# Harbor Freight 90a Flux-core Ac To Dc Conversion



## BGHansen

I have a buddy who impulse bought a Harbor Freight 90A flux-core wire feed welder.  I have to admit to looking at them too, especially when they go on sale for under $90.  They’re probably not a bad deal for the occasional weekend welding project, plus the 110V portability is nice.  But as my dad constantly told me as a kid “you get what you pay for”. 

Bottom line is the welds look terrible.  Penetration is poor and there is a lot of spatter.  Problem with the stock welder is it’s AC output, not DC.  Current flows back & forth from the puddle to the wire which causes the flux-core wire to spatter even more than DC flux-core wire welding.  All other HF wire feed welders are DC if that tells you anything.  I did a little web research, watched a few YouTube videos and found that it’s fairly easy to convert these units from AC to DC.  The two pieces of hardware needed are a full-wave rectifier and an electrolytic filter capacitor.

The rectifier takes the sine wave AC and flips the negative wave to the positive side so the output looks like “hills” running from 0V to peak voltage and back down, then back up again.  The capacitor smooths out the “hills” to a relatively straight line or DC output.  The videos I watched recommended a minimum 47,000 uF electrolytic capacitor with 30V minimum rating (welder outputs 28V).  I found a number of eBay sources for the rectifier rated at 1000V and 100A for $10.  Found a Siemens electrolytic cap rated at 40V and 47,000 uF for $25 delivered. 

Another important component is a shunt resistor across the two leads of the capacitor.  The resistor drains the capacitor after welding, otherwise the wire is still grounded and your ground clamp is still hot!  Some guys on their conversion discharged the capacitor by touching the wire to the ground clamp; expect a nice spark.  The capacitor I found had a shunt resistor already there, drains the cap in about 3 seconds.

The conversion is pretty simple; cut the output leads from the welder’s transformer and jump them to the AC side of the rectifier.  The transformer output leads each have a small lead that runs to the welder’s circuit board, so the splicing takes place downstream from that point. 

The rectifier has two output leads; + positive and – negative.  Those leads run to the appropriate terminals on the capacitor.  For flux-core wire, the welder ground clamp should be DC positive.  So, the welder’s ground clamp lead is tied to the capacitor’s + Positive lead.  The weld gun is tied to the capacitor’s – Negative lead.

One thing I didn’t care for on the on-line conversions is how they mounted the rectifier and capacitor.  All of the examples had the rectifier mounted on the side of the case.  They mounted the capacitor by drilling a couple of holes in the case and tie strapped in down.  I guess that’s probably how Harbor Freight would do it . . .

I chose to mount the rectifier on the back of the case, just looks cleaner to me.  Also made a sheet metal bracket to mount the capacitor next to the transformer.  The cooling fan had to be moved down to make room for the rectifier.  New fan holes were made in the case, in retrospect I should have removed the back panel and punched them on my Roper Whitney #218 press, but went the quicker route of drilling the holes.  Then spent 15 minutes cleaning up burrs . . . 

You can see from the example welds that the DC conversion makes a world of difference on the quality of the welds.  I ran a bead using flux-core wire from my Hobart Handler 190, the HF stock AC and HF after the DC conversion.  Much, much reduced spatter and much better penetration.  The welder sounds better too, just a sizzle while welding instead of lots of spattering.

So what did I learn?  Since everything in my shop will end up being my son’s someday, I went out and bought another Hobart, now have a Handler 140 (110V) to compliment my 190 (220V only).  Sure, the Handler 140 110V welder is $500 compared to the bottom line about $150 for the HF 90A converted to DC.  But I don’t want my son to inherit my shop and say, “Wow, dad bought a lot of cheap junk!”  On the other hand, money doesn’t grow on trees so if you do have one of these HF 90A welders, consider spending around $50 and make some better welds.

Bruce


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## FOMOGO

Nice job on the conversion, and I know what you mean about dc over ac. Always kind of fun to re-engineer something to meet your needs. I borrowed a Hobart 140 from a friend a few yrs back when I was making some gates out of square tube for our winter place in PR. I was really impressed with it, aside from some of the plastic hardware. As far as flux core welders, I think most folks welding skills would benefit from learning on a stick dc machine, and you can find nice examples on Craig's list for dirt cheap. Cheers, Mike


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## Bob Korves

Bruce, you did not mention where you got the rectifier, its part number, and what it cost.  I have one of those welders and would be interested in upgrading it.  Maybe my welds would no longer look like electric steel pigeon droppings...


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## BGHansen

Hi Bob,

I bought the rectifier from eBay seller "casanova*deals".  They currently have one listed under item# 232099994207 for $16.98 including shipping.  Or do an eBay search for "100A rectifier, 1200V" and you'll find many sources.  Another off the shelf seller has them for about $11 shipped from Hong Kong; item# 391595306004.  I bought mine from casanova*deals because they were in the US, naturally it's a Chinese rectifier but I didn't want to have to wait for shipping from China.  The rectifiers are available with or without an aluminum heat sink, I went the heat sink route which added a few $'s also.

Bruce


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## Bob Korves

Thanks, Bruce!

Edit:  Parts ordered.  When I get this working it will hopefully be "goodbye alibi."


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## JimmyA

Being a beginner at this but trying to understand leads me to ask the question: Is there a specific shunt resistor that should be added? There are so many when I search e-bay.


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## markba633csi

Jimmy: Around 250 ohms / 10 watt resistor would bleed down the voltage to a safe level in a few seconds.
Mark S.


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## danbbb

I stumbled on to this (2 year old) thread from a google search and joined the forum just so I could thank the original poster.  THANK YOU!  This relatively simple upgrade has completely transformed my cheapo Harbor Freight welder.  It went from a sloppy "Blat blat blat" to a beautiful sizzle with this fix.  If any one else is on the fence about doing this, absolutely do it!  You will not regret it.


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## Bob Korves

I still have the parts I bought, but so far have not got around to doing the modification.  Maybe the additional kick from danbbb will get me going on it...


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## danbbb

One thing I forgot to mention, I went ahead and bought a 200A bridge rectifier.  I have heard numerous stories of people burning out the 120's so I figured I'd go a bit bigger.   I used it until I ran out of wire on Saturday in the hot sun and had no trouble with it at all.  Welds were significantly more consistent with way less splatter than before.  I felt like I was using a MIG, not some flux-core HF welder.


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## mickri

I just completed converting my HF 125 flux core to DCEN.  It was not hard.  Everything was a tight fit.  I made all of the connections out of 3/16 soft copper tubing.  I flattened the tubing and bent it to shape to make the connections from the rectifier to the capacitor.   3/16 tubing was the perfect fit on the wire so I made the lugs and crimped them onto the wire for the connections on the ground and the electrode to the capacitor.  The picture isn't the best.




Did a couple of before and after test beads and really can't tell much difference.  But then I am not much of a welder.  I have a lot of welding to do on the MG Midget that I am resurrecting.  I'll have a better idea after I am done with that.


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## homebrewed

I did this conversion, too.  Although a total noob when it comes to welding, I've already used the modded welder to repair a garden rake and wing-type weeder.  The beads look pretty awful but the items are functional again.


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## jonesn7

I'm in the same boat. EBay purchases are easy. Finding time and the determination to geterdone is in short supply right now.





Bob Korves said:


> I still have the parts I bought, but so far have not got around to doing the modification. Maybe the additional kick from danbbb will get me going on it...


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## homebrewed

jonesn7 said:


> I'm in the same boat. EBay purchases are easy. Finding time and the determination to geterdone is in short supply right now.



Don't feel too bad.  It took me over a year to get around to actually doing the mod.  Once I got started it didn't take too long, though.  The part that took the longest was figuring out the best path for the added wires.  Being large-gauge they don't bend too easily & I didn't want to put too much stress on the diode and capacitor leads.

I did go to the extra trouble of attaching the diode bridge to a big heat sink.  But it doesn't seem to warm up much at all during welding.  In retrospect I probably could have just bolted it to the case.  I did check to make sure the diode's exposed metal heat spreader was electrically isolated from the diodes.....


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## jonesn7

I may just have some heat sink material (aluminum plate). Maybe sandwich a piece between the case and diode. Plus some good heat sink compond.


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## mickri

The bridge rectifier that I purchased on Ebay came with a heat sink as part of the rectifier.


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## mtb369

Well, I finished the mod last night at about 3am. This morning when I tested it out, I got one spark and then nothing. The fan and wire feed work alright, so I'm assuming the rectifier is blown. (I had a 120A.) 

Now to troubleshoot the system. Good times!


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## mickri

My rectifier is 150 amps.  I almost screwed up the connections.  The diagram on the rectifier had the positive and negative reversed from the markings on the actual contacts/leads/pins whatever you call them coming out of the rectifier.  I didn't notice the error until I was done making the connections when I noticed a "+" on the pin that the diagram indicated was a "-".


