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RE: 60kV 50MHz HV Divider Completed



Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Captain,

I guess "I" have to ask if the resistors are high voltage types too?  At 
10kV each I am sure they are ;-))  But "ordinary" resistors are pretty 
voltage variable!  Here is some data for our 10Mohm 1/2W Yageo carbon film 
resistors of MMC fame:

Voltage (DC)    Current (mA)    Resistance (Mohm)       Power (mW)
250             0.0248          10.0806                 6.2
500             0.0502          9.9602                  25.1
750             0.0759          9.8814                  56.9
1000            0.1026          9.7466                  102.6
1250            0.1303          9.5933                  162.9
1500            0.1593          9.4162                  239.0
1750            0.1899          9.2154                  332.3
2000            0.2229          8.9726                  445.8
2250            0.2579          8.7243                  580.3   Past power 
rating
2500            0.2959          8.4488                  739.8
2750            0.3379          8.1385                  929.2   Thermal drift
3000            0.3989          7.5301                  1195.2  Thermal drift
3250            Thermal failure (core cracked)

For the MMC, we don't really care if the resistance varies 10%, but for a 
voltage divider, they would not work.  It is interesting how voltage 
dependant the resistance is.  Not sure why...

Your voltage divider is great!!!  Neat how you have compensated it!!.  If 
you really want a challenge, try making one for 500kV at like 3pF loading 
without disturbing the coil's fields ;-))  Be great of directly measuring 
top voltage!  I have been trying for about 5 years :o))  A capacitive 
voltage divider up the center is as close as I have thought of (Jennings 
style).  Still messes a lot with the coil, but the effects are predictable...

Cheers,

         Terry




At 10:24 AM 5/15/2003 -0400, you wrote:
> > Hi Dan,
> >
> > I couldn't tell from your schematic what type of caps were
> > used for the 82
> > pF compensation caps.  My assumption is that they are ceramic.
>
>Yes, that is correct.  Both Vishay and AVX makes these type of high voltage
>ceramic capacitors.  These particular
>capacitors are rated 82pF at 10kVDC each.
>
>
> > I once attempted to build a purely capacitive HV divider, also using
> > ceramic caps on the HV side.  When I attempted to scope the
> > output of an
> > NST, the resulting voltage was grossly different than what
> > the capacitive
> > ratio should have produced.  I mentioned this on this List,
> > and I think it
> > was Bert Hickman who pointed out that ceramic dielectrics have an
> > unfortunate property of creating a capacitor that's voltage-dependant.
>
>Actually, ceramic capacitors are pretty much the industry standard for
>commercial high voltage dividers.  Almost all
>your potted high voltage dividers will use these same type ceramic
>capacitors.
>
> > This effect may be less evident if the dominant division is
> > achieved with
> > R's rather than C's, just something to keep in mind (and
> > something that
> > simulation models don't account for).
>
> > BTW, your Marx generator is BEAUTIFUL!  What did you use for the gap
> > electrodes?  Are the support plates for the electrodes
> > constructed so that
> > they are hinged and may be adjusted in unison?
>
>Thanks.  The spark gap electrodes are the home depot special drawer knobs
>which are standard issue at almost any
>Home Depot and sell for price of $2.29 each.  (adds up considering i have 24
>of them), but well worth it. I was going to
>use standard one inch corona balls, but these seemed to mount much nicer in
>they have nice flat flange on the back end to butte up against the support.
>
>The spark gap assembly isn't hinged, but does have the bottom and top
>support plates which can either be replaced with
>longer plates, or spacers installed.  I could have made a hinged design, but
>wanted something a little more permanent.  Initially it will be pain to fine
>tune the gap widths, but in the long run it will prove worthwhile.
>
>I'm finding out by tomorrow whether I'll be going to Kwajalein Missile Range
>(Marshall Islands) for six months, but if I don't i'll definitely be
>bringing the marx generator up to Rochester when I see you in August.  Just
>beware, my minicoil is growing stronger every day!!!!
>
>The Captain
>
>
>
>
>
>
>
> > Regards, Gary
> >
> >
> >
> >   -----Original Message-----
> > From:         Tesla list [mailto:tesla-at-pupman-dot-com]
> > Sent: Thursday, May 15, 2003 1:45 AM
> > To:   tesla-at-pupman-dot-com
> > Subject:      60kV 50MHz HV Divider Completed
> >
> > Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
> > <dhmccauley-at-spacecatlighting-dot-com>
> >
> > I finally got around to completing my 60kV High Voltage
> > Divider.  The design
> > works extremely well (from initial tests) and should serve as a good
> > baseline for others wanting to build hv dividers capable of
> > high frequency
> > measurements.  The frontend features a combination
> > resistive/capacitive
> > divider and
> > a additional box which attaches to the unit to allow fine
> > adjustments of the
> > gain/frequency compensation of the entire hv divider.
> >
> > http://www.spacecatlighting-dot-com/hv_divider.htm
> >
> > Specifications:
> > Maximum voltage:  40kV-60kV (construction dependent)
> > Bandwidth: 54MHz (simulated 3dB roll-off)
> >                     50MHz (measured using impedance analyzer)
> > Ratio: 1000:1
> >
> > Enjoy!
> >
> > The Captain
> >
> >