Modifying the Harbor Freight 120v welder (if you already own one)

Behold some simple, cheap modifications to turn a metal glue gun into a higher performance machine with a rock solid arc that welds like a dream.

I have the 90 dollar 90 amp flux core welder from Harbor Freight (item #68887) that I paid for by doing the muffler job on my truck two and a half years ago. Now I still have the welder, and I’m building a gasifier.

I studied the reviews on the HF website over a period of time before buying it. I asked my brother, an experienced MIG welder about flux core. He said it’s horrible. However, I decided it would work on my truck exhaust system, and it did. I read one reviewer say that he modified his HF welder to weld with DCEN (direct current electrode negative) instead of A/C. I was a bit confused and intimidated by that statement, though, and never seriously considered it. ChrisAdmin and Wayne Keith both highly recommend a quality welder for gasifier building, and I agree. But this is what I have.

Now during the building of my new gasifier I am taking some YouTube courses on flux core welding, and the topic of DCEN comes up over and over. Everyone including Steve Bleile, ChuckE2009, Miller, and everyone else says that flux core welding should be DCEN. Even the Harbor Freight manuals for the 170 and 180 amp units say that flux core should be DCEN. (They are not stupid.) I see the difference in the weld quality on DCEN, and also I see how simple and cheap it actually is to make the conversion.

With A/C flux core the arc changes direction 60 times per second, moving from the wire to the work then from the work to the wire. When it jumps back and forth like that it causes the hottest point to move back and forth. Electrons moving through air cause heat, and when they move from the wire to the work, the hottest point is near the work. This oscillation causes an unstable arc, it causes the wire to pop 60 times per second, and that is the cause of the massive spatter with a stock HF unit (combined with really low quality wire). The weld ends up being colder, less penetration, the slag gets dispersed everywhere causing weld contamination, oxidation, brittle welds. Worm holes. Not a fun time. A lot of this is fixed with DCEN. The arc is stable, heat is concentrated on the work, and with a quality flux core wire: spatter is minimal, the weld is covered evenly with slag, penetration is maximized, and the weld is stronger.

Harbor Freight makes A/C output welders because their target market are people like me who didn’t know the difference, and it’s cheaper to make a welder with A/C output that will glue metal together. It’s a good plan, and they’re making money.

I ordered the parts I needed from eBay to convert my unit to DCEN, and I am very excited about turning my day-late-and-dollar-short welder into a higher performance machine that I can take with me in my truck and weld anywhere. That is an advantage that the flux core wire process has over the MIG processes: welding outdoors in windy conditions.

This conversion requires, bare minimum, a bridge rectifier, a capacitor, some wire, wire connectors, screws, and soldering equipment. Minimum cost is about $13 to $35, depending on the availability of parts and materials. For me it cost closer to $100 since I went ahead and made more upgrades than the bare minimum. I also added some resistors across the capacitor terminals, a toroid core inductor, a 22 foot 10 gauge power cord, and a beefy ground clamp.

I ordered and installed a 150amp three phase 1200 volt bridge rectifier $13, a 68,000 microfarad 63 volt capacitor $20, a toroid core $13, some ceramic resistors $4, a 22-foot 10 gauge cord $34, plug for the cord $10, zinc cable connector $5, 500-amp ground clamp $10 and etc. Also necessary was some miscellaneous hardware like wire, soldering supplies, machine screws, and etc. All told, I spent more than $100 on the upgrades, and that is why it’s not worth buying a new unit to upgrade: for $50 more you can get the $150 220 volt 170 amp welder that is DC from the factory, and also can use shielding gas for MIG welding. That is one of the great things about this little welder: it can be upgraded as you go along.

When I disassembled my unit to do the conversion, the first thing I noticed is that the motor for the cooling fan is mounted to the vent between the fan and the fan blade. The blade is a few inches away from the vent. This allows internal turbulence (in aviation: wing tip vortices) to reduce the cooling ability of the fan. I immediately knew that I would be changing that. I removed the entire assembly to make room inside, and I remounted the motor on the outside. Fortunately there are tapped holes on both sides of the fan motor on my unit. It was a simple matter of unplugging the wires, unscrewing the motor mount screws from the outside, drilling a hole in the fiberglass mounting plate, removing the fan blade from the shaft (pulls right off), and re-mounting the motor on the outside of my case, and then replacing the fan blade back on the motor shaft with just a push. There is a spring that holds the fan blade on. Also, a hole must be drilled in the case to feed the power wires through and reconnect to the main power switch. Now my fan blade is right next the vent holes on the back of my unit, eliminating the internal turbulence, and maximizing the cooling ability of the fan. Also, the bearing started screaming recently, and the externally mounted motor allows easy access to lubricate it.

Some notes on the rest of my modifications: I think a 50volt capacitor is the minimum. I found a 63v unit. The higher voltage rating versus the voltage required makes the unit larger. I would prefer a 50 volt unit, but that’s what I found at the time. A three-phase rectifier was $7 cheaper than a single-phase unit, and works just as well. I believe that the output on this welder is 120 amps on max: the website says the output is 60-120 amps; a 100 amp bridge rectifier will burn up; a 150 amp bridge rectifier will not. I mounted mine to the bulkhead between the lower section where the transformer is and the upper section where the wire spool and wire feed mechanism are. I wanted the heat sink to be in the cooling airstream, to connect to the bulkhead for maximum cooling while keeping it inside the case. I left the power supply for the wire feed motor controller attached to the secondary coil of the main transformer. I may upgrade this in the future, but so far I haven’t had problems with erratic wire feeding. I added an inductor to my unit which is optional for this upgrade. The inductor stabilizes the line current to make it as flat as possible. Unfortunately I clamped my coil into a vise while I was making it, and beat it with a hammer. I broke the toroid core in two, which greatly diminishes the effectiveness of the inductor. It is difficult to wrap 4 gauge solid wire tightly around the toroid core, and I recommend not hitting it with a hammer while clamped in a vice. I went ahead and finished making mine with the cracked core. I may replace this in the future. Also, I added a resistor bank to drain the capacitor slowly when I finish welding so that the tip does not remain hot, even with the power switch off. I noticed that the output voltage dropped with this resistor across the capacitor, so I added another resistor. The voltage dropped again, but not so much. I want to experiment with adding switched resistor circuits across the terminals of the capacitor to fine tune the heat output on my welder, but haven’t got to that point, yet. I got some toggle switches for that purpose, but I’ll need more resistors to do what I’m wanting: five to ten toggle switches to add and remove resistors at will.

The result of these modifications moves my little junk cheap welder from the higher end of the lowest class of wire feed welders into the lower end of the next higher class of $150 to $500 welders. It runs hotter, the arc is rock solid, and by adjusting the wire feed rate I can dial in that sizzling bacon sound that I could never get on A/C output.

I was hoping to be able to lap weld in the horizontal position two pieces of bandsaw blade or 55-gallon drum metal together in one continuous bead without burning through. At first I was not able to do this, but then Steve Bleile said that increasing stickout will decrease the amperage at the weld puddle, thus cooling the weld. The wire acts as a resistor and gets hot. I was able to weld my whole flat bed on my truck with this method in continuous beads without burning through. Now it’s less important to me to dial the heat down. All I have to do is increase stickout. So I will continue welding my gasifier together, experimenting with more stickout and better quality wire before experimenting with more switched resistors on the DC side of my welder.

Other modifications I am considering are a separate power supply for the wire feed mechanism. Then I can experiment with reducing the voltage on the A/C side without harming the wire feed motor. I want to wire in a duplex receptacle so that I can plug things like a grinder into the welder to take advantage of my beefy 22-foot power cord. Also I would like to increase the duty cycle by improving the cooling of the transformer. Firstly I will take my 30 inch industrial fan and move a massive amount of air through the unit whenever I hit the duty cycle limit. (I have noticed that while winter welding I can weld longer, because the cooling system in the welder works better in cold weather, of course.) Later on I can paint the outside white, which will keep it cooler outside. On the extreme side of cooling, there is the possibility of making a liquid cooling system out of copper pipe for my little swelder that could. That is for the more distant future, though. Also, I might add MIG capabilities with a MIG wire feed system.

As a final note, I would recommend avoiding the $90 Harbor Freight welder if possible. If you already have one, you may want to consider converting it to DCEN. It only costs about $13 for the bridge rectifier (get the three-phase 150 amp unit: it’ll work, and not burn up), and $5 to $20 for the capacitor. Wire, cables, and clamps will be extra.

Some photos of my conversion are forthcoming.

Here are two links to two good demonstrations of how to make this conversion:

by ZeroFossileFuel:

by Matt Heere

weld, baby weld!


Here is a picture of my welder after converting to DCEN. The fan motor is on the outside, and I have a big power cord and ground clamp.

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Now I know why that welder is so crappy… AC??

Thanks for the run down. I may do this to mine at some point…

Also, I wouldn’t count the cost of the long cord with the welder. I bought a 12ga x 25’ extension cord from HF for $20, and have used it for lots of other stuff too. 10ga is overkill for that welder. So your actual conversion cost was closer to $50, not bad!

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I didn’t take many pictures of the conversion process. When I open it up again to arrange the resistor circuits I will take more photos. Here is where I put my capacitor:

Here it doesn’t block the air flow through the welder. Capacitors don’t need to be air cooled. And it tucks right in under the circuit board and between two of the rubber foot screws. I used JB Weld to hold it secure and keep it from rubbing on the screws.

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My Harbor Freight welder failed on me yesterday for the very first time. Armed with intimate knowledge of the inner workings of my unit, I did not automatically junk it and go buy another like others. Instead, I opened it up and determined the problem.

