Builbing muself a welder but i'm not sure about the voltages.

[Refrigerator]

Hey.
So the other day i found the remains of a big honkin' transformer and today i finished rewinding the primary.
But now i'm a bit stuck, because i don't know what voltage the secondary should be and if i should make it just a buzzbox or should i add a rectifier for some DC welding.
I want to make two taps on the transformer, one for spot welding and the another one for arc welding, or maybe one tap would be enough ?
About the rectifier though, it's a big-old selenium rectifier but the problem is i have no idea how to hook up or test one of those things.
I'd like to finish this welder during the weekend, because there's alot of stuff that needs to be welded.
Also a pic of the rectifier ( connections numbered for convenience ):

Edit: for the time being i'll make it 30VAC i think it's okay for some 3mm sticks.

[T3sl4co1l]

30V is pretty low for comfortable welding... it can be done but it'll be annoying.  70V open circuit is very typical, in my experience.

The rectifier can be ohmed out.  Looks like it's got two AC pins and two DC pins, just as you'd expect for a FWB, so... as long as you don't put AC on the DC side, you're probably fine.

Tim

[tautech]

I had one with 3 mm primaries and 3 x 10mm flat x  section secondaries for an OC voltage of 65V.
The higher voltage does make for easier striking of an arc.

How do you intend to control the current?

Some rods are easier to start than others, try a few, and settle on what your welder likes.

[Refrigerator]

Thanks for the replies.
I'm a bit poor so i kinda do with what i have, so for now i'm winding my secondary with 0,8 mm wire, of course i'm not stupid enough to just make one winding and expext the thing to not blow up. I've already would four 30VAC windings so i think i'll hook them up in series-parallel the wire is about 20AWG so it should handle 11 amps max, i intend to end up with six 60VAC windings which should handle 66 amps at max which i think would be the max that the transformer can give.

[rob77]

i would rather try to find a old transformer with a thicker wire on some scrap yard and salvage that thicker wire . i did it that way many years ago when my budget was exactly like yours
or at least make more secondary windings  in parallel (not just 6 of them) - make as many as you can fit to the core to share the load across the windings as much as you can.

btw my welder back in the days (20 years ago) secondary was approx 10mm^2 square wire (salvaged wire) - approx 45V open loop (got not enough wire for more voltage )  without rectification and it was usable , but it was a challenge to weld with it

another approach to build a cheap welder was to use a dynamo from a old truck/bus driven by a electric motor. it had to be a dynamo (really old bus/truck) not an alternator.

[G7PSK]

Those selenium rectifiers the usual was the the two outermost and the central was AC and the other 2 were DC but on occasions I have seen it the other way around. For MMA welding you need about 70 to 90 OCV with the transformer having a drooping characteristic,the voltage dropping to around 8 to 24 volts whilst welding.
For resistance weldind 1 to 3 OCV is usual with current in the range of mA for very thin to 30000 Amps for thicker material, I used to have a spot welder that would stick 2 pieces of 10mm steel together at 10000 Amps on 2.5 cycles,(the timing was measured in cycles)
 

[Refrigerator]

i would rather try to find a old transformer with a thicker wire on some scrap yard and salvage that thicker wire . i did it that way many years ago when my budget was exactly like yours
or at least make more secondary windings  in parallel (not just 6 of them) - make as many as you can fit to the core to share the load across the windings as much as you can.

btw my welder back in the days (20 years ago) secondary was approx 10mm^2 square wire (salvaged wire) - approx 45V open loop (got not enough wire for more voltage )  without rectification and it was usable , but it was a challenge to weld with it

another approach to build a cheap welder was to use a dynamo from a old truck/bus driven by a electric motor. it had to be a dynamo (really old bus/truck) not an alternator.

I tried salvaging thick wire but it's just impossible to find a decent lenght of it, for example i have about 6M of 10mm aluminum insulated wire but that will only get me 5V on the output, it's not like i don't have any thick wire it's just that i don't have enough of it, thus i now have this pain in the ass winding to take care of. 
452 turns wound, only 904 more to go...

[sacherjj]

A MIG welder (GMAW and FCAW) is a constant voltage device.  This is the welder with wire fed out the gun and the current varies to keep the voltage constant, making up the differences in arc distance.

A stick welder (SMAW) is a constant current device.  The voltage varies to get a constant current. 

Using the wrong type of regulation with the other process can make an arc that is very hard to control.

[Refrigerator]

A MIG welder (GMAW and FCAW) is a constant voltage device.  This is the welder with wire fed out the gun and the current varies to keep the voltage constant, making up the differences in arc distance.

A stick welder (SMAW) is a constant current device.  The voltage varies to get a constant current. 

Using the wrong type of regulation with the other process can make an arc that is very hard to control.

I know that already, i've watched like 10 tutorials about just that.