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## mtb369

mickri said:


> My rectifier is 150 amps.  I almost screwed up the connections.  The diagram on the rectifier had the positive and negative reversed from the markings on the actual contacts/leads/pins whatever you call them coming out of the rectifier.  I didn't notice the error until I was done making the connections when I noticed a "+" on the pin that the diagram indicated was a "-".



Looks like my rectifier is testing okay. The diodes aren't blown and they appear to have been labeled correctly...

If I'm testing my capacitor correctly, it also appears to be working. Wondering if I blew something within the welder itself?


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## mickri

One thing that I found is that the metal being welded had to have a really good ground connection and where I wanted to weld had to be bright and shiny.  I also found that I had to have the wire pointing pretty much straight down if that makes sense.  Too much of an angle and the wire would just feed out along the surface of the metal.  I am not much of a welder.  Have not done any welding in years.  I am sure that my technique is awful at best.


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## Bi11Hudson

Several (>10) years ago I came into posession of some individual diodes of high current capacity. They were pulls from an industrial DC crane feeders, some 500 Amp, 1000 Volt rating. Being individual components, they took up a* lot *of space. But, installed in a Lincoln 225, the space was there. I built the bus-bars of 1/2" copper tubing, flattened, into a full wave bridge. And appropriate insulators, of course. The results were impressive, to say the least. 

I have worked around DC machines for my entire life. The man who taught me to weld way back when, would write his name on the sheet metal siding on the building, without blowing any holes. Told me when I could do that, I could call myself a welder. Haven't made that yet, still working on it. The point being that with welders, DC *is* the way to go, regardless of size. Such a conversion is well worth the cost, in time or money, however you figure it.

On my Lincoln, I have yet to overheat the transformer from high duty cycle. Conceded, most of my work uses only two or three rods. But have increased the duty cycle at least double. How much more I'm not qualified to say. I have tried several times to use wire welders. Most times I just give up and revert to the stick.

Bill Hudson​


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## homebrewed

I am glad I'm not the only one that has problems using a wire welder.  Yesterday I was making some home-brew jacks that required welding some 3/4" bolt heads to steel plates.  The welds came out pretty horrid looking.  Oh well, at least the bolts are stuck down....which is all that's necessary in this case.


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## jonesn7

Sorry to dredge up an old thread. Just finished the conversion with parts that I've had for over a year. 100amp 1000volt rectifier (I feel the 150 amp 3 phase probably are not fully 150 amp on one phase), 2 paralleled 33000 mfd 50 Volt caps. I used 4 AWG for hook up to/from rectifier and through the choke, which is only 12 turns through a ferite ring. Used 6AWG to connect to the caps (flattened 1/4" copper tubing to join them) with the 50 ohm 10 watt ceramic resistor across them. Recitfier is mounted on ~2"x 6" x 1/2" aluminum plate with high-dollar heat paste that I had a bunch of left over. That is mounted under the mid-deck beside the xformer. The choke (on the +) is up with the wire feed and tapped into the electrode line there. Caps in front of the xformer on the bottom. A Folgers plastic coffee bucket was cut up and used as a fan shroud, and as extra insurance against undesired shorts.

Haven't tried it yet, maybe tomorrow if weather holds.


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## jonesn7

Not pretty. Doesn't need to be.


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## jonesn7

Tried it today. Much better than before, but found that I have to cut the wire right at the exit of the torch. This gives a short delay before the wire makes it to the workpiece, giving the cap time to charge. Otherwise, it just sticks to the workpiece and doesn't start arcing. Did a ~2.5" bead without hitting overload. A lot less sputter and better penetration.


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## homebrewed

I haven't noticed problems with the wire sticking to the workpiece.  Unlike your mod, I didn't add a series choke.  The choke would reduce the initial current flow so that could explain the difference.  

The choke also would smooth out the current flow so overall it should work better -- if I feed the wire too slow, the capacitor charges up enough to blow the wire up in a series of "bangs".  Faster feed rates seem to work OK.


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## AFTRD3TH

What are the details of your shunt resister, "OP"?
I apologize if you have addressed this in your article; I was just curious. I am aware that this is an old post.


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## BGHansen

The cap for my conversion included the resistor, I don't recall the ohms.  Here's a YouTube video of one of the conversions I watched.  He mentions 470 ohms in the video, but I didn't hear the watts.  Looks to be pretty hefty, maybe 10 - 20W.

Bruce


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## mickri

I used a 10 watt 260 ohm resistor in my conversion.  Not being an electrical wiz I don't remember why I picked that size.  It must have been the size recommended by videos that I watched on how to do this.


