Super capacitors

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Mike H
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#16 Re: Super capacitors

Post by Mike H »

Neal wrote: Tue Apr 24, 2018 8:44 pm I would be very careful handling them though and would treat them like a LiPo battery cell. Huge discharge capability and require careful charging IE: current limited or constant current charging and absolutely no over charging.
Kind of what I was alluding to. (Badly. :D )

jack wrote: Tue Apr 24, 2018 9:19 pm They were never designed for this type of operation. As noted, just the stored energy is quite dangerous.
Yep well speaking for myself, to borrow a quote from DTB: pole, touch, long, a, wouldn't, with, rearrange words. :lol:
 
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#17 Re: Super capacitors

Post by pre65 »

From what I can gather Stan's supercap PSU is an energy storage module.

So, given that I have a box full of Kemet 22000uf 63v 105C caps (BIG buggers) I could try that (or them :) ) between my DIY 15v power supply and the SEG.

That would test if "energy storage" was the way to go ?
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#18 Re: Super capacitors

Post by jack »

You'd need a lot of them - a 2F capacitor is 100 times bigger (approx) than a 22,000uF one in terms of stored energy.

E = 1/2 * C * V^2

With V being the same (15V), the relative stored energy is directly proportional to C, the capacity in Farads. i.e. you'd actually need 91 of your big Kemets to have the same effect.

:)
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#19 Re: Super capacitors

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jack wrote: Wed May 09, 2018 12:40 pm You'd need a lot of them - a 2F capacitor is 100 times bigger (approx) than a 22,000uF one in terms of stored energy.

E = 1/2 * C * V^2

With V being the same (15V), the relative stored energy is directly proportional to C, the capacity in Farads. i.e. you'd actually need 91 of your big Kemets to have the same effect.

:)
Yes, I'm sure you are right.

Thing is, how much extra capacitance does one really need ? :?
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#20 Re: Super capacitors

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Yes, I'm sure you are right.
I think we can all agree that 2 / 0.022 = 91. So think we can safely say he IS right.
Thing is, how much extra capacitance does one really need ?
I would guess somewhere between plus and minus some, for some value of some.
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#21 Re: Super capacitors

Post by andrew Ivimey »

I would guess somewhere between plus and minus some, for some value of some.
[/quote]

I'm sure you're right Nick.... snigger.
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#22 Re: Super capacitors

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Ah yes. :lol:

BUT, because my caps are BIG, then my micro farads must be super micro farads as they have more room to work their magic, being extra low ESR helps as well. :wink:
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#23 Re: Super capacitors

Post by jack »

pre65 wrote: Wed May 09, 2018 2:23 pm ... being extra low ESR helps as well. :wink:
It would be a tiny tiny ESR - with all those 50mOhm caps in parallel, you'd end up with an ESR of about 0R00055.

Now THAT IS A LOW ESR !

But it won't really help you at all...

I used just a few similar but HV (500V) RIFAs to build a disk-platter launcher. As the energy stored is proportional to the square of the voltage, having a higher voltage is far more important than having bigger capacitance (in this use case).

One of my projects on this is at http://4hv.org/e107_plugins/forum/forum ... 98629.post - with just 4 capacitors, I got 2200J with an ESR of 0R015 - enough to vaporise a screwdriver, crush a coke can or fire a 3.5" disk platter so high and fast it punches a hole 30ft up in the ceiling of the warehouse we were in... notice the size of the bussbars I was using and the thickness of the main cables :)

FWIW, 2F @ 15V is only about 10% of the energy in this thing - I originally designed it for 8 capacitors, i.e. 4,400J - easily enough to seriously injure or kill if it suffered from spontaneous energetic disassembly...

Schematic is linked to in the top post - most of the design is to do with safety interlocks to try to prevent folk being seriously hurt - the trigger switch was on a lead about 15ft long and the unit was kept behind a polycarbonate safety screen.
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#24 Re: Super capacitors

Post by pre65 »

I've received my Ebay supercapacitor board from China.

Not sure what I'm going to do with it though. :)
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#25 Re: Super capacitors

Post by Neal »

Probably a ‘how long is a piece of string?’ sort of question, picking up on comment by David in a thread on TAS about a Limetree streamer....why do supercap supplies cause more problems than they solve? Or in other word what are their failings and does a good linear supply better a super cap supply.
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#26 Re: Super capacitors

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He is probably quoting me there.

Ok, so you have a super cap, and you need to charge it before use. So, you have to have a current limited supply to charge it otherwise bad things will happen. So, you need a current limited supply, ok, so the cap charges up and you connect the load. So now the load is connected to the current limited reg and the super cap. So the load draws some current. Where does that current come from? The super cap or the current limited power supply? I don't know either, maybe one, maybe both. If its one or both, how is it really any different from powering the load with a regulator with a cap on the end? is there some magic about the supercap? I don't know either. Ahh, you say, but the super cap can provide more current in a pulse than the current limited supply. Well, maybe so, but exactly what load is going to need that big a pulse of current, and no matter what the ESR of the super cap, it needs to get from the cap to the load, so there will still be the resistance and inductance of the connecting leads. So just how big is the pulse going to be that can't be supplied by a normal cap? Ok, lets assume there is a load that does draw that big a pulse? How often does it do it? If its more often than some value, then the current limited regulator that's feeding the supercap is not going to have enough power and the supercap will eventually discharge.

The Berrisford supply is what I described, a simple voltage regulator, followed by a simple current limiter followed by a super cap. Is it better than a cheap voltage regulator? Yes, the cap hides the noise of the reg. Is it a general solution? No, because its current limited in what it can supply by the current limiter to stop the thing blowing up on startup. Is it better than a good voltage regulator with a small cap on the output? I don't know, to my ears no, but I am biased.

So what if we get more tricky and say, ah, we will use a bank of the caps, charge one off line, then switch it to the output, let it discharge while the next one charges? Sounds good, except, the voltage from the cap will reduce as the cap discharges, so we will get a saw tooth wave out of the thing as it switches from cap to cap. Ah you say, we can reduce the problem, by switching faster do the discharge is less. Ok, so now you are just adding more HF noise to the output, one of the points you were trying to avoid at the start. Ok, so you get clever and make it switch at the point the voltage drops a certain amount, so it only switched as fast as it needs to. So still got the sawtooth, but now you have a load dependent HF noise source in series with the output. I know, we can avoid the sawtooth, by switching the caps as before, but send the output via a low noise regulator to remove the sawtooth. Hang on, isn't that just the same as using a cap that charges at 100Hz, and filtering the output from that with a low noise regulator. Hmm.

I am biased as I said, but to me that looks like a solution in search of a problem.
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#27 Re: Super capacitors

Post by jack »

With you completely on that.

It has "super" in the name, so must be good!

The rest is history. ..
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#28 Re: Super capacitors

Post by pre65 »

Interesting post, thanks Nick (G).

Now I know why my Dorado takes so long to charge up. :)
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#29 Re: Super capacitors

Post by Neal »

Thanks for clarifying Nick, I was curious. Maybe another case of emperors new clothes.
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#30 Re: Super capacitors

Post by Nick »

Well, Its just my opinion, but David nagged me to build a supply with them, but no matter ho hard I tried I couldn't think of a general fits all solution that didn't add more problems that it solved. Or at least solved any problems that were not simpler to solve another way.
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