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Compute This! DC Charger SGTC

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LAN_403

Part Scavenger

Thu Sept 28 2006, 01:56AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Well, there's been a sad day in my house, I had to get rid of my 4" coil with the way oversized, leaking, really heavy, power supply.

So, I'm attempting to trade in the couple hundred pound bottle cap/MOT stack/other heavy crap in for what will fit in a computer box.

It's a DC Charger, based heavily on Steve Connor's T2.

Specs:
0-400BPS spark gap
30mH secondary (4.13" x 17")
20pF toroid
34H charging reactor
20nF homemade cap

The spark gap is made from VCR and printer parts, and quietly hums along at 6000RPM. I was suprised at how well it is all aligned. The quadrupler seems to be happy at 60Vin, but I haven't gone full blast yet.

The capacitor is a 22kV, 20nF (30nF if you squish it hard, but that would load the PSU more than I'm wanting ATM). Made from transparency film and aluminum foil. I'm a bit worried about it, I might end up making another and seriesing them for 15nF, 44kV.

The Charging reactor is made from the two MOT's on the left. The black MOT has another secondary wound on so that I can run electronics and the motor. It also runs the quadrupler.

My biggest question is, if you'll note, the lone MOC's to the right will see a peak of 6kV DC and they're rated 2000V AC. I've heard of people pushing these for 8kV+ without fail, but not in this kind of application. Think it will work? I have two more, enough to parallel them and make them .5uF, but in simulation, this really decreases my performance. Any ideas?

EDIT: Just to note: They didn't

Oh, and yes, I'm fixing the naked diodes, they have cases I took off for dianostic purposes.

Marko

Sat Sept 30 2006, 02:25PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

One thing I'm very, very afraid is that such a rotary gap based on small VCR motor is not going to work.

SInce tank cap is charged to enormous voltage there will be great attractive force between electrodes, and gap will just jam still and power-arc.

Even if you manage to make it spin it will be slowed down alot with each strike, again promoting power arcing and possibly ruining your supply in a short period.

I remember once making a pretty nice mini-rotary gap out of high power 28V DC brushless fan.

After overrating it to 36V it drew quite a lot of watts and eye-quess of speed was probably over 5000rpm.
When I hooked it to relatively small 400W coil, with just 6nF cap at 15kV the gap was jammed at place.

People who use large motors and angle grinders for their gaps obivously had a very good reason for it!

Disk and rotor must also be heavy enough so they aren't considerably slowed down each time after firing!

Also: about the capacitor 'squishing' - plates will undergo powerful attraction once the cap is charged, so capacitance is going to increase anyway.

Seriesed caps might be a pretty good icea; TDU seemed t have sucess wth that approach.

Did you use oil or something in the capacitor along with trasparency sheets?

Part Scavenger

Sun Oct 01 2006, 12:26AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Firkragg wrote ...

Did you use oil or something in the capacitor along with trasparency sheets?

No, that's why I was very surprised that it stood up to 22kV! But how long? That's why I want to build another and series them.

Even if you manage to make it spin it will be slowed down alot with each strike, again promoting power arcing and possibly ruining your supply in a short period.

I wondered about that too, but I didn't know so I thought I'd try it. The motor actually has quite a bit of torque (it was from a laser printer I think) but runs at a slow speed, so I stepped it up.

In all honesty, I am expecting several things to go wrong with this coil, and I'm kind of testing "how low can you go" so to speak.

Before I can do much more though, I need new MOC's.

Marko

Sun Oct 01 2006, 08:38AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I just wanted to point how weak such a gap is with motor running few watts and having entire system weighting couple of hundred grams.

For real gap, for your high-power system it needs to be one hell of motor over there (and dangerous too if it flies apart...)

Steve Conner

Sun Oct 01 2006, 11:06AM

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

That thing about the electrostatic force between the electrodes can't possibly be right. The forces are tiny. I bet that Firkragg's fan motor actually stopped because EMI from the spark gap scrambled its control circuit.

I used a 100 watt universal motor from a vacuum cleaner to power my old rotary gap.

Marko

Sun Oct 01 2006, 01:28PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

That thing about the electrostatic force between the electrodes can't possibly be right. The forces are tiny. I bet that Firkragg's fan motor actually stopped because EMI from the spark gap scrambled its control circuit.

I admit my gap looked a bit funny, with too-wide electrodes, wich I measured to have capacitance of some 2pF between.

When charged to 15kV there is a charge of 0,00000003 C.

Force is (9*10^9*Q^2)/r^2.
Distance between electrodes is 1mm.
(IIRC)

F= 0,0000081/0.000001 = 8,1N - not great but definitely not omitable for small 3W DC fan.

It is also amplified by electrodes acting as lever.

Force is very temporary since gap breaks out practically instantly after electrodes get so close, and may be most disturbing while large electrodes are separating and cap recharges again.

My motor worked and spun happily when connected to just obit at low power, and it looked pretty sure that he isn't going to get bothered by EMI.

I would spun it up, but once HV is on gap would ignite, quench, and before it would continue to spin it would be pulled back and break again.

It would just vibrate back and forth arcing radnomly.

Also, bangs from heavy sparks themselves might have significant role here, pushing electrodes away and messing things up. (with large bang energies sparks would toss electrodes around like toys)

Scavenger's gap has mush better constructed electrodes so I think it will work, but i'm not sure if it will mantain constant speed well.

I guess it's better to try it than argue around..

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