The welder was welding particularly cold, and I saw smoke coming from the torch handle. Eventually it failed to arc at all when I pulled the trigger; it only fed the wire. So I touched the wire to the ground clamp and pulled the trigger. Only wire. Then I opened the case, released the wire feed roller, and touched the wire to the positive pole on the capacitor. Trigger pull; no arc. I took a multimeter, set it to Volts DC and tested the poles of the capacitor. It charged up to 30 some volts, so I knew the innards of the welder were working fine, and that there was a fault between the capacitor and the torch tip on the negative side. I opened up the torch and saw smoke residue inside, and where the lead was supposed to be electrically connected to the brass torch tip there was a lot of nasty looking stuff. I pulled at the lead and saw where it was supposed to be threaded, and immediately knew that thar was the problem. So I sanded the surfaces and soldered the connection together, assembled the torch back together, and proceeded to continue welding on my merry way. No cost repair. Powerful knowledge.


Good news Brian .

Thanks for sharing :blush:

Here is an example of my welder on DCEN with the two resistors across the capacitor. The wire is Hobart E71T-11. For this weld I had perfectly prepared work that was bare clean bare metal. The pipe is galvanized. I had virtually no spatter, but two tiny bbs welding these two puffer pipes.

All I did was brush it with a wire brush.

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I was wondering where you found the parts. I looked on eBay but wasn’t sure which parts were correct. Do you have a link?

150 amp three phase bridge rectifier: It seems the design has changed. This is the link to what they sell now:

Capacitor: whatever you can find. Used will work. Look for a 50 volt 50,000 microfarad or better. Or you can get several 2.7 volt or other small capacitors and wire them in series. Here’s one
It’s a little small, but you could get a few cheap 10,000 microfarad capacitor and wire them in parallel:
These are cheap, but this particular lot has many. You should be able to find them in smaller quantities.

And for the inductor, I used bare 4 AWG copper wire and used vinyl electrical tape around it lengthwise for an insulator. The toroid core I bought:

i’m afraid that 150 amps rectifier is half wave, not full and your loosing half of the dc cycles. those 100 amps full wave work just fine, they have around 10-25% tolerance anyway.

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If that were true, and since energy is neither created nor destroyed, then where is the power going? I am not afraid I’m losing half the DC cycles, since my welder is hotter on DC than on AC.

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Would it just cycle back into the grid without any real draw, like a tv that’s turned off?

What is the difference between full wave and half wave ,is it the way the diodes are wired, not enough diodes, or just the type of diode,?

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I dig it. I am so going to do this. It is too easy you almost have to if you have a HF flux core welder.

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Hello Leroy,good luck with your mig welder mod, it should work good for the thinner metals like the barrow housing and the like. I am not sure of your knowege, be carefull with dc voltage,if it rises higher than expected could be real dangerous comeing off the caps,as DC voltage hangs on and dont let go,unlike ac that is safer, my mig welder has 6 caps,and a contious bleed resistor acrrost the caps at all times, so it pulls the high voltage down too safe level, directly after the caps serve there voltage stabilizing purpose.I had some 220 caps on my hho torch, and when i tested the caps for power,it sounded like a 22 pistol pop,zap, jolt, just glad i wasent the ground.The mig welder voltage should be under 30-40 open circuit voltage,much less dangerous, other than the caps.Let us know how it helps, i have a HF that may get the boost sometime.A bigger power fan will add more duty cycle too.

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Thanks for the input on the capacitors. I don’t dig into much without first consulting some of my friends who know better than I. My first bad experience was when I was 15 and I dug the capacitor out of a furnace fan. It got me but not too bad. Just enough to let me know what a capacitor does.


Thats about the way i am when it comes to electric or electronics, I caint remember how i tested the cap. i think i unpluged the water torch, and hung a srewdriver with plastic handle close too the other leg when it scared the out of me. shortly after that i pulled the cap out of the circuit for model changes,and never put it back with the water torch, after testing the torch on my geo metro,it took about 8000 watts too idle the car, it was too much drag on an alternator, Any way i switched too wood gas about a year and half ago when i bought the Wayne Keith plans booklet.I did get a hell of a zap trying too make the trigger circuit work on a home made plasma cutter, that one dident make it much past my arm, though it did heat it up a bit.I am glad i bought a plasma cutter now.

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Well, I looked up what a half-wave rectifier is, and sure enough, it exists. But the thing is, that the three-phase rectifier takes the sine-wave single phase AC and converts it properly into a bouncy DC signal. So there is no need for a full-wave rectifier, thank you very much.


Hi Brian Woodrow,have you welded with the three phase diode yet and or how good do they work, i was thinking on using them for replacements for my HF mig too, are you mounting them too the original heat sink plates,or adding different heat sinks.Happy Easter

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Hey Kevin. I have welded a lot with my welder since I made this mod last year in February. So it’s been over a year. I haven’t opened it up to change the resistors, yet, but I can open it up and take some photos if you’d like.n The three phase diode works fine with a single phase. I think it’s a bit hotter because the signal is steady and on all the time instead of on/off/on/off/on/off. Just my opinion.