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## homebrewed

IIRC, I used something around 500 ohms.  Point is, it's not critical.  It is just meant to discharge the capacitor so the voltage drops pretty quickly, to avoid a shocking surprise later <g>.  The value also depends on how large the capacitor is -- the larger its value, the slower it discharges for a given shunt resistor value.


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## AFTRD3TH

Thank you for the responses where exactly does the capacitor connect I know it's on the DC side but is it after the rectifier leading into the ground and torch. The video kind of confused me????


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## homebrewed

AFTRD3TH said:


> Thank you for the responses where exactly does the capacitor connect I know it's on the DC side but is it after the rectifier leading into the ground and torch. The video kind of confused me????


It attaches to the "+" and "-" terminals of the rectifier.  Same side as the ground and torch.  Putting it on the wrong side of the rectifier could be very dangerous so make sure to get it right.  That also means connecting the plus side of the rectifier to the plus side of the capacitor and similarly for the negative side of each.

By "dangerous", I mean that the capacitor could possibly explode.  They're designed to fail gracefully but putting high-current AC into a big electrolytic capacitor is a pretty extreme overload condition.  It might pop a fuse before going bang, but maybe not....


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## AFTRD3TH

Thank you, Ill make sure that I make sure I do it right, I made a water block and water loop for my rectifier, as i have read that it produces high amount of head when converting AC to DC. Would a water block be beneficial


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## homebrewed

AFTRD3TH said:


> Thank you, Ill make sure that I make sure I do it right, I made a water block and water loop for my rectifier, as i have read that it produces high amount of head when converting AC to DC. Would a water block be beneficial


Not in my experience.  I was concerned about power dissipation in my diode bridge as well so I attached a large convection-type heat sink to mine, but it has never gotten very hot in use.


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## woodchucker

BGHansen said:


> I have a buddy who impulse bought a Harbor Freight 90A flux-core wire feed welder.  I have to admit to looking at them too, especially when they go on sale for under $90.  They’re probably not a bad deal for the occasional weekend welding project, plus the 110V portability is nice.  But as my dad constantly told me as a kid “you get what you pay for”.
> 
> Bottom line is the welds look terrible.  Penetration is poor and there is a lot of spatter.  Problem with the stock welder is it’s AC output, not DC.  Current flows back & forth from the puddle to the wire which causes the flux-core wire to spatter even more than DC flux-core wire welding.  All other HF wire feed welders are DC if that tells you anything.  I did a little web research, watched a few YouTube videos and found that it’s fairly easy to convert these units from AC to DC.  The two pieces of hardware needed are a full-wave rectifier and an electrolytic filter capacitor.
> 
> The rectifier takes the sine wave AC and flips the negative wave to the positive side so the output looks like “hills” running from 0V to peak voltage and back down, then back up again.  The capacitor smooths out the “hills” to a relatively straight line or DC output.  The videos I watched recommended a minimum 47,000 uF electrolytic capacitor with 30V minimum rating (welder outputs 28V).  I found a number of eBay sources for the rectifier rated at 1000V and 100A for $10.  Found a Siemens electrolytic cap rated at 40V and 47,000 uF for $25 delivered.
> 
> Another important component is a shunt resistor across the two leads of the capacitor.  The resistor drains the capacitor after welding, otherwise the wire is still grounded and your ground clamp is still hot!  Some guys on their conversion discharged the capacitor by touching the wire to the ground clamp; expect a nice spark.  The capacitor I found had a shunt resistor already there, drains the cap in about 3 seconds.
> 
> The conversion is pretty simple; cut the output leads from the welder’s transformer and jump them to the AC side of the rectifier.  The transformer output leads each have a small lead that runs to the welder’s circuit board, so the splicing takes place downstream from that point.
> 
> The rectifier has two output leads; + positive and – negative.  Those leads run to the appropriate terminals on the capacitor.  For flux-core wire, the welder ground clamp should be DC positive.  So, the welder’s ground clamp lead is tied to the capacitor’s + Positive lead.  The weld gun is tied to the capacitor’s – Negative lead.
> 
> One thing I didn’t care for on the on-line conversions is how they mounted the rectifier and capacitor.  All of the examples had the rectifier mounted on the side of the case.  They mounted the capacitor by drilling a couple of holes in the case and tie strapped in down.  I guess that’s probably how Harbor Freight would do it . . .
> 
> I chose to mount the rectifier on the back of the case, just looks cleaner to me.  Also made a sheet metal bracket to mount the capacitor next to the transformer.  The cooling fan had to be moved down to make room for the rectifier.  New fan holes were made in the case, in retrospect I should have removed the back panel and punched them on my Roper Whitney #218 press, but went the quicker route of drilling the holes.  Then spent 15 minutes cleaning up burrs . . .
> 
> You can see from the example welds that the DC conversion makes a world of difference on the quality of the welds.  I ran a bead using flux-core wire from my Hobart Handler 190, the HF stock AC and HF after the DC conversion.  Much, much reduced spatter and much better penetration.  The welder sounds better too, just a sizzle while welding instead of lots of spattering.
> 
> So what did I learn?  Since everything in my shop will end up being my son’s someday, I went out and bought another Hobart, now have a Handler 140 (110V) to compliment my 190 (220V only).  Sure, the Handler 140 110V welder is $500 compared to the bottom line about $150 for the HF 90A converted to DC.  But I don’t want my son to inherit my shop and say, “Wow, dad bought a lot of cheap junk!”  On the other hand, money doesn’t grow on trees so if you do have one of these HF 90A welders, consider spending around $50 and make some better welds.
> 
> Bruce
> 
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I did this long ago to the blue one.. the one that has no ability to turn off the torch .. it's always hot.
I did mine with a 200 amp bridge rectifier. I sanded a spot on the bottom of the case and used a thermal paste and attached it. I was never able to find a reasonable capacitor, they were freaking expensive. I added a A/C pancake fan, and I am able to run 100% duty cycle, it never bails on me.  the weld difference and sound were night and day different. my crappy welds became really decent welds for flux core. Now if I can steady my hand, and follow straight... I lose track of the joint sometimes.


