Electronics > Projects, Designs, and Technical Stuff

Snubber for capacitive loads

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Circlotron:
The triac would be better to be in the live line rather than the neutral line.

The MAINS_GND net should all be connected together of course, but definitely not connected to the actual mains ground. Mains ground is at neutral potential so you would effectively have a short across C15 and C28.

What is the inductance of L1? Depending on it's value, you may have crazy high currents.

Where do you expect the energy from the discharged capacitors to go? Least worst thing that can happen is when the caps are at zero volts and the inductor is at maximum current, when you turn off the IGBT it will recharge the caps via the upper mosfet body diode to near the original voltage, minus losses.

What is the purpose of this circuit? Practical application or just an experiment to see what happens with stuff?

You are about to discover that power electronics is rarely easy or safe, hence the old adage  - "with power electronics you are always a microsecond away from misery."

TimNJ:
If you need to fire at up to 20Hz, you’re really going to be slamming the mains very frequently trying to recharge the caps. You’ll need to check the I2T and repetitive surge ratings of applicable components.

Have you considered a PFC boost stage instead of just “passively” charging by switching on mains? This should keep the peak current down, inrush only when you turn it on. This would, in general, reduce stress on the

But even with that...

To me it seems like your power requirement is too high for normal mains. I’m not sure exactly the timing of your system, but let’s say the cycle is to charge and discharge 2*1500uF caps @ 350V each every 50ms (20Hz). Each cap stores 90J @ 350V. So if you fully deplete both, you need to supply 180J every 50ms. That’s 3600W! With a perfectly efficient converter, the average current is 15A. With losses, 17-19A average. And that’s just for one pair of caps.

You’d need to slow down your fire rate to keep the RMS current low enough to not blow a fuse or breaker.

But before getting to that...

This is a pretty dangerous sounding project with lottttsss of stored energy and technically quite difficult due to very high currents in pulse application. A few people have tried to DIY a welder on this forum, and although usually not insanely complicated circuits, very hard to do right.

——

To answer your question about the inrush limiter, those are nice but only provide any limiting when they are cold. If you’re repetitively drawing high current through the NTC, it will get hot and stay hot, and the resistance will practically go to 0.

Regarding capacitor type, if you need very high capacitance, you practically do not have another choice but to use electrolytic. You could check the temperature rise of the cap to see if it’s being ovetstressed due to self heating. Low ESR is best.

OM222O:
Like others said, this seems like an odd project. Why would you need that much energy discharged into the coil so quickly? Depending on the inductwnce and parasitic resitances, the coil might take longer than 50ms to discharge. Also what does the coil interact with? Is it a motor winding near magnets? Is it hanging freely in the air? Part of an actuator of some sort?

Overall this seems like a high voltage H bridge across the coil, is that the goal? Because that means you can simplify things A LOT.

As it stands you need insane anount of cooling and power requirement. I suggest if this is the actual circuit you need, start with smaller caps for testing and then increase the values.

P.S: like others said switching the live with triac is the safer configuration.

ricko_uk:
Thank you Tim, Circlotron, and Om2220 for all the feedback!:)

Answers to all your questions...:

Tim,
yes, I did think about PFC but only briefly because don't have much experience in power electronics and thought the solution in my schematic was the easiest implementation. But after all your feedbacks perhaps that's not the case.

After your reply I looked at ready made PFC modules but they are 300 USD to do what could be done (if I had the knowledge) for maybe 50 USD or less of components (I am referring just to the capacitor charging stage). This is the one that might be suitable while allowing me enough room to try experiments requiring more power (just in case): https://www.artesyn.com/assets/aif_-_pfc_series_11feb2015_a8e76167e7.pdf   
Do you think this is a better way to go about it?
Do you know any other parts/modules perhaps from other suppliers?

A (minor) issue there is that from the datasheet it is not clear what other electronics is needed because there is a large number of pins I have no idea what function they perform and the datasheet does not say - I have emailed them asking for more infos (pins like CMon, CShare, PVAux, PFWadj, LDenable, PFenable).

OM2220,
I looked at full bridge solution in the various suppliers (digikey etc) and this seems the one offering the highest peak current: https://www.st.com/resource/en/datasheet/pwd13f60.pdf

Do you know where/how I could find a higher current rating one? Are there perhaps solutions/components called something other than full bridge?

OM2220 and Circlotron
it just for general experimentation (various ideas like CDIs, discharge coils, spark gaps, engine ignition, etc). Just a bunch of things and ideas I had through the years and never got to try them.

Thank you all again! :)

duak:
R22 is going to dissipate a fair bunch of power depending on how often the charge/discharge cycles occur.  Your caps will also get fairly warm.  Are they rated for this type of service?  Ie., pulse discharge or photoflash?

Crydom made a nice soft start relay setup using two modules years ago that I used in an off line power supply.  I see they have added other features and functions to their line: http://www.crydom.com/en/products/control-relays/mcs-series/  If you don't want to spring for one of these, you should be able to work up a similar function with your existing triac.

Will you be isolating the mains from this circuit with a transformer of some sort?  If not, you may end up with some surprising voltages here and there.

Bleeder resistors and hazardous voltage indicators on the big caps are a Darn Good idea.

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