ignition coil / c-w / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

ignition coil / c-w

Move Thread

LAN_403

attune

Fri Aug 11 2006, 09:38PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

Hello! I am designing a 100kv voltage multiplier to power a large electron tube. I want it to have the lowest ripple possible, so it will be a full wave design with the least amount of stages. A duel mosfet-driven ignition coil setup gives 50kV at a few khz, which would require only one stage. Are there any problems using these coils in a multiplier? The cheapest design for 3, 100kV capacitors seems to be an MMC. I assume that even a large electron tube will be a very small load on the multiplier, so the capacitors can be low capacity. Is there a 'minimum' capacity necessary for multipliers? 20, 6kV 1nF ceramic capacitors in series gives over 100kV at 50pF. Will this setup be efficient? What is the optimum recovery time for the diodes? 100nS seems typical of EBay diodes. Thanks for any help!

Sulaiman

Fri Aug 11 2006, 10:05PM

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

There is a minimum capacitance required for a CW (Greinacher) multiplier;
the capacitors should have much more capacitance than the diodes.

I have just (yesterday) won 6 30 kV diodes off eBay
(HVRT300...30 kV, 30mA avg, 5A pk, 100ns, 10pF max)
and this morning I measured the elecrical parameters for an old ignition coil
to build a 100W 0-20kV power supply,
so I expect to start construction soon,
so if there's any quick test/measurement you want in the near future, I may be able to help.

...

Sat Aug 12 2006, 12:14AM

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

I haven't really used ignition coils, but I was under the impression that they only gave positive pulses (or that the negative swing was much less than the positive) which would make them somewhat unsuited for multiplication... Unless of course you are driving it as a normal transformer; in that case you should be fine.

In order to calculate the size of the caps we need to know how much current you plan to run

Instead of a cw multiplier I would recommend that you look into using the type of voltage doubler used in a microwave oven, as it would greatly reduce the number of components you need.

As to the diodes, at 50khz the pulses are 20us long, so with a .1us recover time you should be fine.

Self Defenestrate

Sat Aug 12 2006, 01:54AM

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

The way I'd do it would be a couple doorknobs and a home wound flyback to give you the exact voltage you need. Check out Steves site,

for what I mean.

Steve Ward

Sat Aug 12 2006, 02:07AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yeah, an ignition coil will require *much* larger capacitors to lower the ripple of the output. Using HF AC (I use 50-100khz) gives you a much smoother supply with far less ripple. Regulation is a whole other story. So far i havent had a need for a well regulated supply, but you could use a voltage divider as feedback to cut down on your drive power. The drawback, possibly, is that you would run a lower input voltage and more stages. I wouldnt try for more than 10kV from a home-made transformer, so plan on 5 - 6 stages to hit your 100kV target with some current draw.

If you must use an ignition coil, go for the highest pulse rate you can, and to lower the voltage, just reduce the pulse width.

Steve Maurer

Sat Aug 12 2006, 02:49AM

Registered Member #133 Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47

I agree, 50 pF of capacitance per stage will be too low considering nominal diode junction capacitances. You may use any diodes that have a recovery time smaller than your intended time frame of operation. (50 kHz = 20 uSec full period or a 10 uSec half period.) 100 nSec rectifiers will work great for what you are trying to accomplish, just make sure that they are rated for the voltage and current that you intend to use. â€œHigh Voltage Laboratory Techniqueâ€ by Craiggs and Meek discusses Cockcroft-Walton multipliers in detail. You may want to view a copy through your local library resources.

Please be careful when applying voltages above 10 kV across electrodes within a vacuum as x-ray hazards may be present. Usually, the low-energy x-rays that are produced with potentials below 10 kV are absorbed within the glass walls of the tube. Placing a 100 kV potential across electrodes in a vacuum tube will surely produce x-rays with enough energy to penetrate the glass envelope and any experiments should only be performed in a controlled (protected) environment by an experienced individual (for obvious safety reasons).

attune

Sat Aug 12 2006, 05:02AM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

Thanks for the great info. As for safety, I will have the device shielded and at a minimum distance of 10 feet from the lab. I understand that the capacitors must have a larger capacitance. The only low cost alternative I can find in 400v 10uf electrolytics. Are there any electrical problems with putting these in series? I will use the circuit driving the ignition coils to power a flyback, which should solve the ripple problem as well. I do not know what the current draw from the tube will be, so I don't know what capacitance is minimal...I have a collection of doorknob caps that I belive are around 500pf, would this be enough? Larger capacity doorknobs exceed my summer-job paycheck.

Sulaiman

Sat Aug 12 2006, 08:55AM

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

Update on the Ignition Coil invertor;
Having measured the ignition coil (typical 'compatible' unit)
it seems that
1) The secondary self-resonant frequency is around 9 kHz
2) I'd need to operate at 1.5 to 9 kHz for my desired throughput

Even though my hearing is not as good as it was,
I don't fancy having such an annoying-sounding power supply
So I'm abandoning this Ignition Coil and I'm going to
wind a Ferrite Core flyback to operate at about 25 kHz instead
(This is waht I hoped to avoid by using an ignition coil,
and the crt flybacks I have aren't happy at 100W+ continuous output)

attune

Sat Aug 12 2006, 02:54PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

This was my thinking too. But the older, disk shaped flybacks can handle over 100w I believe. Ebay.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob