Looking for BIG capacitors.

[itsmoked]

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>

 

[ScottyUK]

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=[&#189;]CV².




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

 

[OperaHouse]

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.

 

[itsmoked]

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?

 

[Warpspeed]

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.

 

[ScottyUK]

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.


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

 

[itsmoked]

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

 

[BrianR]

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.

 

[Warpspeed]

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.

 

[itsmoked]

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??

 

[BrianR]

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.

 

[itsmoked]

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.

 

[ScottyUK]

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.



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

 

[itsmoked]

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

 

[Warpspeed]

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

 

[fsmyth]

'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>

 

[itsmoked]

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!

 

[fsmyth]

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).

 

[itsmoked]

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??!?

 

[BrianR]

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.