Introducing more resistance into the secondary circuit will limit the current but will generate heat, in the car industry water cooling is used to keep things from self destruction and stop the resistance rising which will affect weld quality. From the 70's to 90's control of the primary was effected with SCR control but with ignitrons before that. The firing angle of each half cycle could be delayed to control the primary current and thus the secondary current. The secondary circuit of a weld gun is constructed from aluminium and copper to minimise losses, if you put a big bar of steel in your circuit it might limit your current enough.

Mike

I have made a few new battery packs up and it rejuvenated the power tools no end!

I did try to reuse the tabs, but found that,because new batteries were more powerful, it caused the old tabs to heat up.

So to fix this issue, I used copper pipe cut down the length, flattened then cut into little strips. Polished them up and soldered the tabs with a 100 watt iron.

The soldering is simple. The hard part is keeping all the batteries in the shape of the plastic pack.

well i also found that stainless was hard to solder. That is why i used copper pipe. I ripped the lightweight tabs off, rubbed the battery on some emery and they soldered easy.

cant help with the way you are trying to do it though

Frank

I cannot claim to be an expert on spot welding but I think putting resistance in the output of the transformer is the wrong approach. Resistance welding (which I think is the correct terminology) looks very simple but there are a lot of factors that affect the weld quality. (type of material, electrode mechanical pressure, length of current pulse etc)

All the spot welders I have come across have a control circuit on the primary side, mostly its some sort of timer and the current is fixed at maximum by the design. If your welder was capable of 1000 Amps it could still weld very thin steel shim as long as the current pulse was very short, but mechanical relays and timers would not be precise enough to give repeatable pulses for thin material. I have used 'SSR' (Solid State Relay) modules in conjunction with a 555 timer IC to vary the weld time (switches the primary side of the transformer) but whilst I can weld 0.5mm SS sheet I doubt it would give consistent results on thin battery tabs. If your transformer is the type used for car bodywork repairing (mine was) it is out of proportion for what you want to do.

It might be worth experimenting though with resistance on the primary side of the transformer, 100W lightbulb or a fan heater element for example.

Ian P

It needs to be small in Ohmic value but capable of passing a very high current.

Think about the cross sectional area of the heater element wire in relation to the battery tab (let alone the copper bar the secondary 'winding' is made of, the element wire will be a good fuse!

Ian P

Indeed, but the heater will cope with a massive overload for a second or so, unlike a fuse it is designed to run red hot for hours on end. It's worth a try, and you can always use multiple strands of the wire (I used three to make a shunt for an ammeter once).

Edited By Neil Wyatt on 19/12/2015 09:35:33

This is a nice simple design,

http://www.philpem.me.uk/elec/welder/

http://www.philpem.me.uk/elec/welder/images/schematic.png

You need about 0.6 farads of capacitance, apparently 'I've heard from a few people who have managed to build welders based on the one-farad car-audio capacitors, after removing the blinking-lights display circuitry from the top of them.'

Also useful safety advice.

Neil