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40VDC 58000uF capacitor, any good for can crushing?

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SpaceWyzard

Mon Apr 16 2018, 12:22AM

Registered Member #2318 Joined: Fri Aug 28 2009, 01:22AM
Location:
Posts: 33

I just got a 40vdc 58000uf can type capacitor (electrolytic?) from an old wire welder. I know it isn't high voltage, but I didn't know where else to post.

Would this type and rating of a capacitor work well in a can crushing coil setup? Or would it be better to go with lower capacity, high voltage?

AndreiRS

Mon Apr 16 2018, 03:35AM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

Using some online calculator, your numbers give me 46.4J as result. Lets say in my case where I have 36 microwave oven capacitor, around 35uF and 2kV. It gives me 70J. But getting these high voltage capacitors is a paaaaain, took me years and it is so dangerous. I think your setup looks good and safe.

I can't answer you which one is better because I just don't know. But other people already made this around here and may help.

klugesmith

Mon Apr 16 2018, 07:16AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sorry, SW. To crush cans with that capacitor, you need to hold the capacitor firmly and use it like a hammer.

If Andrei figured the CV^2/2 energy correctly, it's _barely_ enough joules to make a detectable indentation by magneforming, in an efficient design. Tiny ripples at best -- not the hourglass shape often associated with "can crushing".

That ideal design can use a range of capacitance values. 50 uF at 1600 volts, with a 4 or 5 turn coil (around 2 uH) will do (for the tiny ripples). Same cap and coil with 3200-4800 volts can produce "normal" crushed cans.
Larger C can meet the energy requirement with smaller V, but it also needs a lower-inductance work coil to keep the discharge time down. Reference design has a time constant sqrt(LC) of 10 microseconds.

You can't make a work coil with less than one turn. Even if you could, the inductance of the interconnection and of the electrolytic capacitor itself (ESL) would get you. I've seen some reports of respectable indentation using electrolytic caps in 800 volt banks, but I bet they need a lot more joules than a proper high voltage, pulse rated, plastic film or paper in oil capacitor.

dexter

Mon Apr 16 2018, 08:41AM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
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Posts: 195

AndreiRS wrote ...

in my case where I have 36 microwave oven capacitor, around 35uF and 2kV. It gives me 70J.

Microwave caps can take a lot more DC volts
You can easily charge them to 6kV and get 648J

SpaceWyzard

Mon Apr 16 2018, 12:01PM

Registered Member #2318 Joined: Fri Aug 28 2009, 01:22AM
Location:
Posts: 33

Thanks for all the information. What could I do with such a capacitor? If it's no good for crushing?

hen918

Mon Apr 16 2018, 01:56PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Audio amplifier smoothing cap is a good use.

AndreiRS

Mon Apr 16 2018, 05:15PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

A capacitor discharge machine for spot welding batteries and thin metal sheet.

hen918

Mon Apr 16 2018, 09:06PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

AndreiRS wrote ...

A capacitor discharge machine for spot welding batteries and thin metal sheet.

This would be OK, but by my calculations, the cap would only just have enough energy to do nickel strip welding, and would have to be charged after every spot. A rewound MOT is much better for spot welding.

klugesmith

Mon Apr 16 2018, 10:55PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

For small-scale spot welding, I think most designs use a transformer with AC on the primary, and a timer. Internet shows some, both DIY and professional, that discharge a capacitor into the transformer's primary winding. Then energy can be adjusted by initial voltage on the capacitor.

Need to mind the volt-seconds value that will saturate the MOT core. In normal service there's about 30 joules of energy transformed per cycle, with flux swing between -Bmax and +Bmax. For single pulses you get only half the flux swing, but could design for primary current many times higher than normal service. Saturation is a volts thing.

I bet remanent flux from the previous shot would be an enemy, unless it's reset to the opposite sign between shots, or the capacitor connection is reversed between shots. Details are left as an exercise. MOT might become a one-bit nonvolatile data storage core, whose state would be read by seeing if the spot weld is weak or strong.

Dr. Slack

Tue Apr 17 2018, 11:22AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Low voltage electrolytics can work well for disc launching, the appropriate discharge times are orders of magnitude greater than for can crushing. I used several 10,000uF 63v caps (in parallel) with a few turns to launch HD platters.

OTOH, can crushing just doesn't work well with 'lytics, or at least photoflash (PFC) caps. I compared directly 100uF 4kV built with both pulse caps and PFCs. The pulse caps achieved a full neck-down, the PFCs a barely perceptible ripple on the surface.

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