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[moved] 24,000j Capacitor Bank (Now Complete)

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LAN_403

FastMHz

Wed Jan 09 2008, 06:47AM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I used those diodes because those are what I had laying around...all I wanted to do was a quick voltage test....but I still would like to know why the string fried at 4500v when it should have taken that easily.

...

Wed Jan 09 2008, 07:29AM

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

was there any significant load on the mot when the diodes blew? Since you are only halfwave rectifying the output the cycle that is not being used will probably peak much higher than the voltage on the cap bank... Especially with a bank this large that could easliy pull enough current to drop the mots voltage considerably...

Jasonr

Wed Jan 09 2008, 03:14PM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

Did you have any kind of current limiting? Caps will act like a short when quick changes in voltage occur. A variac is a way to attempt to limit the current however dirty brushes or any spike at all on the input is exponential on the output.

You need either a current limit resistor, or a ballast of some kind.

another thing is powering up the MOT's with no load you could have overvolted the diodes very easily.
Jason

Wed Jan 09 2008, 05:39PM

Registered Member #85 Joined: Thu Feb 09 2006, 01:22PM
Location:
Posts: 21

the failure is totally normal, similar things happened to me on a smaller scale.

when you charge caps with a halfwave rectifier, the unused halfwave adds to the voltage of the capacitor, and kills the diode, if it can not stand twice the voltage.

FastMHz

Wed Jan 09 2008, 07:10PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Alright...so it seems to be a halfwave overvolt problem...I suppose my alternative is to make a bridge, or to make a string to handle much higher voltages, with a rather large safety margin.

FastMHz

Sun Jan 13 2008, 08:17PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Great news - my charger appears to be working as needed. I bought some new 8kv diodes and made a bridge rectifier out of them...repeated the same test and charged a 1uf cap up to 5kv, which is a safety margin 500v more than I'll need since my 24kj bank is 4500v.

Next step is to build my remote controlled triggered spark gap. I plan to use a 10kv ignition transformer as the initiator.

Jasonr

Sun Jan 20 2008, 06:14AM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

I wonder if when you test it on your bank you don't over current you diodes. I think you had both over volt and over current happening.
Jason

FastMHz

Tue Jan 22 2008, 07:34AM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

The capacitor bank has been completely bussed together for the first time!!! The system, including charger, went â€œliveâ€ the other day for the first time, charging to 1% of the total energy capacity. I will not charge higher until I complete the remote controlled discharge unit, as I have no other safe way to discharge the system. I am also using an IR thermometer to monitor the diode temps to ensure that I'm not drawing too much current and overheating them.

FastMHz

Wed Jan 30 2008, 08:13PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Iâ€™ve built the charger into a self contained unit, with integrated cooling to keep everything operating at peak performance. With the charger now a self contained unit, I intend to charge the system from a safe distance, in addition to the previously planned remote discharging. Components failâ€¦bad things can happen, and I don't ever want to be near the bank while it's chargedâ€¦the charger:

Hvsupplynr6

Shaun

Wed Jan 30 2008, 09:30PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

What do you use for current limiting on the charger? A big power resistor, or maybe a series capacitor on the AC side?

Without it theres a good chance of overcurrent on the diodes, or even the MOTs. If the diodes really do stand up to 40A, I think they'll survive. But you could blow a breaker; 2400W worth of MOTs will charge this bank in 10s running at maximum power. Thats 20A draw right there, assuming they don't go over...

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