×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Looking for BIG capacitors.

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>

RE: Looking for BIG capacitors.

ItSmoked,

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.


----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: Looking for BIG capacitors.

I have a strain gauge spot welder and it uses the method that Scotty talked about.  The cap was charged up to a variable voltage and discharged into a transformer through a SCR. At least that is what I remember, been about 15 years since I looked inside.  The loss of all those TTL circuits that would draw 100A unfortunantly lead to those caps disappearing.

RE: Looking for BIG capacitors.

(OP)
Let us investigate this a little...

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.

Agree with Scotty, high voltage is your friend when it comes to storing lots of Joules in 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.

Electrolytics won't appreciate being hit time after time with a near-short circuit. Ideally you want a film capacitor designed for pulse duty: the PFC caps are at least film types.

----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: Looking for BIG capacitors.

(OP)
Come on guys... What size tranformer?  I want hours of fun not 50ms of terror... lol

RE: Looking for BIG capacitors.

I'd have to agree with itsmoked, having just finished a magnetic zapper intended to re-magnetize permanent magnets.  Customer specified low (<400V) voltage so the obvious high voltage caps were out.  What I used for about 600J was a bank of 8000uF 450V electros of the type designed for high energy flash discharge work.  They have a long life with repitive hard discharges and these came from Cornell Dublier.  Couple this through a transformer, especially one designed for 400Hz work with lower primary inductance and that should work.

RE: Looking for BIG capacitors.

Good points Scotty and Brian. Early CDI ignitions used to die fairly frequently, mainly due to the capacitor spitting the dummy.  But in this case, the primary of a mains transformer is not going to be such a savage load, and the repetition rate will presumably be fairly low. If the internal connection to the capacitor foil survives the first hit, it may last quite a while. Low ESR caps would be much better, but may not be an option.

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.

(OP)
Okay then I will just take a transformer (120V input) a little bigger than a grapefruit... (technical ay-what?) Saw the primary winding out of it and replace it with um.. ahh.. 5 or so winds of 8AWG.  I will pry open my VFD and tap off the dc bus.  I will run that thru my lemon(yeah I know my descriptions are fruity) sized SCR which I will trigger with a lab supply referenced to the ground of the VFD dc bus.

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.

I got my high voltage by bridge rectifying the mains with a 1:1 isolation txfmr of course.  330VDC and an SCR as big as a coffee mug and a backswing diode nearly as big and this thing will fry your brains.

RE: Looking for BIG capacitors.

(OP)
Sounds like loads of fun Brian.  I don't have to magnetize rocks though :) I just have to heat a spot of nickle 25mil in diameter and 15mil thick to 2,700F for 20ms.

RE: Looking for BIG capacitors.

Be careful if you are connecting to the DC bus - the poles of the DC are sequentially connected to the phases of the AC line as the diodes conduct, so 'ground' will still give you a nasty shock relative to earth.

----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: Looking for BIG capacitors.

(OP)
Thanks for the warning ScottyUK .  I will be moving very slowly.. As I play with the zingle beast.

RE: Looking for BIG capacitors.

Make sure you have a spare set of underwear handy when you trigger that SCR for the first time.

RE: Looking for BIG capacitors.

'smoked, I probably have enough low voltage caps to do what
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.

(OP)
fsmyth; To be honest I know the original cap plan would work without the "invention" effort and I would greatly prefer it.

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.

I find that solder works well as an attachment method.
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.

(OP)
Not supposed to solder them anymore... As they are more fragile due to the fact that they are ALL spot welded now.  :(

What's wrong with invention -> motorcycles??!?

RE: Looking for BIG capacitors.

ScottyUK, I noticed that risk of the o/p being connected to the mains while diodes conduct and I disconnect the mains during zapps.  (Although getting connected to >10KuF @330V is possibly worse than a touch on the mains - off course you don't want them added together).

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.

(OP)
BrianR is that the stuff in a little gray can?  I may have some.  Everything I'm reading is saying that there are membranes in the batteries that will be damaged, especially Li-Ions.  When the cost $11 a piece I am wary of damaging one in the middle of a stack only to discover it by having a laptop explode on my lap... bugeyed

RE: Looking for BIG capacitors.

Somewhere in the dim recesses of my rather murky memory, I remember reading about electrically conductive epoxy. It contained silver powder, (or something similar).

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.

itsmoked, my lot is in a black plastic bottle and is a clear liquid.
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.

itsmoked,
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.

For a low voltage spot welder, you could try using a car battery. I seem to recall some simple low duty cycle discharge-welders actually had a lead-acid battery as the source with a charger to keep the battery up. These used a mechanical means to drive and pull back an electrode against the two pieces of metal to be spot welded.

RE: Looking for BIG capacitors.

"What's wrong with invention -> motorcycles??!?"
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.

(OP)
Thanks dino40.

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.

'smoked, the 6-cell (12v) version will work, but I have
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.

(OP)
Thanks for the voltage tip fsmyth.  It does appear that the entire spot welding scheme is a battle of resistances.  The key is that the 'big resistance' be the location you want welded.  Well when the place you want welded is a good conductor to another good conductor you see the problem.  It often comes down to the cable, electrode, and termination details.  I suspect if you go not "the extra mile" but two miles towards the cabling aspect then the energy/voltage situation gets somewhat less important.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources