He is running it through the primary of the second transformer, and then using the secondary of the second transformer to feed the motor. The other wire should go to ground. It seems like you could just use a big cap (+resistor) to smooth it out. It is a neat way to do it, it just seems like overkill.
I’m also not convinced the rectifier is needed. I have a hard time imagining AC with mig working at all. My guess is someone mistook pulsed DC for AC.
Sean
This is what I gleaned from his video.
As I mentioned, I understood everything he did till he turned the welder around. It appears to me (I could be wrong) that the 120 volt hot side is being feed by tapping into the trigger relay and then he is tapping into a neutral wire somewhere on the back side. Since he mentioned the Fan, I assume he figured out which wire going to the fan was the neutral and tapped the other side of the primary into that.
Is my diagram correct to what he was doing?
I am a bit confused too. 
He is splicing the main transformer primary after the switch relay as the primary to the second transformer, and connecting the two secondary wires of the 2nd transformer to the motor. Which he said he was going to change 1 of the wires to the center tap to get 12v. I assume the pot/transistor for the wirespeed is -after- where he cut the wires from the board.
http://jacobs-online.biz/understanding_transformers.htm
I am a bit unsure about the toroid inductor, and that maybe why they have the rectifier to protect the main transformer from back current. You can use bare grounding wire then coat it, or you can run multiple smaller wires in parallel for the same effect.
Given I have seen 3 different schematics for the welder, I would put a multimeter on the transformer output leads to see if it is pulsed dc or ac. IIRC ac will show up as both positive and negative using the dc setting on the multimeter and pulsed dc will just go to 0 (at least on my multimeter.) I am guessing the rectifier is built in the power supply and that is why it isn’t showing in the schematic, but I could be wrong.
I just watched "Zero"s video for the third time. I have watched other videos that he has made (dealing with induction heating) and he is a pretty smart guy. I think he is correct in that the welder, as received, IS indeed an AC output machine. I also looked up specs on it and they say “duty cycle 20% @ 90 AC amps”.
It probably would be best to do a screen capture of the schematic that he shows at the beginning of his video, rather than attempting to redraw parts of it.
Then watch the video again try to follow what he SAYS, making notes as you go right on that schematic.
You WILL have to draw in a few things (that are missing from the schematic) as you go.
Another approach, depending on your comfort with electrical circuits like these, might be to start by simply making the DC rectifier change first. That one looks pretty straight forward. Then see if that works okay. Then move on to the next change, etc…
I have one of the very much older versions of that welder, so old that it doesn’t have any wire speed control at all. It also has the “hot” gun, but I put up with it in the few cases where I can’t get my old BetaMig 250 close enough.
If you already have one of these welders, the users manual has a schematic toward the end of it. That should help you to understand what’s in there as you get it (before any modifications).
And— if you don’t have the manual, you can always get an electronic copy by going to the Harbor Freight listing for that product. Go down toward the bottom of the page and you will find an area that has clickable tabs like this:
Customer overview, Product Reviews, Warranty, User Manual and Quick Start Guide, Buying Guide.
Click the "User Manual’ tab and you get the manual. The schematic is toward the end of it.
I make no guarantee that your machine will exactly fit, but it’s worth a look.
Actually I believe he is still letting the two leads connected to the main circuit board connect to the wire feed motor. Then I believe he has one of the leads from the second transformer’s primary side spliced into the trigger relay. The two leads from the second transformer’s secondary side appears to be connected to the circuit board. That leaves one lead from the second transformer primary connection. This seems to be the mystery lead.
Obviously I could be all wrong about theses connection points. He seem to show his handy work as if everyone knew what he had done instead of teaching, but following him along, the above diagram appears to be correct.
If anyone knows for sure, we would all like to know.
Madflower69:
After watching his video 4 more times, I think you are correct. It does appear that he re-routed the feed motor wires to the secondary of the second transformer.
Since he apparently spliced into the trigger relay on the motherboard and connected that lead to one of the primary leads on the second transformer, it looks like he spliced into the the blue fan wire (its blue in my HF) on hot side of the switch and connected that to the other lead on the primary side of the second transformer. So assuming the trigger relay is the AC neutral and the fan wire splice is the AC hot side, then that all makes sense now.
Have I got it right?
I believe there is a newer version and an older version of the welder in the different videos. I never changed anything to do with the small transformer. I only added the rectifier, the choke, and the resisters to the transformer outputs to change them from ac to dc.
I would be tempted to put the motor on a separate circuit using the second transformer. The trick would be finding a way to tap the trigger relay input signal with a second wire to another relay that controls the new circuit. It might be to tricky to do and it might have been his original intent.
Brian,great job on the welder will do mine also now, by the way the bridge rectifier (MDS150A 3-Phase Diode Bridge Rectifier 150A Amp 1600V) is in fact a full wave rectifier.To get half wave rectification you use only one diode from ac, this is not the case here,where two diodes are used for each phase.
At risk of starting an argument, You can do better. One of the good things about Harbor freight, they are one of the OK Cheep brands. You might be able to get some parts for some models…
Check into used Lincoln, Miller, Hobart, ESAB machines. Check less expensive but well supported machine brands like Lotos, Eastwood, Everlast, USAWeld (HTP) and some others turning up the heat on the main-stream brands. Always check the internet for reviews before buying, wisdom comes with many opinions averaged out! 
With the mods, you are essentially upgrading it. I got an old well-used mig century model, for what I could have gotten a HF model for, but I looked for about a year for it and it leaks some gas, but I don’t use it often enough to justify buying a new gun/hose kit for it.
I have 125 amp Harbor Freight Flux Core welder. I bought it for $89. Weld was splattery (like dog’s slobber) and sucky. I decide to check out Welder mods. I found your site as well as other. I ordered 150amp Full Bridge Rectifier and 68000uf 63v Big Can Capacitor. Bought couple foot of black and red 6 guage wires, connector ect… I put it all together and made it neat and organize. I use old Pentuim III heatsink (4 inches silver heat sink with tiny cooling fan removed) and put thermal paste on its back, I drilled hole and bolted it to Full Bridge Rectifier. I trimmed Secondary Transformer wire that it will hold Rectifier without any contact around the welder case and cooling fin facing toward to welder cooling fan. Capacitor laid on bottom of welder and hooked up 6 gauges wires (red and black) according to its polarity to Capacitor and from there I hooked Electrode to Negative and Clamp to Positive. I haven’t add resistor to drain Capacitor yet but will do that soon. I tested welded the scrap using cursive “e” and setting #4 speed and Min setting. Weld on scrap look so beautiful, enough to make me cry. No splatter!! Soon, I will order Toroid cores and 6 Guage enameled wire to wrap around. I did not need to unsolder anything, just unscrew capacitor and re connect to Toroid and then I’m ready to weld. I will post Picture of my “design”. So it works perfect for the first time I’ve put together. (Of Course I’m electronic Technician myself).
Matt,
Why add toroid if allready beautiful? I get thats the way its usually done, but…
Rindert
LOL. Just want to have smoother DC current. I’ve built a micro DCEN experiment on breadboard. All old Electronic scraps from junks and luckily had a big fat toroid coil with 16 guage wire wrapped around from 600 watts high performance PC power supply (it had a lot of Toroids in it). I use 120v to 28v ac step down transformer, 20 amps full wave rectifier, 3 of 680uf 25v, big fat toroid, and 1k ohm resister (for draining capacitor). Fired it up and use volt meter and oscilloscope. It show that without Toroid, I can see flicker of ac on dc line, with Toroid, dc line is smooth. I posted it on YOutube. https://youtu.be/iN5UCXu79hg
Unfortunately, mini DCEN can’t weld but it put out big spark when tapping “electrode” end to ground…
(update) I just put up youtube video of my welder mod. it’s just silent video but observe! On end of video, you may see
video moved to edge of welder to see how flush the mods are. I will put thick tapes on connector so it won’t touch panel. Just my common sense…
Good luck
I have been struggling over the past year with power sources for my welding projects. I have a new-in-the-box buzz box 240 volt AC/DC stick welder and I made an extension cord to plug it into a range outlet, but I never had the opportunity to do so yet. I tried running my HFT flux core machine off of my 1500 watt inverter, but the inverter refuses to power it. So I ran an extension cord to my neighbor’s house while I wait for my 5000 watt 240 volt inverter.
But yesterday I was welding away on a stool for my gasifier and my flux core machine started buzzing loudly and lost a lot of power. It is unusable as-is. Upon examination there is either a short in the transformer windings or a bad connection at the spade connectors. So I used my deep cycle batteries to stick weld finish my stool. But I was left without a machine for welding thin sheet metal.
Today I wired four solar panels in parallel directly to the high current output of my wire feed machine before the inductor and disconnected the transformer. I was able to heat up and melt the wire, but I only had around 16 amps at 32 volts coming from the panels. I had a social function to attend so I didn’t connect more panels, but I did get a load center today and 10 circuit breakers to connect 10 to 15 panels together. After today’s proof of concept I am confident I will be welding wire feed powered by the sun tomorrow.
Here is a close-up of the guts of my welder. The two big wires on the left are positive and negative from the load center. Positive connects to the (lower) positive post of the capacitor and goes on to the ground clamp. Negative goes to the inductor through the other side and then connects to the negative post of the capacitor and on up the cables to the gun. The two wires from the secondary coil of the transformer are detached and taped off. I also removed the resistors that I had to drain the capacitor. I am now back to a hot tip gun, but I think that helps to kick start the arc.
