Help!Capacitor that burns out after a while!

[Peregrine]

Help!
Hi everyone, I need to use this circuit, but after a while the capacitor burns out!
the power supply comes from a voltage multiplier, and the capacitor is 50kv 10nF, the coil 10uH
what can I do to protect it? thank you.

[Mads Barnkob]

It sounds like you are using a capacitor beyond its voltage, current or frequency specifications. Please provide all details about the application and capacitor.

[Peregrine]

the capacitor is a doorknob 50kv/10nf , I need a quick discharge on the coil...
the oscillation frequency is about 500khz but the discharges on the sparkgap are few hertz

[MRMILSTAR]

If this is the schematic for a spark gap coil, you need to swap the position of the capacitor and spark gap. I don't think this is causing your capacitor problem but it could cause problems for your transformer. As Mads stated, it sounds like you are exceeding the voltage rating of your capacitor. You want at least a 2x and preferably a 3x voltage rating for your capacitor.

[klugesmith]

I stand with Steve about it being easier on the transformer to drive across spark gap instead of across capacitor. Maybe not a lot easier, because of the 10M resistor.

As Mads said, there's more to learn about how you are abusing the capacitors, or if the capacitors are over-rated fakes (?).

What is the RF current in each spark, the tank circuit energy, and the average power?

Power input is partitioned somehow between losses in L, C, and spark.
Do L or C get hot when you operate the circuit at power below the destructive level?

[Peregrine]

If this is the schematic for a spark gap coil, you need to swap the position of the capacitor and spark gap. I don't think this is causing your capacitor problem but it could cause problems for your transformer. As Mads stated, it sounds like you are exceeding the voltage rating of your capacitor. You want at least a 2x and preferably a 3x voltage rating for your capacitor.

no, the position of the sparkgap and coil are exact. This circuit is not to make a tesla coil but a therapy device with pulsed magnetic fields (as in the picture)... and then I don't use a transformer as a power supply but a 30-40.000v direct current generator.
then for the voltage of the capacitor maybe you are right but I used doorknob which told me that they are not really suitable for fast impulses while I should take the polypropylene ones, but apart from this problem, would you have other tips to avoid damage?

I stand with Steve about it being easier on the transformer to drive across spark gap instead of across capacitor. Maybe not a lot easier, because of the 10M resistor.

As Mads said, there's more to learn about how you are abusing the capacitors, or if the capacitors are over-rated fakes (?).

What is the RF current in each spark, the tank circuit energy, and the average power?

Power input is partitioned somehow between losses in L, C, and spark.
Do L or C get hot when you operate the circuit at power below the destructive level?

the power is less than 10J, but before the capacitor broke, I saw a flash of lightning go through it ...

in tesla coils, a so-called "terry filter" is often used but there is an alternating transformer and it is used to avoid RF or that could damage it, instead mine is a DC circuit....

[Weston]

Ceramic doornknob capacitors with a 50kv 10nF rating are uncommon. Do you have a picture you could share? I suspect that the capacitor could also be unsuitable for tesla coil use.

[Peregrine]

Ceramic doornknob capacitors with a 50kv 10nF rating are uncommon. Do you have a picture you could share? I suspect that the capacitor could also be unsuitable for tesla coil use.

[MRMILSTAR]

Even though this isn't a spark gap tesla coil I would still swap the positions of the spark gap and the capacitor. Doing so will make it less likely for any high frequency high voltage oscillations to get back into your power supply and possibly damaging it. From your schematic, the functionality should be the same other than the capacitor charging through the additional wire resistance of the inductor. The additional inductor wire resistance is negligible compared to your 10M ohm charging resistor.

I would want at least a 2x voltage margin on that capacitor. With a rating of 50 KV you aren't close. That flash that you describe sounds like the capacitor breakdown voltage was exceeded. Four of those 50 KV capacitors in series-parallel would yield a 100 KV rating with the same capacitance which would give you a comfortable 2.5x voltage rating.

