Electronics > Projects, Designs, and Technical Stuff
Snubber for capacitive loads
OM222O:
If it's for experimentation then where did the 20Hz switching frequency come from? Either relax that to lower cycles per second or use smaller caps (both of which reduce the power needed). There are half bridge FET drivers, I'm not sure how well they work for IGBTs though, I never used them like that, but you can use two half bridges to make an H bridge. Also adding 1 Meg resistors across the caps seems like a good idea. They won't interfere much with normal operation but discharge the caps when not in use.
Now to the simplification: why not use a bridge rectifier to get a high voltage DC input from mains, which is then smoothed by your high value capacitors, connected directly to an H bridge that then carries out the pulsing? This way a lot of the current comes directly from the mains, and the caps only compensate for when voltage is below peak. This eliminates a lot of the complexity (zero crossing detection, opto triacs, normal triacs, high inrush currents, pulse grade caps, etc etc). Is there specific reansons for which you need to isolate the caps before discharging them? They'll be at mains potential with a nasty shock regardless so isolation doesn't really add much safety.
Circlotron:
--- Quote from: ricko_uk on July 29, 2020, 07:45:02 pm ---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! :)
--- End quote ---
Ignitions??? Oh my goodness! Why didn’t you say so??? My favourite subject! I have made literally thousands. Let’s talk ignitions. :-+
ricko_uk:
Thank you Circlotron, Duak and OM222O :)
Sorry for replying only now but took some time off.
Replies below:
Circlotron:
LOL, I started my electronics journey making ignitions, but that by now I need quite a bit of refreshing. Good that you are passionate about them, maybe we can have separate conversations about those when I start playing with them!! :)
OM222O:
Yes, it looks like I will have to do either of those, smaller caps and/or lower frequencies. I am putting together a spreadsheet to try to balance all the variables. I will share it here when done.
--- Quote from: OM222O on July 29, 2020, 08:50:57 pm ---Now to the simplification: why not use a bridge rectifier to get a high voltage DC input from mains, which is then smoothed by your high value capacitors, connected directly to an H bridge that then carries out the pulsing? This way a lot of the current comes directly from the mains, and the caps only compensate for when voltage is below peak. This eliminates a lot of the complexity (zero crossing detection, opto triacs, normal triacs, high inrush currents, pulse grade caps, etc etc).
--- End quote ---
That's a good idea but:
1) it would half the voltage available...?
2) how can I make sure that when discharging I always get the same voltage level fed into the bridge?
--- Quote from: OM222O on July 29, 2020, 08:50:57 pm ---Is there specific reansons for which you need to isolate the caps before discharging them? They'll be at mains potential with a nasty shock regardless so isolation doesn't really add much safety.
--- End quote ---
3) what do you mean by "isolate the caps before discharging them"? Do you mean the 2 diodes or the "charging" mosfet?
4) If you are referring to the "charging" mosfet than to avoid shorting the mains. Is that wrong in some way?
DUAK:
--- Quote from: duak on July 29, 2020, 08:32:31 pm ---Are the caps rated for this type of service? Ie., pulse discharge or photoflash?
--- End quote ---
5) I could not find any pulse-rated caps that size. Only ones I could find are electrolytic and those I found do not say anything about pulse rating. Are there any electrolytic rated for pulsing applications?
--- Quote from: duak on July 29, 2020, 08:32:31 pm ---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.
--- End quote ---
6) I assume you mean a 1:1 isolation transformer?
7) I wasn't planning to. Would Why would that cause "surprising voltages here and there" and how would the transformer help?
Many thanks again to you all! :)
OM222O:
Why would it half the voltage? In the other case you have 1 cap charged to mains peak and one connected basically to GND, it'll be the same with just using an H bridge. If you want double the voltage, then you should use two bridge rectifiers and charge a set of caps to + mains peak and another set to - mains peak. If using 240vRMS, that's about ~700v. What the actual fuck would require that amount of power? You'd blow the coil up / melt it.
To ensure you always get the same voltage, you can use a peak detect circuit and switch on the H bridge at peak voltage.
I was refering to the triac and disconnecting the caps from mains. Seems pointless since there will be high voltages involved anyways. It is a good idea to have a switch for fault conditions, like short circuit, but those switches are called circuit breakers (ideally followed by GFID) not triacs.
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