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Newbie here, want a little help with CW multiplier

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LAN_403

Th3_uN1Qu3

Sat Jan 09 2010, 09:35AM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

Yesterday i built a basic half-wave CW multiplier to try and draw some small arcs. I have no flyback (silly me didn't recover it when my old TV broke), so i am running directly off 230v AC 50Hz, from an isolation transformer. My multiplier had 8 stages at first, which was supposed to get me 3kV, enough to go 1mm through air. Indeed, it did. Leaving the output leads very close to each other resulted in sustained sparking, with a rate of 5-10Hz estimated. The rate wasn't constant, it would pause every now and then, but it worked. I then built 4 more stages. Powering those up resulted in failure. It gave off one fat spark, then no more distance jumping, and touching the two leads resulted in very very small sparks, like shorting a small battery. It now behaves the same every time i plug it in. One spark, then nothing.

I have checked the stages separately and it seems like the failure is in the first two stages - however, wiring the other ones up by themselves only results in really small sparks, as to why, i'm puzzled. My meter needs a new set of probes and a battery so i can't measure the diodes either.

The diodes were mostly 1n5408 and similar, pulled from fried ATX power supplies. The capacitors were either Y capacitors from the same power supplies, or some other caps pulled out from a monitor. They are rated 1kV, most are 2.2nF or 4.7nF but i also have some 0.1uF ones in the first stages, as i read that larger caps in the first stages are better. Strangely, i had a few 400v caps and even a couple 250v ones in my first 8 stage CW, and none of them fried. They are supposed to be rated for 2x peak AC voltage right?

Now, my isolation transformer could also play a part in this. It is built out of two small mains transformer, one feeding the others' secondary so i get my 230v back, isolated and current limited. The transformers are very small, they only give out 200mA at 24v, so assuming 75% efficiency my supply can only deliver 3.6W, which makes for 1.2mA at 3kV. I had no limiting resistors in my 8 stage that worked, and i didn't fry the diodes even though i shorted it many times, so i suspect the transformer does play a large part in this.

I read that 1N4007 diodes are good enough. They are dirt cheap so i'd buy 60 of them for a 30 stage multiplier. Does that sound okay? Also, what capacitance should i use? I didn't find much about optimum capacitance in a CW multiplier. I'm not much interested by arc length (5-6mm will be enough for now), but i want the arc to burn through wood if possible. I assume i'm going to need some current to make the arc burn, correct? That means a larger transformer but that can be taken care of.

In the event that i decide to go for a flyback design, i just read about someone who built a flyback out of a SMPS transformer. I have about 8 of those lying around, so that would be an option. Could someone please point me in the right direction about modding those? I wasn't able to find much info.

Thanks,
Andrew.

Proud Mary

Sat Jan 09 2010, 09:52AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A useful C&W xls spreadsheet calculator may be found here:

Th3_uN1Qu3

Sat Jan 09 2010, 09:55AM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

I was using this one. Edit: Just factored in the 50Hz frequency and i do realize now that my multiplier was terribly inefficient with those small caps. :) It seems like a 555 timer circuit is in order. I'll go buy one on Monday along with the other stuff i need like the probes for my meter.

Edit 2: There's something funky going on with my isolation transformers. I put a rectifying bridge on the 230v output and a 6.3v 330uF cap on the bridge's output. Guess what? I've been waiting for that cap to blow for five minutes already. Not having a working meter sucks, but even without it i'm pretty sure that the xf's are either busted (they aren't), or they're giving out way too little power to be of any use.

Edit 3: I know i had to eventually wire my CW up straight to the mains, and after having no luck trying to find a suitable limiting resistor (i only had a 22k 5W one which let too little current through), i realized i had forgotten the oldest trick in the book. That is, a lightbulb in series with the circuit. I dropped in one 40W bulb (40W seemed like a safe power to me, it should give 8.3mA at 4.8kV which is what i'm looking at with my current multiplier), wired the bridge up and short circuit tested it. The bulb lit up, the spark was small, and the bridge did not suffer any damage. Good. I then wired a 1uF 50v cap onto the bridge. 5 seconds - nothing. 8 seconds - smoke started coming out of the cap. 12 seconds - POP. The cap... Wait there's no more cap.

