DC Welder off Car Batteries

[zacwolf]

I'm working on a DC welder that will run off two automotive batteries in series [24V].

It's based around a spool gun:
http://www.lincolnelectric.com/assets/servicenavigator-public/lincoln3/imt913.pdf

This gun pulls max 120Amps off two series batteries[24V].

I want to build a small circuit that works off the spool gun trigger so that the welder tip is not always "live".

I've created a basic schematic at:
http://www.digikey.com/schemeit/#efp

Would anyone be willing to give me any recommendations on anything I may have missed or misunderstood?

THANKS!

 

[zacwolf]

I'm confused most on if diode are even necessary?

Without diodes:
http://www.digikey.com/schemeit#efr

I would think that I would need something to allow the contractor to be un-energized?

 

[poor mystic]

I'd personally prefer to use a big copper knife switch rather than electronics, because the currents involved are going to be enormous.

 

[zacwolf]

Yeah, but I went this tied into the trigger control.

The contactor I chose is rated at 350amp, which is just under three time what I'll need, so that part I'm not concerned about at all.

The part I'm not fully understand is what, if any, blocking diode isolation I need between the welding circuit, and the circuit being used to control the coil and the wire driver motor...

 

[zacwolf]

Let me try to ask the question in a different way.

Do you size a diode's amperage based on the amperage traveling THROUGH the diode, or based on the amount of amperage it would need to BLOCK?

 

[duke37]

A diode passes current in one direction only. The current rating should be well in excess of the average current for safety.

The diode should have a voltage rating sufficient to stop any reverse current. With a supply of 25V, go for 100V rating.

Welding involves bad connections and pulses of current with perhaps significant inductance so do not try to use diodes which do not have spare ability.

 

[zacwolf]

Thanks for checking this out!

Could you take a look at:
http://www.digikey.com/schemeit#efu

You'll notice that the entire circuit/square in the upper right is portion will carry up to 120amps at a pretty stable 24V. The rest of the circuit, will be carrying the same 24v but less than six amps max.

The contactor is this one:
http://www.electriccarpartscompany....r-5060-Hz-AC-Contactor-br-15-Leads_p_200.html

...so it's able to carry the load, and has really good characteristics for back-talk, sparking, etc.

What I don't fully understand is does the D1 in my schematic just need to be a basic 50V/10amp Schlotsky [since that's the max that would ever be "pulled" by the contactor coil and drive motor parts of the circuit]

OR

Does it need to be this:
http://www.ebay.com/itm/121104490126?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

...so that it can resist any back-feeding from the welding circuit?

Or am I misunderstand everything and the diode isn't even necessary?

 

[woodchips]

I see this is a TIG welding gun.

Whilst the welding transformer puts out 24VDC this is not the same as two 12V lead acid batteries in series. The transformer will have current limiting, the batteries most certainly will not. The instructions talk about 0.035" aluminium wire which will act as a fuse with the batteries, not a welder.

Must say I haven't read all the way through the attached PDF, but I can't see it working.

Bob

 

[duke37]

I cannot get at the digikey site. Why not sketch the schematic and then scan it or photograph it and attach it to the thread?

 

[zacwolf]

Woodchips: I'm working on a mig welding gun, and it works just fine:
http://www.readywelder.com/

I'm just not willing to pay $600+ dollars, and as a bonus I'm creating a spool gun that will be compatible with a battery setup, as well as work with my normal Lincoln welder setup.

duke37: I'll figure out how to get a copy of the schematic. Thanks!

 

[zacwolf]

Schematic

I've attached a screenshot of the schematic.

The source is two 12V car batteries connected in series to 24V.

Then two DC/DC power supply/regulators:

24V[unregulated]->12V[10A regulated]->24V[3A regulated]

The spoolgun drive motor runs off the 24V[3A regulated].

I have a trigger that controls two relay coils: The relay that powers the 24V drive motor, and a 350A contactor that supplies power to the welder tip.

I've also added a 150amp/6000V diode across the tip circuit (after the contactor) to help with any inductance/spiking from the welding tip.

I did leave out that there is also a 150amp circuit breaker (auto-reset) off the battery pos.

 

[duke37]

An inductance in series with the arc can generate a high voltage if the arc dies, with AC the arc extinguishes twice per cycle this will allow it to restrike. Putting a diode across the arc will suppress the high voltage but this may not be a problem with a DC source.

 

[zacwolf]

Thanks Duke, that's what I was unsure of. None of the other "plans" for a DC welder I had seen had such a diode, but I just wasn't sure...

Would it "hurt" anything by being there? I understand it'll cut the voltage a small amount, but that's not really import for the welding. I have a nice huge heat sink that it connects to (and that I'm also connecting the regulators too), so heat shouldn't be an issue.

THANKS!
-Zac