Thank you guys for your reply
SeaMonkey, do you relay mean I can reform a 30V rated capacitor to 74 by charging it with a 50k resistance in series? if this is so, then you have saved me a lot of troubles, I'm trying to use available items from the market and over 40 V capacitors with large capacity is very difficult to find. AC capacitors are up to 440 V but with only 16 mf.
Any way, I think I should not deliver 74V to a 12 V battery, so may be I'd better make the PS gives me only 24 V. Again, and if I understood the label on the welding machine (the switched mood supply) it should deliver 24 Vmax when on load, and 74 when not on load i.e. oped circuit voltage. For some reason (maybe as the load is considered very small by the power supply) so it keeps outputting 74 V, if this is the case, then shouldnt I use a lamp as a load but in parallel not in series? shouldnt this trick the PS to think there is a load and then steps down to 24 V? If what I'm saying is correct then should I use a 220v/100w lamp ? Maybe I should make some tests and see.
If I put a resistance in series, this will limit the V as you said, but then, when the output steps down to 24 V, the resistance will still limit the V and it may drop to less than desired.
Meal, If I can some how get say 80 V rated capacitors, which I'm not certain I can get, then the circuit will need to be more complex, I hope I can spare the output semiconductors so I hope I would control the volt coming from the switched mood supply to avoid complicating the circuit. As you advised, I keep a PC fan permanently on the transistors.
I bought that welding machine that uses switched mood instead of transformer as all the local experts warned me of buying any thing that has a transformer as their wires are of low quality copper that heats up and the insulation would melt and then an internal short will happen so it will not last, if the wounds are not from copper they will be from aluminum which is even worst (I'm talking about the local market of course - importing a transformer from digy key or something will cost triple original price due to shipping)
The machine's data: 999 sanjiu welding - model zx7-160