Author Topic: Help!Capacitor that burns out after a while!  (Read 2105 times)

Offline davekni

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Re: Help!Capacitor that burns out after a while!
« Reply #20 on: February 20, 2020, 05:14:43 AM »
Simulating spark gaps is a bit tricky.  For most uses, a simple switch is sufficient.  Instantaneous elements such as switches are problematic for analog simulation. LTSpice does reasonably well with instantaneous events, but still sometimes gets stuck (step time becomes too small until the simulation errors out).    LTSpice does have switch elements along with arbitrary behavioral voltage and current sources that can be used to control the switch. Sometimes I've used thyristors instead (NPN and PNP connected into a latching circuit) with diode breakdown defining the trigger voltage.
David Knierim

Offline Uspring

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Re: Help!Capacitor that burns out after a while!
« Reply #21 on: February 20, 2020, 02:06:26 PM »
At the bottom of this page is a scope shot, that shows, what you might see in your circuit. http://www.laushaus.com/tesla/vortexgap.htm It is the voltage in a primary tank of a spark gap Tesla coil with the secondary removed. In this example, the decay of voltage is mostly due to spark gap losses and approximately linear. Your waveforms would look similarly, if only a small fraction of the energy is burned up in the cap ESR. If the decay is faster and exponentially sloped, you probably dissipate the energy in the cap.


Offline Peregrine

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Re: Help!Capacitor that burns out after a while!
« Reply #22 on: February 29, 2020, 05:32:24 AM »
I detected this signal with a probe, what does it make you understand?  What are those peaks on each ridge?  It is not seen well but at the beginning of the oscillations there is a signal of 17mhz while the resonance frequency is about 400khz

Offline davekni

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Re: Help!Capacitor that burns out after a while!
« Reply #23 on: March 01, 2020, 05:35:02 AM »
The 17MHz is probably resonance of wiring inductance and capacitance (wiring from the cap through the spark gap and coil and back to the cap).

The spikes appear at the voltage peaks, which are the current zero-crossing points.  At those points, the spark gap has no current, so partially extinguishes (ions recombine).  It then requires a bit more voltage drop to return to normal arc conduction.  Those momentary spark-gap voltage increases are likely what you are seeing as the spikes.

Polypropylene capacitors would be best for your application, a long string of lower-voltage caps.  A 33-long string of the Chinese induction-cooker caps, 0.33uF 1200V, would last longer than any ceramic.  (I've abuse tested a couple of these 1200V caps with bursts of +-1700V at 80kHz.  Your frequency is higher, but your bursts are shorter.)  They are available multiple places, but here's one EBay link:
https://www.ebay.com/itm/10PCS-New-BM-Capacitor-MKPH-0-33uF-630VAC-1200VDC-for-Induction-cooker-P-30-5/272271654633?hash=item3f64a7bee9:g:PJ4AAOSw9eVXXQsg
If a single 33-long string doesn't last long enough, 132 parts in a 66s2p array would almost certainly survive long-term.  (You may find that the ring lasts longer using PP caps, as the ceramic parts are likely a significant part of your total power loss.)

Is the sharp initial start to the ring waveform necessary for your application?  If not, a DRSSTC-style circuit (series-fed resonance from an H-Bridge drive) would be much quieter, more efficient, and more stable.
David Knierim

Offline Peregrine

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Re: Help!Capacitor that burns out after a while!
« Reply #24 on: March 01, 2020, 03:03:44 PM »
thank you David for your answer and competence. I would also use another circuit, but in your opinion, with the electronics you are talking about, you can get large peaks of energy (30.50w) discharged on a coil in a few microseconds or better, nanoseconds? or are there other techniques? thank you

Offline Uspring

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Re: Help!Capacitor that burns out after a while!
« Reply #25 on: March 01, 2020, 05:13:19 PM »
It looks, like you're loosing about a factor of 2 in amplitude for each cycle. That cannot be expected from losses in the spark gap alone. The damping you see corresponds to a series resistance in either the cap or the coil of about 7 ohm. This is probably also much larger, than can be expected from your coil. My guess is, that a very high percentage of the input power is dissipated in the cap, which would be about 20W assuming 5J per discharge and 4 discharges per second. Too much for a doorknob.

Handling 10's of kV at a 1000A with silicon is probably not very economical. I'd stay with a spark gap, but make sure, that you have flat or better rounded electrodes. That will allow to move the electrodes closer together for a given breakdown voltage, which will reduce losses after the gap fires. And, of course, heed the the previous advice regarding the caps.

Offline davekni

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Re: Help!Capacitor that burns out after a while!
« Reply #26 on: March 02, 2020, 12:46:42 AM »
It would help to know your goals more precisely.  You refer to "peaks of energy", but list 30.50w, where "w" usually refers to watts, which is a unit of power and not energy.

