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Rectifying a TC (a bit?)

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LAN_403

Tesladownunder

Sun Apr 08 2007, 03:32AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

We know that TC's don't make your hair stand on end like a van der Graaff generator. This is because charges don't accumulate but are dissipated in the reverse direction continuously with the spark ring down which occurs at the TC frequency of perhaps 100kHz.
This became obvious to me with the non-performance of my 14 foot spinner.

Consider a spark from a needle point to a plane surface. The voltage at which this breaks down is lowest when the needle is negative. From my experiments, this is perhaps as much as 1.4 times as great as the opposite polarity. Unlike a diode, though, when the spark fires, the diode is short circuited (in both directions) and any accumulated charge is lost.

Consider the situation where there is just a corona, depositing charge without the spark. It may be possible to accumulate some charge. This might show up with the equivalent of a head full of hair which is an electrometer just two foils together which separate if a constant DC charge is present.

My experimental setup is based on Coke Zero cans. (I have not replicated this with any other cans like Coke plain or other generic Colas

). By twisting and cutting the ringpull, a sort of needle point is obtained. This is brought up to 1.5cm from another cans base which is concave. In this way a string of cans can be made, each with a polarising gap and with few sharp edges exteriorly to generate spurious corona. It is run from my mini TC (2 inch sparks) connected to the right can. Earth is the left. Note that this is below sparking threshhold.

By the use of an aluminium foil electrometer I can show (with constant TC output)
Pic 1 Eathed side: No electrometer reading (only few mm)
Pic 2 Central can: Electrometer shows wide separation
pic 3 TC side: Electrometer shows mild separation

I interpret this to show some charge accumulation, presumably positive, since this is the charge that can't escape.

Anyone have any thoughts or how to progress this?

TDU

1176003101 10 FT0 Tesladcelectometerearth

1176003101 10 FT0 Tesladcelectometercenter

1176003101 10 FT0 Tesladcelectometertcend

haulinick

Fri Apr 13 2007, 10:16AM

Registered Member #619 Joined: Sat Mar 31 2007, 05:26AM
Location: San Jose, CA
Posts: 16

very interesting idea. good thinking. I am slightly puzzled by the whole point to it but I get the idea of how the rectifier works.

Tesladownunder

Fri Apr 13 2007, 01:39PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Rectifying a TC would be able to supply large DC voltages at significant currents which are far higher than van der Graaffs can give. Make big x-ray stuff, charge huge objects, make Marx multipliers to get even higher voltages, run powerful ionic spinners etc.

Modelling what is happenning here with a sort of diode draining charge on to a capacitor (the can) at 200KHz with multiple units all in series is something I need to get my head around first.

TDU

Steve Conner

Fri Apr 13 2007, 01:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Instead of grounding the first can directly, try grounding it through a milli- or micro- ammeter. A DC meter with a capacitor shunted across it, so it only reads the DC component of the output, and passes the AC through to ground. That will give you an idea of how well your rectifier is working.

The output of a Tesla coil is limited by breakout from the topload or breakdown of the secondary, just like a Van De Graaff is limited by corona from its top terminal or arcing down the column. So I don't really see how a Tesla coil could output "Far higher" voltages than a similarly sized VDG.

*edit* oops, I see you said "Currents" which are far higher, not voltages, my mistake

HV Enthusiast

Fri Apr 13 2007, 03:13PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

We actually have several very high voltage hi-pot machines at work which are essentially tesla coils with the output rectified. they work pretty good and we have units up to 500kV. the units we have are probably at least 30 years old, so the technology has been around for quite a while. you can probably search old IEEE archives (or similar) to find more specifics about how this is done if you don't wish to devise your own solution.

Justin H.

Fri Apr 13 2007, 03:21PM

Registered Member #641 Joined: Fri Apr 13 2007, 02:45AM
Location:
Posts: 3

Why not rectify the output of an SSTC (CW) with a string of HV high speed diodes?

Connect the diode string between the SSTC's toroid and another (highly polished) toroid mounted on a mega-insulator. Then tune the coil to the setup.

See any reason that 1/2 megavolt DC or better couldn't be had?

See any reason that we couldn't get a very good measurement of the actual output voltage of a tesla coil itself, using this rectifier arrangement + a serious HV divider? The coil + diodes would charge the polished "filter" toroid to peak voltage, where it could be measured via string of R's and high-Z-input buffer (such as AD8641...with a 0.25picoamp input bias current!!!)

Gotta watch out for the high-frequency HV output of the coil driving AC current into the second toroid. Just separate them I guess...

(EDIT: Hi Dan, didn't see your post until after I posted mine. 500kV is impressive. That's good news, it means it's do-able...)

Tesladownunder

Fri Apr 13 2007, 04:34PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Justin H. wrote ...

Why not rectify the output of an SSTC (CW) with a string of HV high speed diodes?
.....See any reason that we couldn't get a very good measurement of the actual output voltage of a tesla coil itself, using this rectifier arrangement + a serious HV divider? ....

I have at least 50 9 amp 600V diodes that have a 60ns recovery if I recall. Even if they were avalanche rated I doubt that they would survive long (unlike red LED's

)
Don't forget that to rectify 500kV into a capacitative load you need to use a diode rated at 1MV minimum.

ragnar

Sat Apr 14 2007, 04:27AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

BY8418 is your best friend, guys -- 18kV 5mA

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