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colin heath

Sat Jan 27 2007, 06:06PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi All,
Here is a pic of a Cockcroft Walton I removed from an X-Ray machine at work. Puts out around 80KV and has onboard divider for voltage measurement.
Problem I have is how to figure out the frequency I should be driving the ferrite transformer at?
I was thinking I should measure primary inductance for a clue but not sure.
Any help much appreciated.
Colin

RickR

Sat Jan 27 2007, 06:33PM

Registered Member #93 Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64

How many stages are there in that thing? I thought there was a practical limit (~8) to the number of stages in a CW multiplier. Also what's the voltage rating on those ceramic caps?

As far as determining the proper driving frequency (and duty cycle), the best I can suggest is to set up a variable frequency driver (e.g. TL494) pushing a half bridge and tune it as you observe the output voltage through the divider. The few experiences I've had with this type of circuit suggest a driving frequency of 25-50 kHz with a 50% duty cycle, but this is based on a really limited sample set.

Rick

Simon

Sat Jan 27 2007, 11:17PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

RickR wrote ...

I thought there was a practical limit (~8) to the number of stages in a CW multiplier.

The killer is ripple. Ideally the CW output will be perfectly flat but, as you draw more current from it, the ripple will increase until you're practically working with an AC output.

Ripple goes up with the number of stages and the current draw and down with capacitance and frequency. A quick web search will bring up a hundred formulae so no need to labour it here.

Nice toy, Colin, BTW.

Electroholic

Sun Jan 28 2007, 04:20AM

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

theres still a voltage drop, its not exactly vin * # of stage.
again, google will tell you

colin heath

Sun Jan 28 2007, 08:17AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi,
It has 11 stages. all the caps you see are in parallel. I still don't see how any of this will tell me what frequency the transformer is meant to be run at

Sorry just read post about tuning

I will give that try later on today with 18V input and measure output with my HV probe on Multimeter
Cheers
Colin

Bjørn

Sun Jan 28 2007, 08:38AM

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

Look at the transformer design. Is the core laminated iron or ferrite of some sort?

colin heath

Sun Jan 28 2007, 08:39AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi Bjorn,
It's Ferrite so will be HF.
Cheers
Colin

Steve Conner

Sun Jan 28 2007, 11:12AM

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

Somebody mentioned tuning, but a device like this shouldn't operate in a resonant mode. There should be a band of frequencies that it works "OK" over. Too low, and the transformer will saturate before you can get the rated 80kV output. Too high, and leakage inductance in the transformer will limit the output current.

If it did operate in a resonant mode, I think the clue would be a smallish capacitor in series with the transformer primary, a la Thor CCPS or Steve Ward's versions of it.

Sulaiman

Sun Jan 28 2007, 03:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

What follows is guesswork;
There are 12 stages so the input voltage must be >80kV/12 = 6667 volts/stage
So transformer output > +/- 3333 V pk, or 2357 Vrms.
Allowing for say 20% droop on full output, Vsec = 3 kVrms.

So the MINIMUM operating frequency is when 3kVrms can be produced at the transformer secondary
without core saturation.

The MAXIMUM frequency may be determined by losses in the diodes recovery time or capacitors dissipation;
as a first estimate, starting from a discharged state,
Vsec will be applied to the first stage and an rms current of approximately Vsec x (2 x PI x F x C1) will flow initially.
In this case C! is the value of three of those capacitors in parallel. (I can't see the values, say 10nF each)
Suppose those hv diodes can handle 100 mArms (short-term)
So F < 0.1 / (Vac x 2 x PI x C1) = 17.6 kHz.

I would start at about 20 kHz (because I can't hear it, and I could use an audio PA to test it)
Run for a while on no-load and after discharging check how hot the transformer, diodes and capacitors are.
P.S. Be sure to limit discharges via a resistor, a spark discharge may kill the diodes.

Anyway, judging by the transformer size I'd guess 80kV at a few mA is the best you can expect.

Dr. Shark

Sun Jan 28 2007, 03:55PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

To give you a ballpark value, most ferrite transformers I have seen work at 20-40kHz, and you are unlikely to break your CW if you start in this range.

Unlike Steve I do believe there is some tuning involved here, since on one hand a higher frequency will give you a potentially higher voltage out of the transformer (you can have more volts / turn), and more current through the cascade. On the other hand too high a frequency will increase losses in the diodes and capacitors. You will have to tune for what gives you the biggest sparks.

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