AC Welder to DC conversion.

[BloomingtonMike]

I have heard stories of converting old skool AC welders to DC using WW2 surplus diodes. Anyone ever done this? Anyone have any info for me to copy on how to do this to my Lincoln IdealArc 250 AC welder? Looking for a way to surface mount the hookups for DC. Thanks for any info.

 

[Taylortractornut]

I worked for a company when I was 16 that had some old welders from the 30's that were still used to weld up dies and worn punches. they were GE and Westinhouse. they had all been converted over to DC with a diode on the input side one I think had a large DC motor with Its leads run to the welders power cord then and ac motor was attached to the front of the DC motor to make it generate DC current. We used those old welders alot in the maintenace shop for heavy gouging and other hot jobs.

 

[PILOON]

AC to DC is relativly simple to do.
4 diodes properly connected will make a full wave bridge.

The problem then is to get diodes that will carry the amperage (like 250 amp diodes).
Also I believe that a 'reactor' is required in the curcuit.

I found some BIG scr's that I plan to use as diodes to make just that same conversion.
If I can locate the diagram, I'll scan and post it.

 

[djradz]

SCRs require a circuit to "turn on" whereas a diode does not. You might want to consider using something else.

 

[PILOON]

djradz
I was led to believe that an SCR could be used as a straight diode sort of like 'wide open' if not controled.
Would hope that I'm right as these are brutes. They are mounted on 1/2" studs with 5/16 leads.
Either way a set of 4 was a mere $5.00 so no great loss.
On the other hand maybe I just need to feed them some DC to have them work as a diode.
Guess I need more research.

 

[SPIKER]

to make the scrs act as simple diodes you should be able to connect the firing lead dirrectly to the input side of the scr so that as the wave form comes up it is trigering the scr, as it goes back down it automatically shuts off. on 0 crossing.

the diode pack is easy assembly for full wave rectification. ac across the leads and dc out the bases, +/- then...

how much power does these scrs hold? ratings? where did you get them? what voltage are they rated at?

mark M

 

[Soundguy]

I don't mess with triacs or silicon controlled rectifiers much.. but.. Think of the SCR as a solid state switch. You need to supply a small current at the gate terminal and then hook up anode and cathode as for a diode application. Add some resistance to the gate line.. it doesn't need a huge jolt. Once the scr is triggered, it will remain on till the current thru the SCR drops below a minimun threshold.. which will be identified as Ih or holding current listed on the package.

So you will need a small power supply, a 'exciter; possibly momentary push button switch, and then run a wire from that switch to all 4 gates in your full wave bridge. The kicker is.. you will have to push the exciter to get the arc started, and as soon as you loose the arc, you will need to re-excite. I'm not sure if you could put a large value resistor across the + and - of the load to keep holding current flowing... might have to expirement with a resistor that allowed enough holding current to flow.. yet wasn't a heavy parasitic load that would burn up with long welding runs. You might expirement with just keeping the gates excited with a on/off switch. DO read up on the characteristics of yor SCR.. see if they will tolerate constant gate voltage.. and if so.. try to provide not much mor ethan the minimum to excite them...

Soundguy


Soundguy

 

[Soundguy]

</font><font color="blue" class="small">( you should be able to connect the firing lead dirrectly to the input side of the scr so that as the wave form comes up it is trigering the scr, as it goes back down it automatically shuts off. on 0 crossing )</font>

I'm not so sure they work like that. The data in my semiconductor ref manual does not indicate that negative bias on the scr gate will shut it off. It said that as long as minimum holding current ( I h ) was flowing thru the scr, it stayed on. Cutting the supply to the scr will shut it down, assuming the gate is also not being continously triggered.

You may be thinking more of a Triac.. which will handle AC.. which is essentially 2 scr's connected in reverse parallel. ( note that triacs still have a triggerable gate. )

Soundguy

 

[Highbeam]

Lot's of stuff I don't ever want to know about here. I am also interested in the simple and safe way to turn an AC welder into a DC welder. Has anyone actually done it? My welder is an old but good Lincoln AC225.

Walmart now sells them for 250$.

 

[BloomingtonMike]

Spiker, Soundguy, Pillion

There are a set of 275 amp dodes on Ebay for $60. First off how is that pricing - if high do you recommend a place I buy them?

Second I really do not know much about A welder electricity. I know the basics of AC only really. I have two external connectors on my welder - one for teh electrode and one for the ground clamp. How would i wire these with diodes to make DC?

REally newbie at this so sorry for the questions (wrong, inaccurate, stupid - all questions will apply with me).

Thanks for any newbie level help.

Mike