >> > Cheers,
>> >> >
>> >> >         Terry
>> >> > __________________________________________________________________
>> >> >
>> >>
>> >>
>> >>
>> >
>>
>
>
>