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## homebrewed

I believe the torch on my HF version is hot all the time, too.  The only thing the trigger controls is the motor that runs the wire feeder.  That can get "interesting" if you put the torch down on your welding table.

I'm not sure I would do the conversion now.  The cost of the parts is _almost_ as much as what I recently paid for an inverter-style stick welder; and you get to enjoy DC welding right away.  Plus it weighs so much less than the HF welder and it has a lot more flexibility, since I can adjust the current (along with a couple of other welding parameters).  I'm not going to get rid of my wire welder but I suspect it won't be my go-to welder any more.


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## woodchucker

homebrewed said:


> I believe the torch on my HF version is hot all the time, too.  The only thing the trigger controls is the motor that runs the wire feeder.  That can get "interesting" if you put the torch down on your welding table.
> 
> I'm not sure I would do the conversion now.  The cost of the parts is _almost_ as much as what I recently paid for an inverter-style stick welder; and you get to enjoy DC welding right away.  Plus it weighs so much less than the HF welder and it has a lot more flexibility, since I can adjust the current (along with a couple of other welding parameters).  I'm not going to get rid of my wire welder but I suspect it won't be my go-to welder any more.


at the time the BR was only $16-$20 , I bought new connectors to attach the wires, clamp down style... maybe $10.. so not that expensive.


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## homebrewed

woodchucker said:


> at the time the BR was only $16-$20 , I bought new connectors to attach the wires, clamp down style... maybe $10.. so not that expensive.


The capacitor was the expensive part, as you commented earlier.  Folks that want to include the inductor may need to buy the core, too (I didn't go that far with my mod).


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## AFTRD3TH

Hello everyone.
I have upgraded my 125 AMP Harbor Freight (Chicago Electric) welder from AC to DCEN.
A little background: I'm originally from Australia, and I grew up on a Huge farm, often known as a ranch In the US.

With the need to repair farm equipment on a regular basis, welding and fabrication come naturally.

I used a stick and mostly MIG/TIG 240 Volt when I was growing up.
I moved to PA in 2017 I left all of my tools in Australia as shipping was expensive,
I purchased the Harbour Light 125 Amp Welder in 2018 and used it once and I thought it was TRASH I made a home made Stick welder instead.
 I was recommended a YouTube video about the conversion, so I decided to do some research on it. That's how I discovered this community.

Parts used
Bridge rectifier, 1600V, 300A
4 Gauge Gun and Ground leads
5 X 63V 22000 uF Capacitors
A dead microwave's 110v fan 
2 X 4" heat sinks.
Copper piping from a broken portable air conditioner.
HART battery drill motor as a new feeder motor.
NEMA Power cord.
New heavy duty ground clamp

My welder is double the breadth of a typical welder since I mounted it to the base of a broken portable air conditioner base plate.
I mounted the bridge rectifier vertically on an alloy plate with two thermal paste-coated heat sinks and a microwave cooling fan.

Bridge rectifier temperature does not exceed 70F.

At a later time, I will upload images. The welder performed like a 220v DCEN MIG without spatter.


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