Looking for BIG capacitors.
Looking for BIG capacitors.
(OP)
I am building a capacitive spot welder. Over the years I have thrown away maybe a hundred ~47,000uF screw post type/panel mount type caps. Now I find that I need 6 to 700,000uF at 16 to 25V worth of caps..
I expect to find these things laying around, but now suddenly, no sign of them.
I have looked at maybe 30 spots on the web. Ebay has them but in ones and twos in Bulgaria, etc. :(
Anyone have a handle on a spot where I can get some/all of these? I don't want to give Digikey $20 a piece for 10 of them. <groan>
I expect to find these things laying around, but now suddenly, no sign of them.
I have looked at maybe 30 spots on the web. Ebay has them but in ones and twos in Bulgaria, etc. :(
Anyone have a handle on a spot where I can get some/all of these? I don't want to give Digikey $20 a piece for 10 of them. <groan>





RE: Looking for BIG capacitors.
This isn't a direct answer, but many years ago I worked on a machine we knew as the 'projection welder' which was a machine which welded lids onto hermetic packages for hybrids. This item of plant used a capacitor bank charged up to several kV which was dumped into the primary of a pulse transformer through an HV contactor. The secondary winding was a few turns connected to a pair of huge hydraulically-actuated electrodes which clamped the hermetic package together before welding the lid and base together. The transformer core was nothing out of the ordinary - just iron lamination IIRC.
It might be easier to get hold of a number of PFC capacitors or similar and use this technique rather than build a bank of nearly a Farad. The energy storage is more effective at higher voltage, being given by E=½CV2.
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Your body might be a temple. Mine is an amusement park...
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
I have a couple of transformers and they're certainly easier to come by than big Farads.
I have a VFD I can gut for high voltage caps.
I also have an SSR the size of a lemon laying around.
Q1) You can ram one big decaying exponential thru an iron transformer with no more than 10 or 15% loses?
Okay..
E = ½CV2
= 1/2 x 0.68F x 18V2
= 110 Watt Seconds
Q2) What's the math relation needed to figure the core cross section for a pulse delivering 110Ws?
RE: Looking for BIG capacitors.
The VFD capacitors sound ideal, charge them straight off the mains through a resistor and bridge rectifier, and that part is solved.
A big nasty SCR should discharge that into maybe an old battery charger transformer. It should keep you entertained for hours.
RE: Looking for BIG capacitors.
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Your body might be a temple. Mine is an amusement park...
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
Unexpected violent capacitor venting can be either fun or terrifying, depending on your nerves and state of sobriety at the time.
As for the transformer, it will be doing pulse duty, so the resistance, inductance, and ratio of the turns will probably have more influence on the outcome than losses in the core.
RE: Looking for BIG capacitors.
I will also turn on the VFD. Once its up, I will turn it off then trigger the SCR. This is to limit the available energy(reduce the fire).
Comments??
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
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Your body might be a temple. Mine is an amusement park...
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
you originally intended (low voltage spot welds), from years
of salvage - I just cannot stand tossing all the absolutely
wonderful parts I run across. :)
I also have a few not-quite-up-to-spec SCR's (600v, 200A,
IIRC), but you would have to build a mounting jig to put
the correct pressure on the case).
A few points: if you drive a transformer with any solid-
state device, you are going to see some pretty healthy
inductive reactions. I am sure that you are aware of this,
but the magnitude in this sort of application may surprise
you. SSR's that I have used in the past seem particularly
sensitive to spikes, and die quickly. Good snubbers are
definitely in order, regardless of the driver.
One thing you might consider: a deep-discharge battery or
three, coupled to a few tens of turns of AWG6, feeding a
healthy SCR triggered by a de-bounced pushbutton.
(Moving the choke to the output of the SCR would be
eliminate the need to use large soak components, but I
like it there so I can add monitoring).
Simple, easy to implement, and effective.
<als>
RE: Looking for BIG capacitors.
This is a spot welder for battery tabs, like the ones in my laptop and my Hilti hammer drill. I have the replacement batteries, all 36 of them, that need to be welded together. Maybe we could get a group welder together and circulate it around, since one individual doesn't need it but briefly, occasionally!
RE: Looking for BIG capacitors.
Use the 300W gun, and there are no cell heating problems.
A high percentage of silver adds joint strength. I use
the same solder I got long ago to repair old HP 'scopes.
