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Looking for advice on homemade spot welder design

Hi. Lately i've become interested in building a small spot welder for DIY use. The idea is to end up with a (relatively) portable unit, with controlable power, suitable for spot-welding anything from battery contacts to 1.5mm sheet metal. I've ruled out buying a commercial welder - mainly because they're rather expensive where i live, and also because i happen to like the challenge :)

So, right now i'm contemplating two different designs, and having a hard time deciding which one to go for:

  • Capacitor discharge welder: The idea is to have a rather large (100,000 to 300,000uF) capacitor bank which is discharged by a large triac or similar electronic switch into the weld contacts. A cheap microcontroller with an integrated ADC would control the charging voltage, allowing for variable power, plus triggering the output device.

  • Mains transformer welder: The idea is to modify (or pay to have built) a large (1 kVA to 3 kVA) transformer with only a few turs in the secondary to get a low voltage, high current output. The power controlling would also be handled by a microcontroller timing a relay in the mains winding - i've had good sucess in switching off relays very quickly (arround 5ms) by using a zenner in series with the back- EMF catching diode.

I've considered also running a high voltage cap discharge through a transformer to get a strong amperage pulse, as done in commercial welders, but ruled it out - if anything, because it's overly complex and overkill for a simple device as intended. Anyway, in both cases i'm planning to build a simple clamp arm with copper contacts as weld tips.

Right now i'm favoring the transformer option, if only because it ends up being way cheaper than buying large electrolyitc caps + SCR. I have concerns about it being used to weld small parts, as i feel that a capacitor bank allows for much finer power control - yet, caps won't perform as well in thicker parts... or will they?

So, my questions are... does anyone with experience with this type of welders have any advice to share? If i go with the transformer option, should i look into limiting the output surge current with an inductor of some kind? How long can one expect electrolytic caps to last when stressed with charge-discharge cycles like these? Any comments that can help me make up my mind will be appreciated, thanks!

Reply to
plisandro
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I can share my unsuccessful experiences: I wound a roughly 1kVA (continuous rating) transformer with a few (tried 1 to 5) turns of welder cable (also tried paralleled lengths of 10mm2 wire. I put a toggle clamp with copper contacts on this thing. What I found was: most of the power was dissipated in the secondary winding, it was way too thin, and, I was able to weld thin metal, but only if I pressed very lightly on the electrodes so that they made bad contact with lots of resistance. When I pressed down hard, there was not enough resistance in the material to be welded and so the weld area stayed cool whilst the secondary got hot. It was impossible to get consistent results because the pressure and surface condition of the metal to be welded caused the contact resistance to be either too high or too low many times.

I have also used a proper commercial spot welder which had a *much* bigger transformer, much fatter secondary, much more pressure on the (water cooled) electrodes, and which did much better welds.

You could try asking on sci.engr.joining.welding

Chris

Reply to
Chris Jones

Sounds like a 1kVA transformer doesn't offer enough output current to weld consistently. This was also a fear of mine - that a trasformer- based device wouldn't offer enough power or would do so only for a brief moment...

I just did, thanks for the heads up!

Reply to
plisandro

I have a small spot welder that uses a small Variac to charge a largish capacitor, and a pressure-switch that activates a contacter to dump the cap through a transformer when the electrodes clamp down hard enough. It's designed for tack-welding small s/s wires in orthodontics before they're brazed into position. I can send internal photos if needed... however:

It doesn't work worth a damn on anything more conductive than stainless steel, or on anything above a couple of mm thick, and the parts must be touching between where the electrodes touch. In short, it's great for its intended purpose, but not much use for anything else.

If you want to make one, I'd modify a microwave oven transformer to replace the high voltage secondary with a few turns of heavy-duty flat copper sheet, or flattened tube. Bolt the I side of the transformer core together before grinding through the welds that attach them to the E side, then you can load your new secondary easily before re-attaching the I cores (clamp tight and weld again).

Clifford Heath.

Reply to
Clifford Heath

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