Yup, we got power.

Time to test my multiplier then.

Edit 4: It works.

Nothing too spectacular due to not enough capacitance but it works. Also it seems to use less than 40w as the bulb doesn't light up. I'll make a high frequency driver in the next few days and be back with vids.

LutzH

Sun Jan 10 2010, 10:57AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

If I may ask: What is the purpose of your multiplier? I am asking because if you are just trying to get a few KV then other methods may be much more easy, and more reliable. If your purpose is to just have fun with the circuit and learn then its another matter entirely.

The number one reason for most VM failures, is normally diode failure, due to surge currents and transients. Dealing with this is not hard, you have to know the ifsm of the diode. This is the single cycle surge rating. If you put enough resistance either at the output in series, or in the circuit in series to stay well under the ifsm, then you should be ok.

Because good quality very fast switching < 100nS HV diodes are now so common, and affordable, I do not mess with stringing together a mess of small diodes anymore. Maybe if we understood your objective better then it would be more easy to know, what to write.

Take Care.......Lutz

Th3_uN1Qu3

Sun Jan 10 2010, 06:00PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

I'm just having fun. And yes i know there are better ways to get a few kV, if i were to build a tesla coil i would use a flyback of course. I'm trying to understand the relationship between capacitance, ripple and frequency. Who knows, maybe it'll result in me eventually building a switching power supply (i hate them because i don't understand them properly).

By the way, i'm open to ideas on how to get mains voltage at 20kHz or higher. I checked out Uzzors2k's multipurpose inverter, but it seems a little complicated for what i want. I do have the circuitry on hand but i would like to do without winding those gate drive transformers for the mosfets. Is that possible? I haven't worked with mosfets so far.

LutzH

Mon Jan 11 2010, 02:59AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

There are a few short cuts to HF, but the ones by Uzzor's, and especially the ones by Steve Ward are very well made, and thought out, better than I could make! I have however on occasion just needed some HF to test something etc. What worked for me was to "hack" into the HF portion on an inverter PCB. My inverter takes 12VDC, chops it to HF, then raise the voltage to about 170V, then it rectifies the result, and re-chops it at 60Hz. I was able to "tap" into the HF easy.

The same goes for computer power supplies, but these tend to be at around 40-75Khz. On these do not remove the final step down transformer, it is needed for regulation, just tap into the primary cricuit of this transformer, and put a small load like a 12V bulb on the PS to keep the circuit loaded.

If you want more power check out the recent thread on inverter microwave power supplies :) Talk about simple HF.

Happy Experimenting......Lutz

Th3_uN1Qu3

Tue Jan 12 2010, 05:56AM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

I did tap into a computer PSU primary, right at the transformer, however it didn't work. The sparks were much smaller than on mains, maybe my diodes aren't fast enough.

I did however manage to build a simple boost converter. The 555-based controller didn't work (for some reason the timer outputs DC), however i injected a 1kHz square wave from an audio amp to the gate of the mosfet, and i am getting 200v out with 9v input.

I have tried different freqs, but 1kHz seems to work best. I'm using the primary of an AT PSU transformer for an inductor.

Now i just need to get that control circuitry working. Edit: Fixed, i had forgotten to wire power to the resistors making the 555 astable... Now i got one quiet power supply. The only problem is that with the 555 the voltage gets limited to 80v, i'll have to mess around with that voltage divider that keeps the timer in check. Also, the voltage drops rather dramatically under load (40W bulb), to a measly 23v. But it did that with external signal too. I think i need a better inductor, there's no way i can't get 40W out of this thing.

Edit 2: Pulled a random inductor out of a PSU - bam, instant 300v. Now it's also able to light that 40W bulb a bit, and load voltage drops to "just" 70v. Still work to do but i'm on the right track.

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