[ElectroXa]

Hi 

capacitor  failed maybe from resonance ringing, as this phenomenon boosts voltage and current in the circuit, leading to failure.
So I recommend you to use capacitors in series to have a large voltage  headroom, to avoid flashover, as for example using 2* 50kV capacitors in series to have a rating of 100kV.
I also recommend using pulse film caps as they can withstand high pulse current. 

[klugesmith]

Just confirmed your estimate of ringing frequency, 500 kHz.
Associated impedance sqrt(L/C) is about 32 ohms.
That means that if spark gap fires at 32 kV, the RF current hits 1000 amperes in first cycle.
How fast does the ringing decay?  You could measure that with oscilloscope and a sense coil or pickup antenna loosely coupled to the tank circuit.

If spark gap fires at 32kV, initial energy in C is about 5 joules.
If a discharge happens 4 times each second, that would be 20 watts being dissipated in C+L+spark
(and I think another 20 watts in the charging resistor).
How hot do the R, C, and L get, and how fast do they get hot?

[Peregrine]

Thanks friends for your advice ... I looked better at my circuit and saw that, since I had already burned many, I had put 2 40kv capacitors in series, thus obtaining a total value of 80kv 5nf ... So the  problem I don't think it is the sizing of the capacitor ... Keep in mind that during the discharge a discharge bang occurs so strong that at 1m away the ears hurt ... therefore it may be that this kind of condenser is not suitable for these  discharged or is it another problem? I add that the device must be kept on for at least 20 minutes, ... For a short time it is good for over 20 minutes it is not reliable

[rikkitikkitavi]

Sounds like your caps are overheating internally due to the discharge currents and then something goes bang as it takes some time.

/Rickard

[Uspring]

Overheating seems a likely cause. Do you have ESR specs for the cap or a tan(d) ? If not, place a small pickup coil near the inductor and take a scope shot of the voltage. From the ring down of the amplitude you can get a rough estimate of the dissipation. You'll loose a few hundred volts amplitude in the tank for each cycle due to the spark gap voltage drop. At your high voltages, this might be a few percent initially. If the ring down is much faster, than the cap ESR will take most of the energy.

If that is the case, paralleling caps instead of serialising them might be the better idea.

[MRMILSTAR]

You haven't told us what you are trying to do with this circuit. If you are only charging and discharging this capacitor infrequently, once a minute for example, I would disconnect the power supply from the circuit before firing in order to protect it. If this is firing rapidly then obviously you can't do that and will have to rely on the charging resistor for protection.

[Teravolt]

your capacitor is not rated for pulse duty if you can find some high voltage mica caps you will be better off. The foil atached to the ceramic slug on the inside typicaly will degrade with pulse duty on door knob caps. they are better for marxes or tripplers

[Peregrine]

You haven't told us what you are trying to do with this circuit. If you are only charging and discharging this capacitor infrequently, once a minute for example, I would disconnect the power supply from the circuit before firing in order to protect it. If this is firing rapidly then obviously you can't do that and will have to rely on the charging resistor for protection.

the capacitor does not charge and discharge it once a minute  but continuously, as I said, even for 20 minutes ... in fact the resistance heats up a lot but now I will try to divide it into more values to have less power dissipated ...

[Peregrine]

do you know if there is a circuit in LTSpice that can simulate a sparkgap well? it would be very useful ...

[Peregrine]

I have a probe that reduces the voltage read by 1000 times ... do you think I could use it to connect it to the oscilloscope to see the signals? or to read the currents at stake?

[klugesmith]

You don't need a probe that touches the circuit. 
I would question the accuracy of any ordinary 1000:1 voltage probe at 500 kHz.

As a couple of us have suggested,
* Any probe close to your pulser will provide a signal with which you can accurately measure the frequency and damping. 
* You can get the initial voltage accurately enough from spark gap geometry, or from known power supply voltage and measured repetition rate.
* You can get the first-cycle current accurately enough from voltage and tank impedance sqrt(L/C).