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Harbor Freight 90a Flux-core Ac To Dc Conversion

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BGHansen

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#1
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

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#2
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
 

Bob Korves

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#3
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...
 

BGHansen

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#4
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

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Thanks, Bruce!

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

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#6
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.
 

markba633csi

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

danbbb

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#8
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.
 

Bob Korves

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#9
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...
 

danbbb

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#10
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.
 

mickri

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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.

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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.
 

homebrewed

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#12
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.
 

jonesn7

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

homebrewed

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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.....
 

jonesn7

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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.
 

mickri

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#16
The bridge rectifier that I purchased on Ebay came with a heat sink as part of the rectifier.
 
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