Electrolytic vs supercap for CD spotwelder? Answered
I need to make a cheap spotwelder to rebuild some battery packs
(e.g,. welding thin nickel strips).
My options are:
(a) Try to find most of a farad at up to 25V or so on eBay. This could
take a while. (Buying new electrolytics in the 0.1F range and above
at these voltages is pretty expensive.)
(b) Go for supercaps from Digikey (e.g). They've got some 5V 1F models
for only a few bucks, and some "fast-discharge" ones for 10x more.
Of course, if I go for more than 10V or so, I'll have to build
some sort of charge-balancing circuit around the series caps,
and I'd rather not unless it's really the best way to go---
though it might be as simple as a resistor in parallel to each cap.
(c) Try one of these weirdo "digital supercapacitors" that are all
over eBay for the crowd who fill their entire trunk with stereo
amps. These look like they're rated at 1-5F at 13.8V and (I would
hope) include some sort of charge balancer. Dunno if they're
suitable for the sort of instant-discharge I want for a welder,
I'm having difficulty figuring out:
(a) How fast a discharge I need for a spotwelder anyway; I'm going to
be using an SCR to trigger it and presumably want the lowest-inductance
arrangement I can get, but are we talking milliseconds or microseconds?
(b) What can I get out of a supercap? Milliseconds or microseconds?
I haven't worked with supercaps before, and most people who seem
to be designing circuits with them are using them as batteries.
Other random questions:
My reference design had 0.5 to 1F at 0-25V through a 600V 50A TO-65 SCR; what
worries me are things like http://www.avxcorp.com/docs/Catalogs/bestcap.pdf,
whose page 3 shows that non-ultra-low-ESR supercaps (e.g., the
affordable ones) seem to have virtually no capacitance for pulse
widths of 10ms or less. (The BestCaps in that datasheet claim
something like 60% of nominal instead, which is pretty good!)
I dunno how narrow the pulse width needs to be to get good welds,
but I'm guessing pretty narrow---for example, one guy claimed in
http://www.philpem.me.uk/elec/welder/ that just the difference in
putting the SCR on the low side and not the high side (so the trigger
for it didn't have to go through the wires, electrodes, and workpiece)
worked a whole lot better, which implies to me that he might be
talking microseconds. But I just don't know.]
Of course, I also don't know whether I need 0.5 F at 10V or 1F at
25V for the things I'm considering.
Not to mention---the 50F caps at http://dkc3.digikey.com/PDF/T072/P1354.pdf
(some of which claim ESR's of 0.025 ohm at 1kHz) have these skinny little
0.5mm leads on them. How in hell would these leads not be vaporized if I
charged up the cap and then shorted it, as a spot-welder would be doing?
(And sure, I could try to attach #4 copper wire to them---somehow---and
send that to the sharpened tips on my welding electrodes, but boy I feel
silly doing that---seems like whatever part of the cap's leads aren't
attached to the heavy wire will just evaporate when shorted, if they can
really dump that kind of current, even if it -is- just for milli-to-
The Instructables site has some rambling about using supercaps for CD
spotwelders, but it's just rambling---nobody who's actually built one
using them, for example, or who seems to talk about discharge rates,