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Apparently around 20 microwave caps is enough to explode stuff?

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Mads Barnkob

Sat Dec 13 2008, 02:21PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

First I tried chargeging a small MOC bank with a solid state NST 4kV / 20kHz fullbridge rectified, but this only resulted in 3kVDC on 5 capacitors, the high frequency gave big losses in the diodes and the nst was too weak and got pulled down about 100-200V per capacitor I added to the bank.

So I looked into using a MOT to charge with instead, I have a 2100Vac MOT ballasted on the primary side by a shorted MOT, on the secondary side I got a voltage doubler using 2 MO diodes and 2 MO capacitors, measuring over the capacitor bank shows about 6kVDC over 4 MOCs.

So what is the best way to protect my chargeging circuit from the voltage ringing in the circuit as I dishcharge the bank into a coil/whatever.

I have made 3 ideas, the less I have to be near the charged bank, the better, so the switch idea is not ideal, but could maybe be made with a couple of contactors, unless they cant withstand the DC voltage, which I really doubt they can. The biggest contactor I got 80A is rated Uimp 8kV.

1229178112 1403 FT58754 Motpsuformocbattery

Billybobjoe

Sat Dec 13 2008, 10:20PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

MadsKaizer wrote ...

First I tried chargeging a small MOC bank with a solid state NST 4kV / 20kHz fullbridge rectified, but this only resulted in 3kVDC on 5 capacitors, the high frequency gave big losses in the diodes and the nst was too weak and got pulled down about 100-200V per capacitor I added to the bank.

So I looked into using a MOT to charge with instead, I have a 2100Vac MOT ballasted on the primary side by a shorted MOT, on the secondary side I got a voltage doubler using 2 MO diodes and 2 MO capacitors, measuring over the capacitor bank shows about 6kVDC over 4 MOCs.

So what is the best way to protect my chargeging circuit from the voltage ringing in the circuit as I dishcharge the bank into a coil/whatever.

The contactor would probably work because if you are charging the bank to the same voltage as your supply, when its fully charged there will be little voltage across it that it has to interrupt (there will be some due to the internal cap. resistors). However, as I mentioned before the capacitors will drain themselves very quickly so you'll have to be fast with opening the contactor (there are high voltage relays that would work well here as well) then closing the main switch. Although a 555 IC could easily be set up to put a small automatic delay between the two.

A modification of your third idea is probably the simplest/most practical. I say a modification because I don't know exactly what the voltage ratings are for MO diodes and if they'll be sufficient here. A string of BY255's could easily replace them and you wouldn't need any other diode strings (your string in the pic doesn't really do anything because a voltage reversal will pass right through it).

If you have any other questions feel free to PM me - we are sort of hijacking this thread but no one seems to mind yet . . .

Fraggle

Sat Dec 13 2008, 11:24PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I use a spark gap with one spring loaded electrode. When you pull back the electrode (with your bit of string) it touches the charger contact, when you let it go it snaps over to the load electrode. There is a nut I can adjust to set the load gap separation when closed and the charger `gap` never welds anyway.

Mads Barnkob

Sun Dec 14 2008, 10:32AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Fraggle wrote ...

I using a spark gap with one spring loaded electrode. When you pull back the electrode (with your bit of string) it touches the charger contact, when you let it go it snaps over to the load electrode. The is a nut I can adjust to set the load gap separation when closed and the charger `gap` never welds anyway.

Thats also a very easy and good method.

Dr. Dark Current

Sun Dec 14 2008, 10:45AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Connect it like this.
1) The two MO caps in the doubler will provide balasting.
2) You can even leave the charger powered while you "shoot", as the current will be limited (and any reverse voltage will get damped on the MOT secondary)

Edit: realising the reverse peak might be a few A, it might be better to construct each of the HV rectifiers of 12 1N5408's in series.

LithiumLord

Sun Dec 14 2008, 02:18PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

...just don't forget the fact that in case any if the caps in the bank fail all the energy remaining in the bank will be applied to the failed cap and will eventually blow it apart ;)

Mads Barnkob

Fri Dec 19 2008, 09:48PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

My moc bank is done, 21 mocs (18,5uF) at 6kVDC.

I will post more pictures and write about it later, here is a little first blast teaser :)

PS. this is FUN!

Arcstarter

Fri Dec 19 2008, 10:59PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

this is FUN!

Indeed it is! Some people may just say it is just sparks and blowing stuff up, but it is sooo fun. I really wish my cap was not blown, i really want to do some 465 joule shots now :).

Fraggle

Sat Dec 20 2008, 09:19PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I wish I could get access to a camera and post some pics of my bank. It`s 24 MOCs plus 2 pairs of defibrillator caps in series, about 50uF but the defibrillator caps get a bit smaller each time :D I charge to 9kV.

Arcstarter

Sun Dec 21 2008, 01:09AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Fraggle wrote ...

I wish I could get access to a camera and post some pics of my bank. It`s 24 MOCs plus 2 pairs of defibrillator caps in series, about 50uF but the defibrillator caps get a bit smaller each time :D I charge to 9kV.

9kv you say? Wow, that is pretty good :). All that paralleled up is 2kj! Pretty sweet for a few MOC's and a few diffib caps ;). But how is it with stuff that requires high peak current? In other words, how is the discharge time? Loud as hell, or kinda 'soft'? When i discharged my bank of lytic caps that was about 600-700 joules, it was MUCH quieter than my 12.5uf cap charged to 2kv, which is only 25 joules. I would like a low value cap with insanely low inductance/resistance. Maybe something like 8kv and 5-10uf, that would be 160 to 320 joules. With very low inductance/resistance that would have high enough peak current to crush a can. If i could slightly do it with 25 joules and really do it with 100 joules, without even messing with coil turns, that would sure work.

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