This has worked well for the last ten years.
Not near as much fun as spot welding, though. :)
As an aside, I did use the method I suggested earlier
(albeit with a knife switch instead of an SCR) using small
2" pieces of selected solid wire as a fusible link to
control spot heat. Worked, but soldering was quicker.
As far as circulating a group welder, it seems to me that
shipping would be prohibitive, and would almost certainly
leave out our friends across the pond. Couldn't have that!
<als>
P.S. - I find that the "invention effort" is generally
more entertaining and instructive than the end use of
the object in question (unless you are talking about
motorcycles or airplanes). :)
RE: Looking for BIG capacitors.
What's wrong with invention -> motorcycles??!?
RE: Looking for BIG capacitors.
itsmoked, Oh dear, I have just soldered a bunch of NiMh cells together using an ordinary mid sized electronics soldering iron. Use a wipe of zinc chloride flux and they solder easily. This stuff is sold here as Bakers soldering fluid but it's hydrochloric acid neutralised with zinc and makes soldering chromed bits practical too. You need to clean off the flux residue afterwards.
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
Anyone know about this stuff ? And could it be used to "glue" tabs on batteries without incinerating the internal structure ?
RE: Looking for BIG capacitors.
I have also used it to solder stainless steel but a lot of heat is needed and a 2% silver solder helps a lot too.
I don't know about the cell structure but am suspicious that what you say is correct and I might have a short life for these cells.
RE: Looking for BIG capacitors.
If your still doing the Cap route, Check with anybody that works with old computer main frames they may have a few old caps laying about form the power supplies.
RE: Looking for BIG capacitors.
RE: Looking for BIG capacitors.
Nothing. The point was that the end use is more fun in
those two categories than the inventive process. :)
<als>
RE: Looking for BIG capacitors.
Comcokid.... I really like that idea because I know the typical voltage used is about 11-13V and what is a lead acid but a reallly big cap?
I have certainly welded my share of dipsticks, etc, across batteries. I even have two new six volt Trojans sitting in my lab. I also have a bin of IGBTs, the big brick ones yanked from a VFD. Perhaps I could run an IGBT with a triggered signal generator to do this whole thing.
RE: Looking for BIG capacitors.
found it easier to control the weld diameter and depth
with a higher (36 to 48) voltage source. Plus, your
choice of electrodes is broader as the voltage rises.
Your IGBT's should be capable of handling 50v pulses
easily.
Note that I have never used this in any production type
setting, merely toyed with it as a sort of experiment.
As far as soldering, I have soldered NiCad, NiMH, SLA,
Li-Ion, and any other small cells used in packs, with
no apparent problems. I do use the "solder the tab on
one end, then to the other cell, and fold" method,
sometimes using wire if small strips were not handy.
I have not found it necessary to use any special or
unusual flux; merely lightly sand or scrape the spot
to be soldered, tin it quickly, and solder the tab or
wire after it has cooled. If any cells have been
damaged by this method, it has not shown up yet (and
some rebuilt packs are old enough that the usual aging
effects are starting to appear).
Not trying to throw sand on your spot-welding effort
(I have used far less in the past to justify experimenting
with a new or different technology/field/method/process),
but if the objective is to construct a battery pack, then
I'd say solder it and be done. If there are any negative
effects of soldering cells, I would certainly appreciate
someone jumping in with an explanation.
<als>
<als>
RE: Looking for BIG capacitors.
This article used a microwave oven transformer.
http://w
Is the coffee mug sized scr more of an apple sized mug or
a grapefruit size mug?
Hope this helps.
RE: Looking for BIG capacitors.
fsmyth; I believe I could safely do my multiple 15 cell nicad packs with solder {evil lead solder :)}.
My concern is more with the relatively new Liions. I think(could be wrong) that they are not vented because water vapor entering them may react badly. Hence the reason they can explode more easily, (witness recent cellphone injuries), and that specifically they have the membranes in them that are not meant to take any heat.
Have you (fsmyth) or anyone else soldered LIions successfully? Or just nicads and NiMh?
Madcow; Yeah I saw that welding article a while ago. Tiz not a spot welder :( I like the two 115V cooling fans wired in series. Haha!
I fished out my SCR and found that I inflated the size estimate some what... It is actually the size of a grade AAA Jumbo Egg or possibly a small tangerine.