The energy stored in the coil is proportional to the square of the magnetic field strength for a given coil geometry.  The spark-gap into resonant circuit is making an oscillating field with multiple magnetic-energy peaks of alternating polarity (alternating magnetic field direction).  Once you are using good PP caps, the oscillation will continue well longer.  Are those multiple peaks helpful?  Or, are you after a single peak?  If the latter, then it will be much more efficient if the oscillation can be terminated after a half-cycle, saving most of the energy from the coil back into the capacitors for the next pulse.  (Thyristors would be great for that case, but most are too slow in reverse-recovery time.)

An H-Bridge series-driving your resonant circuit will make the current ramp up much as it ramps down after the spark.  So, the whole burst will be roughly twice as long, having a ramp-up followed by a ramp-down.  If each magnetic-energy peak of the 400kHz resonance is useful, there's twice as many in each burst.  If you are after a single half-cycle (single uni-polar magnetic field pulse), then an H-bridge isn't a good topology.  BTW, if you do go with an H-Bridge, it needs to handle the ~1000A, but only a few hundred volts, as in DRSSTC drive.  There's lots of information here on building DRSSTC H-Bridges using IGBT bricks.
David Knierim

Offline klugesmith

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Re: Help!Capacitor that burns out after a while!
« Reply #27 on: March 02, 2020, 08:57:34 PM »
Uspring wrote:
>>Handling 10's of kV at a 1000A with silicon is probably not very economical. I'd stay with a spark gap...

I was about to say: or use a coil of the same size, with more turns, so less current for the same magnetic field energy.  Proportionally smaller capacitance, to keep the frequency unchanged.

But then the voltage would need to be even higher, by the same factor.  This is not helping to make a semiconductor-switched solution workable.

I like Dave's suggestion: drive the tank with AC voltage source in series, at the resonant frequency.  Can get tank voltage much higher than the driver voltage.  Tank impedance, sqrt(L/C), can be chosen to suit the driver.


As others have said:  What is this for?
Suppose that with much better capacitor and spark gap, your 5 joule discharge rings for 40 cycles (100 us).
That's a loss rate of 50 kW.  To ramp it up DRSSTC-style in 40 cycles would need a 50 kW driver.
« Last Edit: March 02, 2020, 09:09:56 PM by klugesmith »

Offline ritaismyconscience

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Re: Help!Capacitor that burns out after a while!
« Reply #28 on: March 03, 2020, 07:12:53 AM »
Is high voltage necessary? It sounds like you just want to create a pulsed magnetic field. A simpler solution might be to use 1000V instead of 40000V, and use an SCR to discharge the capacitor (along with a diode if you want it to oscillate more). You could get a 1000V capacitor that is specifically designed to deal with pulsed applications like this for very cheap.

Here is something you might find interesting:
https://www.repairfaq.org/sam/other/sgtms1/sgtms1.htm
« Last Edit: March 03, 2020, 07:16:42 AM by ritaismyconscience »

Offline davekni

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Re: Help!Capacitor that burns out after a while!
« Reply #29 on: March 04, 2020, 06:16:58 AM »
I mentioned thyristors (SCRs) too, but they won't work at the frequency of interest here, 400kHz.  The "critical rate of rise of current" specification is just too low for 400kHz.  (I've searched many parts, as I'm planning to use such for increasing MMC capacitance during each burst on my DRSSTC eventually, at 80kHz dropping to 55kHz.)
David Knierim

Offline ritaismyconscience

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Re: Help!Capacitor that burns out after a while!
« Reply #30 on: March 04, 2020, 06:47:31 AM »
I mentioned thyristors (SCRs) too, but they won't work at the frequency of interest here, 400kHz.  The "critical rate of rise of current" specification is just too low for 400kHz.  (I've searched many parts, as I'm planning to use such for increasing MMC capacitance during each burst on my DRSSTC eventually, at 80kHz dropping to 55kHz.)

OP never said he needed 400kHz, he just said he wanted to make a pulsed magnetic field for a therapy device (probably something related to TMS). In that case, a lower frequency would work fine.

Offline johnf

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Re: Help!Capacitor that burns out after a while!
« Reply #31 on: March 04, 2020, 09:25:15 AM »
Ive used Behlke High voltage switches before to switch ion beams before an inflexion magnet. A modified half bridge constructed out of two SPDT switches shifting + 10kV to 0 volts then to minus 14kV and back to 0 volts into the cap plates in the magnet. Cap = 1200pf. I used non inductive 200 ohm resistors to limit current to around 20 amps. This switching was done at several kHz and we needed the voltage to settle sub Microsecond so that we could do our measurements
Behlke make solid state switches up to 1600 amps and up to at least a 100kV

Offline John123

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Re: Help!Capacitor that burns out after a while!
« Reply #32 on: March 05, 2020, 07:38:49 PM »
As a quick fix what about a low ohm resistor or/and ferrite bead to limit the peak currents from destroying the capacitor?

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Re: Help!Capacitor that burns out after a while!
« Reply #32 on: March 05, 2020, 07:38:49 PM »

 


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